[PATCH RESEND 1/2] dma-fence: allow signaling drivers to set fence timestamp

[Daniel Vetter]

On Thu, Nov 12, 2020 at 7:27 PM Veera Sundaram Sankaran
<veeras at codeaurora.org> wrote:
>
> Some drivers have hardware capability to get the precise timestamp of
> certain events based on which the fences are triggered. This allows it
> to set accurate timestamp factoring out any software and IRQ latencies.
> Move the timestamp parameter out of union in dma_fence struct to allow
> signaling drivers to set it. If the parameter is not set, ktime_get is
> used to set the current time to fence timestamp during dma_fence_signal.
>
> Signed-off-by: Veera Sundaram Sankaran <veeras at codeaurora.org>

So with they "why?" question fully resolved, I think this is a bit too
much a hack. I think much better if we pass the timestamp explicitly,
in a new dma_fence_signal_timestamp variant. That means a bit more
work, but I think it will handle this special case cleaner.

Also means we need to wire the timestamp through the entire call stack
on the drm side too. So we need a drm_send_event_locked_timestamp
variant too for send_vblank_event.
-Daniel

> ---
>  drivers/dma-buf/dma-fence.c | 18 ++++++++++--------
>  include/linux/dma-fence.h   | 15 +++------------
>  2 files changed, 13 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 43624b4..7cef49a 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -4,6 +4,7 @@
>   *
>   * Copyright (C) 2012 Canonical Ltd
>   * Copyright (C) 2012 Texas Instruments
> + * Copyright (c) 2020 The Linux Foundation. All rights reserved.
>   *
>   * Authors:
>   * Rob Clark <robdclark at gmail.com>
> @@ -329,7 +330,6 @@ void __dma_fence_might_wait(void)
>  int dma_fence_signal_locked(struct dma_fence *fence)
>  {
>         struct dma_fence_cb *cur, *tmp;
> -       struct list_head cb_list;
>
>         lockdep_assert_held(fence->lock);
>
> @@ -337,16 +337,18 @@ int dma_fence_signal_locked(struct dma_fence *fence)
>                                       &fence->flags)))
>                 return -EINVAL;
>
> -       /* Stash the cb_list before replacing it with the timestamp */
> -       list_replace(&fence->cb_list, &cb_list);
> -
> -       fence->timestamp = ktime_get();
> +       /* set current time, if not set by signaling driver */
> +       if (!fence->timestamp)
> +               fence->timestamp = ktime_get();
>         set_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags);
>         trace_dma_fence_signaled(fence);
>
> -       list_for_each_entry_safe(cur, tmp, &cb_list, node) {
> -               INIT_LIST_HEAD(&cur->node);
> -               cur->func(fence, cur);
> +       if (!list_empty(&fence->cb_list)) {
> +               list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
> +                       INIT_LIST_HEAD(&cur->node);
> +                       cur->func(fence, cur);
> +               }
> +               INIT_LIST_HEAD(&fence->cb_list);
>         }
>
>         return 0;
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index 09e23ad..a9eebaf 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -4,6 +4,7 @@
>   *
>   * Copyright (C) 2012 Canonical Ltd
>   * Copyright (C) 2012 Texas Instruments
> + * Copyright (c) 2020 The Linux Foundation. All rights reserved.
>   *
>   * Authors:
>   * Rob Clark <robdclark at gmail.com>
> @@ -70,26 +71,16 @@ struct dma_fence {
>          * release the fence it is unused. No one should be adding to the
>          * cb_list that they don't themselves hold a reference for.
>          *
> -        * The lifetime of the timestamp is similarly tied to both the
> -        * rcu freelist and the cb_list. The timestamp is only set upon
> -        * signaling while simultaneously notifying the cb_list. Ergo, we
> -        * only use either the cb_list of timestamp. Upon destruction,
> -        * neither are accessible, and so we can use the rcu. This means
> -        * that the cb_list is *only* valid until the signal bit is set,
> -        * and to read either you *must* hold a reference to the fence,
> -        * and not just the rcu_read_lock.
> -        *
>          * Listed in chronological order.
>          */
>         union {
>                 struct list_head cb_list;
> -               /* @cb_list replaced by @timestamp on dma_fence_signal() */
> -               ktime_t timestamp;
> -               /* @timestamp replaced by @rcu on dma_fence_release() */
> +               /* @cb_list replaced by @rcu on dma_fence_release() */
>                 struct rcu_head rcu;
>         };
>         u64 context;
>         u64 seqno;
> +       ktime_t timestamp;
>         unsigned long flags;
>         struct kref refcount;
>         int error;
> --
> 2.7.4
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch