GNU Linux-libre 5.19-rc6-gnu

[releases.git] / drivers / platform / surface / aggregator / ssh_request_layer.c

// SPDX-License-Identifier: GPL-2.0+
/*
 * SSH request transport layer.
 *
 * Copyright (C) 2019-2021 Maximilian Luz <luzmaximilian@gmail.com>
 */

#include <asm/unaligned.h>
#include <linux/atomic.h>
#include <linux/completion.h>
#include <linux/error-injection.h>
#include <linux/ktime.h>
#include <linux/limits.h>
#include <linux/list.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/types.h>
#include <linux/workqueue.h>

#include <linux/surface_aggregator/serial_hub.h>
#include <linux/surface_aggregator/controller.h>

#include "ssh_packet_layer.h"
#include "ssh_request_layer.h"

#include "trace.h"

/*
 * SSH_RTL_REQUEST_TIMEOUT - Request timeout.
 *
 * Timeout as ktime_t delta for request responses. If we have not received a
 * response in this time-frame after finishing the underlying packet
 * transmission, the request will be completed with %-ETIMEDOUT as status
 * code.
 */
#define SSH_RTL_REQUEST_TIMEOUT                 ms_to_ktime(3000)

/*
 * SSH_RTL_REQUEST_TIMEOUT_RESOLUTION - Request timeout granularity.
 *
 * Time-resolution for timeouts. Should be larger than one jiffy to avoid
 * direct re-scheduling of reaper work_struct.
 */
#define SSH_RTL_REQUEST_TIMEOUT_RESOLUTION      ms_to_ktime(max(2000 / HZ, 50))

/*
 * SSH_RTL_MAX_PENDING - Maximum number of pending requests.
 *
 * Maximum number of requests concurrently waiting to be completed (i.e.
 * waiting for the corresponding packet transmission to finish if they don't
 * have a response or waiting for a response if they have one).
 */
#define SSH_RTL_MAX_PENDING             3

/*
 * SSH_RTL_TX_BATCH - Maximum number of requests processed per work execution.
 * Used to prevent livelocking of the workqueue. Value chosen via educated
 * guess, may be adjusted.
 */
#define SSH_RTL_TX_BATCH                10

#ifdef CONFIG_SURFACE_AGGREGATOR_ERROR_INJECTION

/**
 * ssh_rtl_should_drop_response() - Error injection hook to drop request
 * responses.
 *
 * Useful to cause request transmission timeouts in the driver by dropping the
 * response to a request.
 */
static noinline bool ssh_rtl_should_drop_response(void)
{
        return false;
}
ALLOW_ERROR_INJECTION(ssh_rtl_should_drop_response, TRUE);

#else

static inline bool ssh_rtl_should_drop_response(void)
{
        return false;
}

#endif

static u16 ssh_request_get_rqid(struct ssh_request *rqst)
{
        return get_unaligned_le16(rqst->packet.data.ptr
                                  + SSH_MSGOFFSET_COMMAND(rqid));
}

static u32 ssh_request_get_rqid_safe(struct ssh_request *rqst)
{
        if (!rqst->packet.data.ptr)
                return U32_MAX;

        return ssh_request_get_rqid(rqst);
}

static void ssh_rtl_queue_remove(struct ssh_request *rqst)
{
        struct ssh_rtl *rtl = ssh_request_rtl(rqst);

        spin_lock(&rtl->queue.lock);

        if (!test_and_clear_bit(SSH_REQUEST_SF_QUEUED_BIT, &rqst->state)) {
                spin_unlock(&rtl->queue.lock);
                return;
        }

        list_del(&rqst->node);

        spin_unlock(&rtl->queue.lock);
        ssh_request_put(rqst);
}

static bool ssh_rtl_queue_empty(struct ssh_rtl *rtl)
{
        bool empty;

        spin_lock(&rtl->queue.lock);
        empty = list_empty(&rtl->queue.head);
        spin_unlock(&rtl->queue.lock);

        return empty;
}

static void ssh_rtl_pending_remove(struct ssh_request *rqst)
{
        struct ssh_rtl *rtl = ssh_request_rtl(rqst);

        spin_lock(&rtl->pending.lock);

        if (!test_and_clear_bit(SSH_REQUEST_SF_PENDING_BIT, &rqst->state)) {
                spin_unlock(&rtl->pending.lock);
                return;
        }

        atomic_dec(&rtl->pending.count);
        list_del(&rqst->node);

        spin_unlock(&rtl->pending.lock);

        ssh_request_put(rqst);
}

static int ssh_rtl_tx_pending_push(struct ssh_request *rqst)
{
        struct ssh_rtl *rtl = ssh_request_rtl(rqst);

        spin_lock(&rtl->pending.lock);

        if (test_bit(SSH_REQUEST_SF_LOCKED_BIT, &rqst->state)) {
                spin_unlock(&rtl->pending.lock);
                return -EINVAL;
        }

        if (test_and_set_bit(SSH_REQUEST_SF_PENDING_BIT, &rqst->state)) {
                spin_unlock(&rtl->pending.lock);
                return -EALREADY;
        }

        atomic_inc(&rtl->pending.count);
        list_add_tail(&ssh_request_get(rqst)->node, &rtl->pending.head);

        spin_unlock(&rtl->pending.lock);
        return 0;
}

static void ssh_rtl_complete_with_status(struct ssh_request *rqst, int status)
{
        struct ssh_rtl *rtl = ssh_request_rtl(rqst);

        trace_ssam_request_complete(rqst, status);

        /* rtl/ptl may not be set if we're canceling before submitting. */
        rtl_dbg_cond(rtl, "rtl: completing request (rqid: %#06x, status: %d)\n",
                     ssh_request_get_rqid_safe(rqst), status);

        rqst->ops->complete(rqst, NULL, NULL, status);
}

static void ssh_rtl_complete_with_rsp(struct ssh_request *rqst,
                                      const struct ssh_command *cmd,
                                      const struct ssam_span *data)
{
        struct ssh_rtl *rtl = ssh_request_rtl(rqst);

        trace_ssam_request_complete(rqst, 0);

        rtl_dbg(rtl, "rtl: completing request with response (rqid: %#06x)\n",
                ssh_request_get_rqid(rqst));

        rqst->ops->complete(rqst, cmd, data, 0);
}

static bool ssh_rtl_tx_can_process(struct ssh_request *rqst)
{
        struct ssh_rtl *rtl = ssh_request_rtl(rqst);

        if (test_bit(SSH_REQUEST_TY_FLUSH_BIT, &rqst->state))
                return !atomic_read(&rtl->pending.count);

        return atomic_read(&rtl->pending.count) < SSH_RTL_MAX_PENDING;
}

static struct ssh_request *ssh_rtl_tx_next(struct ssh_rtl *rtl)
{
        struct ssh_request *rqst = ERR_PTR(-ENOENT);
        struct ssh_request *p, *n;

        spin_lock(&rtl->queue.lock);

        /* Find first non-locked request and remove it. */
        list_for_each_entry_safe(p, n, &rtl->queue.head, node) {
                if (unlikely(test_bit(SSH_REQUEST_SF_LOCKED_BIT, &p->state)))
                        continue;

                if (!ssh_rtl_tx_can_process(p)) {
                        rqst = ERR_PTR(-EBUSY);
                        break;
                }

                /* Remove from queue and mark as transmitting. */
                set_bit(SSH_REQUEST_SF_TRANSMITTING_BIT, &p->state);
                /* Ensure state never gets zero. */
                smp_mb__before_atomic();
                clear_bit(SSH_REQUEST_SF_QUEUED_BIT, &p->state);

                list_del(&p->node);

                rqst = p;
                break;
        }

        spin_unlock(&rtl->queue.lock);
        return rqst;
}

static int ssh_rtl_tx_try_process_one(struct ssh_rtl *rtl)
{
        struct ssh_request *rqst;
        int status;

        /* Get and prepare next request for transmit. */
        rqst = ssh_rtl_tx_next(rtl);
        if (IS_ERR(rqst))
                return PTR_ERR(rqst);

        /* Add it to/mark it as pending. */
        status = ssh_rtl_tx_pending_push(rqst);
        if (status) {
                ssh_request_put(rqst);
                return -EAGAIN;
        }

        /* Submit packet. */
        status = ssh_ptl_submit(&rtl->ptl, &rqst->packet);
        if (status == -ESHUTDOWN) {
                /*
                 * Packet has been refused due to the packet layer shutting
                 * down. Complete it here.
                 */
                set_bit(SSH_REQUEST_SF_LOCKED_BIT, &rqst->state);
                /*
                 * Note: A barrier is not required here, as there are only two
                 * references in the system at this point: The one that we have,
                 * and the other one that belongs to the pending set. Due to the
                 * request being marked as "transmitting", our process is the
                 * only one allowed to remove the pending node and change the
                 * state. Normally, the task would fall to the packet callback,
                 * but as this is a path where submission failed, this callback
                 * will never be executed.
                 */

                ssh_rtl_pending_remove(rqst);
                ssh_rtl_complete_with_status(rqst, -ESHUTDOWN);

                ssh_request_put(rqst);
                return -ESHUTDOWN;

        } else if (status) {
                /*
                 * If submitting the packet failed and the packet layer isn't
                 * shutting down, the packet has either been submitted/queued
                 * before (-EALREADY, which cannot happen as we have
                 * guaranteed that requests cannot be re-submitted), or the
                 * packet was marked as locked (-EINVAL). To mark the packet
                 * locked at this stage, the request, and thus the packets
                 * itself, had to have been canceled. Simply drop the
                 * reference. Cancellation itself will remove it from the set
                 * of pending requests.
                 */

                WARN_ON(status != -EINVAL);

                ssh_request_put(rqst);
                return -EAGAIN;
        }

        ssh_request_put(rqst);
        return 0;
}

static bool ssh_rtl_tx_schedule(struct ssh_rtl *rtl)
{
        if (atomic_read(&rtl->pending.count) >= SSH_RTL_MAX_PENDING)
                return false;

        if (ssh_rtl_queue_empty(rtl))
                return false;

        return schedule_work(&rtl->tx.work);
}

static void ssh_rtl_tx_work_fn(struct work_struct *work)
{
        struct ssh_rtl *rtl = to_ssh_rtl(work, tx.work);
        unsigned int iterations = SSH_RTL_TX_BATCH;
        int status;

        /*
         * Try to be nice and not block/live-lock the workqueue: Run a maximum
         * of 10 tries, then re-submit if necessary. This should not be
         * necessary for normal execution, but guarantee it anyway.
         */
        do {
                status = ssh_rtl_tx_try_process_one(rtl);
                if (status == -ENOENT || status == -EBUSY)
                        return;         /* No more requests to process. */

                if (status == -ESHUTDOWN) {
                        /*
                         * Packet system shutting down. No new packets can be
                         * transmitted. Return silently, the party initiating
                         * the shutdown should handle the rest.
                         */
                        return;
                }

                WARN_ON(status != 0 && status != -EAGAIN);
        } while (--iterations);

        /* Out of tries, reschedule. */
        ssh_rtl_tx_schedule(rtl);
}

/**
 * ssh_rtl_submit() - Submit a request to the transport layer.
 * @rtl:  The request transport layer.
 * @rqst: The request to submit.
 *
 * Submits a request to the transport layer. A single request may not be
 * submitted multiple times without reinitializing it.
 *
 * Return: Returns zero on success, %-EINVAL if the request type is invalid or
 * the request has been canceled prior to submission, %-EALREADY if the
 * request has already been submitted, or %-ESHUTDOWN in case the request
 * transport layer has been shut down.
 */
int ssh_rtl_submit(struct ssh_rtl *rtl, struct ssh_request *rqst)
{
        trace_ssam_request_submit(rqst);

        /*
         * Ensure that requests expecting a response are sequenced. If this
         * invariant ever changes, see the comment in ssh_rtl_complete() on what
         * is required to be changed in the code.
         */
        if (test_bit(SSH_REQUEST_TY_HAS_RESPONSE_BIT, &rqst->state))
                if (!test_bit(SSH_PACKET_TY_SEQUENCED_BIT, &rqst->packet.state))
                        return -EINVAL;

        spin_lock(&rtl->queue.lock);

        /*
         * Try to set ptl and check if this request has already been submitted.
         *
         * Must be inside lock as we might run into a lost update problem
         * otherwise: If this were outside of the lock, cancellation in
         * ssh_rtl_cancel_nonpending() may run after we've set the ptl
         * reference but before we enter the lock. In that case, we'd detect
         * that the request is being added to the queue and would try to remove
         * it from that, but removal might fail because it hasn't actually been
         * added yet. By putting this cmpxchg in the critical section, we
         * ensure that the queuing detection only triggers when we are already
         * in the critical section and the remove process will wait until the
         * push operation has been completed (via lock) due to that. Only then,
         * we can safely try to remove it.
         */
        if (cmpxchg(&rqst->packet.ptl, NULL, &rtl->ptl)) {
                spin_unlock(&rtl->queue.lock);
                return -EALREADY;
        }

        /*
         * Ensure that we set ptl reference before we continue modifying state.
         * This is required for non-pending cancellation. This barrier is paired
         * with the one in ssh_rtl_cancel_nonpending().
         *
         * By setting the ptl reference before we test for "locked", we can
         * check if the "locked" test may have already run. See comments in
         * ssh_rtl_cancel_nonpending() for more detail.
         */
        smp_mb__after_atomic();

        if (test_bit(SSH_RTL_SF_SHUTDOWN_BIT, &rtl->state)) {
                spin_unlock(&rtl->queue.lock);
                return -ESHUTDOWN;
        }

        if (test_bit(SSH_REQUEST_SF_LOCKED_BIT, &rqst->state)) {
                spin_unlock(&rtl->queue.lock);
                return -EINVAL;
        }

        set_bit(SSH_REQUEST_SF_QUEUED_BIT, &rqst->state);
        list_add_tail(&ssh_request_get(rqst)->node, &rtl->queue.head);

        spin_unlock(&rtl->queue.lock);

        ssh_rtl_tx_schedule(rtl);
        return 0;
}

static void ssh_rtl_timeout_reaper_mod(struct ssh_rtl *rtl, ktime_t now,
                                       ktime_t expires)
{
        unsigned long delta = msecs_to_jiffies(ktime_ms_delta(expires, now));
        ktime_t aexp = ktime_add(expires, SSH_RTL_REQUEST_TIMEOUT_RESOLUTION);

        spin_lock(&rtl->rtx_timeout.lock);

        /* Re-adjust / schedule reaper only if it is above resolution delta. */
        if (ktime_before(aexp, rtl->rtx_timeout.expires)) {
                rtl->rtx_timeout.expires = expires;
                mod_delayed_work(system_wq, &rtl->rtx_timeout.reaper, delta);
        }

        spin_unlock(&rtl->rtx_timeout.lock);
}

static void ssh_rtl_timeout_start(struct ssh_request *rqst)
{
        struct ssh_rtl *rtl = ssh_request_rtl(rqst);
        ktime_t timestamp = ktime_get_coarse_boottime();
        ktime_t timeout = rtl->rtx_timeout.timeout;

        if (test_bit(SSH_REQUEST_SF_LOCKED_BIT, &rqst->state))
                return;

        /*
         * Note: The timestamp gets set only once. This happens on the packet
         * callback. All other access to it is read-only.
         */
        WRITE_ONCE(rqst->timestamp, timestamp);
        /*
         * Ensure timestamp is set before starting the reaper. Paired with
         * implicit barrier following check on ssh_request_get_expiration() in
         * ssh_rtl_timeout_reap.
         */
        smp_mb__after_atomic();

        ssh_rtl_timeout_reaper_mod(rtl, timestamp, timestamp + timeout);
}

static void ssh_rtl_complete(struct ssh_rtl *rtl,
                             const struct ssh_command *command,
                             const struct ssam_span *command_data)
{
        struct ssh_request *r = NULL;
        struct ssh_request *p, *n;
        u16 rqid = get_unaligned_le16(&command->rqid);

        trace_ssam_rx_response_received(command, command_data->len);

        /*
         * Get request from pending based on request ID and mark it as response
         * received and locked.
         */
        spin_lock(&rtl->pending.lock);
        list_for_each_entry_safe(p, n, &rtl->pending.head, node) {
                /* We generally expect requests to be processed in order. */
                if (unlikely(ssh_request_get_rqid(p) != rqid))
                        continue;

                /* Simulate response timeout. */
                if (ssh_rtl_should_drop_response()) {
                        spin_unlock(&rtl->pending.lock);

                        trace_ssam_ei_rx_drop_response(p);
                        rtl_info(rtl, "request error injection: dropping response for request %p\n",
                                 &p->packet);
                        return;
                }

                /*
                 * Mark as "response received" and "locked" as we're going to
                 * complete it.
                 */
                set_bit(SSH_REQUEST_SF_LOCKED_BIT, &p->state);
                set_bit(SSH_REQUEST_SF_RSPRCVD_BIT, &p->state);
                /* Ensure state never gets zero. */
                smp_mb__before_atomic();
                clear_bit(SSH_REQUEST_SF_PENDING_BIT, &p->state);

                atomic_dec(&rtl->pending.count);
                list_del(&p->node);

                r = p;
                break;
        }
        spin_unlock(&rtl->pending.lock);

        if (!r) {
                rtl_warn(rtl, "rtl: dropping unexpected command message (rqid = %#06x)\n",
                         rqid);
                return;
        }

        /* If the request hasn't been completed yet, we will do this now. */
        if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state)) {
                ssh_request_put(r);
                ssh_rtl_tx_schedule(rtl);
                return;
        }

        /*
         * Make sure the request has been transmitted. In case of a sequenced
         * request, we are guaranteed that the completion callback will run on
         * the receiver thread directly when the ACK for the packet has been
         * received. Similarly, this function is guaranteed to run on the
         * receiver thread. Thus we are guaranteed that if the packet has been
         * successfully transmitted and received an ACK, the transmitted flag
         * has been set and is visible here.
         *
         * We are currently not handling unsequenced packets here, as those
         * should never expect a response as ensured in ssh_rtl_submit. If this
         * ever changes, one would have to test for
         *
         *      (r->state & (transmitting | transmitted))
         *
         * on unsequenced packets to determine if they could have been
         * transmitted. There are no synchronization guarantees as in the
         * sequenced case, since, in this case, the callback function will not
         * run on the same thread. Thus an exact determination is impossible.
         */
        if (!test_bit(SSH_REQUEST_SF_TRANSMITTED_BIT, &r->state)) {
                rtl_err(rtl, "rtl: received response before ACK for request (rqid = %#06x)\n",
                        rqid);

                /*
                 * NB: Timeout has already been canceled, request already been
                 * removed from pending and marked as locked and completed. As
                 * we receive a "false" response, the packet might still be
                 * queued though.
                 */
                ssh_rtl_queue_remove(r);

                ssh_rtl_complete_with_status(r, -EREMOTEIO);
                ssh_request_put(r);

                ssh_rtl_tx_schedule(rtl);
                return;
        }

        /*
         * NB: Timeout has already been canceled, request already been
         * removed from pending and marked as locked and completed. The request
         * can also not be queued any more, as it has been marked as
         * transmitting and later transmitted. Thus no need to remove it from
         * anywhere.
         */

        ssh_rtl_complete_with_rsp(r, command, command_data);
        ssh_request_put(r);

        ssh_rtl_tx_schedule(rtl);
}

static bool ssh_rtl_cancel_nonpending(struct ssh_request *r)
{
        struct ssh_rtl *rtl;
        unsigned long flags, fixed;
        bool remove;

        /*
         * Handle unsubmitted request: Try to mark the packet as locked,
         * expecting the state to be zero (i.e. unsubmitted). Note that, if
         * setting the state worked, we might still be adding the packet to the
         * queue in a currently executing submit call. In that case, however,
         * ptl reference must have been set previously, as locked is checked
         * after setting ptl. Furthermore, when the ptl reference is set, the
         * submission process is guaranteed to have entered the critical
         * section. Thus only if we successfully locked this request and ptl is
         * NULL, we have successfully removed the request, i.e. we are
         * guaranteed that, due to the "locked" check in ssh_rtl_submit(), the
         * packet will never be added. Otherwise, we need to try and grab it
         * from the queue, where we are now guaranteed that the packet is or has
         * been due to the critical section.
         *
         * Note that if the cmpxchg() fails, we are guaranteed that ptl has
         * been set and is non-NULL, as states can only be nonzero after this
         * has been set. Also note that we need to fetch the static (type)
         * flags to ensure that they don't cause the cmpxchg() to fail.
         */
        fixed = READ_ONCE(r->state) & SSH_REQUEST_FLAGS_TY_MASK;
        flags = cmpxchg(&r->state, fixed, SSH_REQUEST_SF_LOCKED_BIT);

        /*
         * Force correct ordering with regards to state and ptl reference access
         * to safe-guard cancellation to concurrent submission against a
         * lost-update problem. First try to exchange state, then also check
         * ptl if that worked. This barrier is paired with the
         * one in ssh_rtl_submit().
         */
        smp_mb__after_atomic();

        if (flags == fixed && !READ_ONCE(r->packet.ptl)) {
                if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
                        return true;

                ssh_rtl_complete_with_status(r, -ECANCELED);
                return true;
        }

        rtl = ssh_request_rtl(r);
        spin_lock(&rtl->queue.lock);

        /*
         * Note: 1) Requests cannot be re-submitted. 2) If a request is
         * queued, it cannot be "transmitting"/"pending" yet. Thus, if we
         * successfully remove the request here, we have removed all its
         * occurrences in the system.
         */

        remove = test_and_clear_bit(SSH_REQUEST_SF_QUEUED_BIT, &r->state);
        if (!remove) {
                spin_unlock(&rtl->queue.lock);
                return false;
        }

        set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state);
        list_del(&r->node);

        spin_unlock(&rtl->queue.lock);

        ssh_request_put(r);     /* Drop reference obtained from queue. */

        if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
                return true;

        ssh_rtl_complete_with_status(r, -ECANCELED);
        return true;
}

static bool ssh_rtl_cancel_pending(struct ssh_request *r)
{
        /* If the packet is already locked, it's going to be removed shortly. */
        if (test_and_set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state))
                return true;

        /*
         * Now that we have locked the packet, we have guaranteed that it can't
         * be added to the system any more. If ptl is NULL, the locked
         * check in ssh_rtl_submit() has not been run and any submission,
         * currently in progress or called later, won't add the packet. Thus we
         * can directly complete it.
         *
         * The implicit memory barrier of test_and_set_bit() should be enough
         * to ensure that the correct order (first lock, then check ptl) is
         * ensured. This is paired with the barrier in ssh_rtl_submit().
         */
        if (!READ_ONCE(r->packet.ptl)) {
                if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
                        return true;

                ssh_rtl_complete_with_status(r, -ECANCELED);
                return true;
        }

        /*
         * Try to cancel the packet. If the packet has not been completed yet,
         * this will subsequently (and synchronously) call the completion
         * callback of the packet, which will complete the request.
         */
        ssh_ptl_cancel(&r->packet);

        /*
         * If the packet has been completed with success, i.e. has not been
         * canceled by the above call, the request may not have been completed
         * yet (may be waiting for a response). Check if we need to do this
         * here.
         */
        if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
                return true;

        ssh_rtl_queue_remove(r);
        ssh_rtl_pending_remove(r);
        ssh_rtl_complete_with_status(r, -ECANCELED);

        return true;
}

/**
 * ssh_rtl_cancel() - Cancel request.
 * @rqst:    The request to cancel.
 * @pending: Whether to also cancel pending requests.
 *
 * Cancels the given request. If @pending is %false, this will not cancel
 * pending requests, i.e. requests that have already been submitted to the
 * packet layer but not been completed yet. If @pending is %true, this will
 * cancel the given request regardless of the state it is in.
 *
 * If the request has been canceled by calling this function, both completion
 * and release callbacks of the request will be executed in a reasonable
 * time-frame. This may happen during execution of this function, however,
 * there is no guarantee for this. For example, a request currently
 * transmitting will be canceled/completed only after transmission has
 * completed, and the respective callbacks will be executed on the transmitter
 * thread, which may happen during, but also some time after execution of the
 * cancel function.
 *
 * Return: Returns %true if the given request has been canceled or completed,
 * either by this function or prior to calling this function, %false
 * otherwise. If @pending is %true, this function will always return %true.
 */
bool ssh_rtl_cancel(struct ssh_request *rqst, bool pending)
{
        struct ssh_rtl *rtl;
        bool canceled;

        if (test_and_set_bit(SSH_REQUEST_SF_CANCELED_BIT, &rqst->state))
                return true;

        trace_ssam_request_cancel(rqst);

        if (pending)
                canceled = ssh_rtl_cancel_pending(rqst);
        else
                canceled = ssh_rtl_cancel_nonpending(rqst);

        /* Note: rtl may be NULL if request has not been submitted yet. */
        rtl = ssh_request_rtl(rqst);
        if (canceled && rtl)
                ssh_rtl_tx_schedule(rtl);

        return canceled;
}

static void ssh_rtl_packet_callback(struct ssh_packet *p, int status)
{
        struct ssh_request *r = to_ssh_request(p);

        if (unlikely(status)) {
                set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state);

                if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
                        return;

                /*
                 * The packet may get canceled even though it has not been
                 * submitted yet. The request may still be queued. Check the
                 * queue and remove it if necessary. As the timeout would have
                 * been started in this function on success, there's no need
                 * to cancel it here.
                 */
                ssh_rtl_queue_remove(r);
                ssh_rtl_pending_remove(r);
                ssh_rtl_complete_with_status(r, status);

                ssh_rtl_tx_schedule(ssh_request_rtl(r));
                return;
        }

        /* Update state: Mark as transmitted and clear transmitting. */
        set_bit(SSH_REQUEST_SF_TRANSMITTED_BIT, &r->state);
        /* Ensure state never gets zero. */
        smp_mb__before_atomic();
        clear_bit(SSH_REQUEST_SF_TRANSMITTING_BIT, &r->state);

        /* If we expect a response, we just need to start the timeout. */
        if (test_bit(SSH_REQUEST_TY_HAS_RESPONSE_BIT, &r->state)) {
                /*
                 * Note: This is the only place where the timestamp gets set,
                 * all other access to it is read-only.
                 */
                ssh_rtl_timeout_start(r);
                return;
        }

        /*
         * If we don't expect a response, lock, remove, and complete the
         * request. Note that, at this point, the request is guaranteed to have
         * left the queue and no timeout has been started. Thus we only need to
         * remove it from pending. If the request has already been completed (it
         * may have been canceled) return.
         */

        set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state);
        if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
                return;

        ssh_rtl_pending_remove(r);
        ssh_rtl_complete_with_status(r, 0);

        ssh_rtl_tx_schedule(ssh_request_rtl(r));
}

static ktime_t ssh_request_get_expiration(struct ssh_request *r, ktime_t timeout)
{
        ktime_t timestamp = READ_ONCE(r->timestamp);

        if (timestamp != KTIME_MAX)
                return ktime_add(timestamp, timeout);
        else
                return KTIME_MAX;
}

static void ssh_rtl_timeout_reap(struct work_struct *work)
{
        struct ssh_rtl *rtl = to_ssh_rtl(work, rtx_timeout.reaper.work);
        struct ssh_request *r, *n;
        LIST_HEAD(claimed);
        ktime_t now = ktime_get_coarse_boottime();
        ktime_t timeout = rtl->rtx_timeout.timeout;
        ktime_t next = KTIME_MAX;

        trace_ssam_rtl_timeout_reap(atomic_read(&rtl->pending.count));

        /*
         * Mark reaper as "not pending". This is done before checking any
         * requests to avoid lost-update type problems.
         */
        spin_lock(&rtl->rtx_timeout.lock);
        rtl->rtx_timeout.expires = KTIME_MAX;
        spin_unlock(&rtl->rtx_timeout.lock);

        spin_lock(&rtl->pending.lock);
        list_for_each_entry_safe(r, n, &rtl->pending.head, node) {
                ktime_t expires = ssh_request_get_expiration(r, timeout);

                /*
                 * Check if the timeout hasn't expired yet. Find out next
                 * expiration date to be handled after this run.
                 */
                if (ktime_after(expires, now)) {
                        next = ktime_before(expires, next) ? expires : next;
                        continue;
                }

                /* Avoid further transitions if locked. */
                if (test_and_set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state))
                        continue;

                /*
                 * We have now marked the packet as locked. Thus it cannot be
                 * added to the pending or queued lists again after we've
                 * removed it here. We can therefore re-use the node of this
                 * packet temporarily.
                 */

                clear_bit(SSH_REQUEST_SF_PENDING_BIT, &r->state);

                atomic_dec(&rtl->pending.count);
                list_move_tail(&r->node, &claimed);
        }
        spin_unlock(&rtl->pending.lock);

        /* Cancel and complete the request. */
        list_for_each_entry_safe(r, n, &claimed, node) {
                trace_ssam_request_timeout(r);

                /*
                 * At this point we've removed the packet from pending. This
                 * means that we've obtained the last (only) reference of the
                 * system to it. Thus we can just complete it.
                 */
                if (!test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
                        ssh_rtl_complete_with_status(r, -ETIMEDOUT);

                /*
                 * Drop the reference we've obtained by removing it from the
                 * pending set.
                 */
                list_del(&r->node);
                ssh_request_put(r);
        }

        /* Ensure that the reaper doesn't run again immediately. */
        next = max(next, ktime_add(now, SSH_RTL_REQUEST_TIMEOUT_RESOLUTION));
        if (next != KTIME_MAX)
                ssh_rtl_timeout_reaper_mod(rtl, now, next);

        ssh_rtl_tx_schedule(rtl);
}

static void ssh_rtl_rx_event(struct ssh_rtl *rtl, const struct ssh_command *cmd,
                             const struct ssam_span *data)
{
        trace_ssam_rx_event_received(cmd, data->len);

        rtl_dbg(rtl, "rtl: handling event (rqid: %#06x)\n",
                get_unaligned_le16(&cmd->rqid));

        rtl->ops.handle_event(rtl, cmd, data);
}

static void ssh_rtl_rx_command(struct ssh_ptl *p, const struct ssam_span *data)
{
        struct ssh_rtl *rtl = to_ssh_rtl(p, ptl);
        struct device *dev = &p->serdev->dev;
        struct ssh_command *command;
        struct ssam_span command_data;

        if (sshp_parse_command(dev, data, &command, &command_data))
                return;

        if (ssh_rqid_is_event(get_unaligned_le16(&command->rqid)))
                ssh_rtl_rx_event(rtl, command, &command_data);
        else
                ssh_rtl_complete(rtl, command, &command_data);
}

static void ssh_rtl_rx_data(struct ssh_ptl *p, const struct ssam_span *data)
{
        if (!data->len) {
                ptl_err(p, "rtl: rx: no data frame payload\n");
                return;
        }

        switch (data->ptr[0]) {
        case SSH_PLD_TYPE_CMD:
                ssh_rtl_rx_command(p, data);
                break;

        default:
                ptl_err(p, "rtl: rx: unknown frame payload type (type: %#04x)\n",
                        data->ptr[0]);
                break;
        }
}

static void ssh_rtl_packet_release(struct ssh_packet *p)
{
        struct ssh_request *rqst;

        rqst = to_ssh_request(p);
        rqst->ops->release(rqst);
}

static const struct ssh_packet_ops ssh_rtl_packet_ops = {
        .complete = ssh_rtl_packet_callback,
        .release = ssh_rtl_packet_release,
};

/**
 * ssh_request_init() - Initialize SSH request.
 * @rqst:  The request to initialize.
 * @flags: Request flags, determining the type of the request.
 * @ops:   Request operations.
 *
 * Initializes the given SSH request and underlying packet. Sets the message
 * buffer pointer to %NULL and the message buffer length to zero. This buffer
 * has to be set separately via ssh_request_set_data() before submission and
 * must contain a valid SSH request message.
 *
 * Return: Returns zero on success or %-EINVAL if the given flags are invalid.
 */
int ssh_request_init(struct ssh_request *rqst, enum ssam_request_flags flags,
                     const struct ssh_request_ops *ops)
{
        unsigned long type = BIT(SSH_PACKET_TY_BLOCKING_BIT);

        /* Unsequenced requests cannot have a response. */
        if (flags & SSAM_REQUEST_UNSEQUENCED && flags & SSAM_REQUEST_HAS_RESPONSE)
                return -EINVAL;

        if (!(flags & SSAM_REQUEST_UNSEQUENCED))
                type |= BIT(SSH_PACKET_TY_SEQUENCED_BIT);

        ssh_packet_init(&rqst->packet, type, SSH_PACKET_PRIORITY(DATA, 0),
                        &ssh_rtl_packet_ops);

        INIT_LIST_HEAD(&rqst->node);

        rqst->state = 0;
        if (flags & SSAM_REQUEST_HAS_RESPONSE)
                rqst->state |= BIT(SSH_REQUEST_TY_HAS_RESPONSE_BIT);

        rqst->timestamp = KTIME_MAX;
        rqst->ops = ops;

        return 0;
}

/**
 * ssh_rtl_init() - Initialize request transport layer.
 * @rtl:    The request transport layer to initialize.
 * @serdev: The underlying serial device, i.e. the lower-level transport.
 * @ops:    Request transport layer operations.
 *
 * Initializes the given request transport layer and associated packet
 * transport layer. Transmitter and receiver threads must be started
 * separately via ssh_rtl_start(), after the request-layer has been
 * initialized and the lower-level serial device layer has been set up.
 *
 * Return: Returns zero on success and a nonzero error code on failure.
 */
int ssh_rtl_init(struct ssh_rtl *rtl, struct serdev_device *serdev,
                 const struct ssh_rtl_ops *ops)
{
        struct ssh_ptl_ops ptl_ops;
        int status;

        ptl_ops.data_received = ssh_rtl_rx_data;

        status = ssh_ptl_init(&rtl->ptl, serdev, &ptl_ops);
        if (status)
                return status;

        spin_lock_init(&rtl->queue.lock);
        INIT_LIST_HEAD(&rtl->queue.head);

        spin_lock_init(&rtl->pending.lock);
        INIT_LIST_HEAD(&rtl->pending.head);
        atomic_set_release(&rtl->pending.count, 0);

        INIT_WORK(&rtl->tx.work, ssh_rtl_tx_work_fn);

        spin_lock_init(&rtl->rtx_timeout.lock);
        rtl->rtx_timeout.timeout = SSH_RTL_REQUEST_TIMEOUT;
        rtl->rtx_timeout.expires = KTIME_MAX;
        INIT_DELAYED_WORK(&rtl->rtx_timeout.reaper, ssh_rtl_timeout_reap);

        rtl->ops = *ops;

        return 0;
}

/**
 * ssh_rtl_destroy() - Deinitialize request transport layer.
 * @rtl: The request transport layer to deinitialize.
 *
 * Deinitializes the given request transport layer and frees resources
 * associated with it. If receiver and/or transmitter threads have been
 * started, the layer must first be shut down via ssh_rtl_shutdown() before
 * this function can be called.
 */
void ssh_rtl_destroy(struct ssh_rtl *rtl)
{
        ssh_ptl_destroy(&rtl->ptl);
}

/**
 * ssh_rtl_start() - Start request transmitter and receiver.
 * @rtl: The request transport layer.
 *
 * Return: Returns zero on success, a negative error code on failure.
 */
int ssh_rtl_start(struct ssh_rtl *rtl)
{
        int status;

        status = ssh_ptl_tx_start(&rtl->ptl);
        if (status)
                return status;

        ssh_rtl_tx_schedule(rtl);

        status = ssh_ptl_rx_start(&rtl->ptl);
        if (status) {
                ssh_rtl_flush(rtl, msecs_to_jiffies(5000));
                ssh_ptl_tx_stop(&rtl->ptl);
                return status;
        }

        return 0;
}

struct ssh_flush_request {
        struct ssh_request base;
        struct completion completion;
        int status;
};

static void ssh_rtl_flush_request_complete(struct ssh_request *r,
                                           const struct ssh_command *cmd,
                                           const struct ssam_span *data,
                                           int status)
{
        struct ssh_flush_request *rqst;

        rqst = container_of(r, struct ssh_flush_request, base);
        rqst->status = status;
}

static void ssh_rtl_flush_request_release(struct ssh_request *r)
{
        struct ssh_flush_request *rqst;

        rqst = container_of(r, struct ssh_flush_request, base);
        complete_all(&rqst->completion);
}

static const struct ssh_request_ops ssh_rtl_flush_request_ops = {
        .complete = ssh_rtl_flush_request_complete,
        .release = ssh_rtl_flush_request_release,
};

/**
 * ssh_rtl_flush() - Flush the request transport layer.
 * @rtl:     request transport layer
 * @timeout: timeout for the flush operation in jiffies
 *
 * Queue a special flush request and wait for its completion. This request
 * will be completed after all other currently queued and pending requests
 * have been completed. Instead of a normal data packet, this request submits
 * a special flush packet, meaning that upon completion, also the underlying
 * packet transport layer has been flushed.
 *
 * Flushing the request layer guarantees that all previously submitted
 * requests have been fully completed before this call returns. Additionally,
 * flushing blocks execution of all later submitted requests until the flush
 * has been completed.
 *
 * If the caller ensures that no new requests are submitted after a call to
 * this function, the request transport layer is guaranteed to have no
 * remaining requests when this call returns. The same guarantee does not hold
 * for the packet layer, on which control packets may still be queued after
 * this call.
 *
 * Return: Returns zero on success, %-ETIMEDOUT if the flush timed out and has
 * been canceled as a result of the timeout, or %-ESHUTDOWN if the packet
 * and/or request transport layer has been shut down before this call. May
 * also return %-EINTR if the underlying packet transmission has been
 * interrupted.
 */
int ssh_rtl_flush(struct ssh_rtl *rtl, unsigned long timeout)
{
        const unsigned int init_flags = SSAM_REQUEST_UNSEQUENCED;
        struct ssh_flush_request rqst;
        int status;

        ssh_request_init(&rqst.base, init_flags, &ssh_rtl_flush_request_ops);
        rqst.base.packet.state |= BIT(SSH_PACKET_TY_FLUSH_BIT);
        rqst.base.packet.priority = SSH_PACKET_PRIORITY(FLUSH, 0);
        rqst.base.state |= BIT(SSH_REQUEST_TY_FLUSH_BIT);

        init_completion(&rqst.completion);

        status = ssh_rtl_submit(rtl, &rqst.base);
        if (status)
                return status;

        ssh_request_put(&rqst.base);

        if (!wait_for_completion_timeout(&rqst.completion, timeout)) {
                ssh_rtl_cancel(&rqst.base, true);
                wait_for_completion(&rqst.completion);
        }

        WARN_ON(rqst.status != 0 && rqst.status != -ECANCELED &&
                rqst.status != -ESHUTDOWN && rqst.status != -EINTR);

        return rqst.status == -ECANCELED ? -ETIMEDOUT : rqst.status;
}

/**
 * ssh_rtl_shutdown() - Shut down request transport layer.
 * @rtl: The request transport layer.
 *
 * Shuts down the request transport layer, removing and canceling all queued
 * and pending requests. Requests canceled by this operation will be completed
 * with %-ESHUTDOWN as status. Receiver and transmitter threads will be
 * stopped, the lower-level packet layer will be shutdown.
 *
 * As a result of this function, the transport layer will be marked as shut
 * down. Submission of requests after the transport layer has been shut down
 * will fail with %-ESHUTDOWN.
 */
void ssh_rtl_shutdown(struct ssh_rtl *rtl)
{
        struct ssh_request *r, *n;
        LIST_HEAD(claimed);
        int pending;

        set_bit(SSH_RTL_SF_SHUTDOWN_BIT, &rtl->state);
        /*
         * Ensure that the layer gets marked as shut-down before actually
         * stopping it. In combination with the check in ssh_rtl_submit(),
         * this guarantees that no new requests can be added and all already
         * queued requests are properly canceled.
         */
        smp_mb__after_atomic();

        /* Remove requests from queue. */
        spin_lock(&rtl->queue.lock);
        list_for_each_entry_safe(r, n, &rtl->queue.head, node) {
                set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state);
                /* Ensure state never gets zero. */
                smp_mb__before_atomic();
                clear_bit(SSH_REQUEST_SF_QUEUED_BIT, &r->state);

                list_move_tail(&r->node, &claimed);
        }
        spin_unlock(&rtl->queue.lock);

        /*
         * We have now guaranteed that the queue is empty and no more new
         * requests can be submitted (i.e. it will stay empty). This means that
         * calling ssh_rtl_tx_schedule() will not schedule tx.work any more. So
         * we can simply call cancel_work_sync() on tx.work here and when that
         * returns, we've locked it down. This also means that after this call,
         * we don't submit any more packets to the underlying packet layer, so
         * we can also shut that down.
         */

        cancel_work_sync(&rtl->tx.work);
        ssh_ptl_shutdown(&rtl->ptl);
        cancel_delayed_work_sync(&rtl->rtx_timeout.reaper);

        /*
         * Shutting down the packet layer should also have canceled all
         * requests. Thus the pending set should be empty. Attempt to handle
         * this gracefully anyways, even though this should be dead code.
         */

        pending = atomic_read(&rtl->pending.count);
        if (WARN_ON(pending)) {
                spin_lock(&rtl->pending.lock);
                list_for_each_entry_safe(r, n, &rtl->pending.head, node) {
                        set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state);
                        /* Ensure state never gets zero. */
                        smp_mb__before_atomic();
                        clear_bit(SSH_REQUEST_SF_PENDING_BIT, &r->state);

                        list_move_tail(&r->node, &claimed);
                }
                spin_unlock(&rtl->pending.lock);
        }

        /* Finally, cancel and complete the requests we claimed before. */
        list_for_each_entry_safe(r, n, &claimed, node) {
                /*
                 * We need test_and_set() because we still might compete with
                 * cancellation.
                 */
                if (!test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
                        ssh_rtl_complete_with_status(r, -ESHUTDOWN);

                /*
                 * Drop the reference we've obtained by removing it from the
                 * lists.
                 */
                list_del(&r->node);
                ssh_request_put(r);
        }
}