[PATCH] dma-buf: add dma_fence_timestamp helper

[Alex Deucher]

On Sat, Sep 30, 2023 at 7:06 PM Christian König
<ckoenig.leichtzumerken at gmail.com> wrote:
>
> When a fence signals there is a very small race window where the timestamp
> isn't updated yet. sync_file solves this by busy waiting for the
> timestamp to appear, but on other ocassions didn't handled this
> correctly.
>
> Provide a dma_fence_timestamp() helper function for this and use it in
> all appropriate cases.
>
> Another alternative would be to grab the spinlock when that happens.
>
> v2 by teddy: add a wait parameter to wait for the timestamp to show up, in case
>    the accurate timestamp is needed and/or the timestamp is not based on
>    ktime (e.g. hw timestamp)
> v3 chk: drop the parameter again for unified handling
>
> Signed-off-by: Yunxiang Li <Yunxiang.Li at amd.com>
> Signed-off-by: Christian König <christian.koenig at amd.com>
> Fixes: 1774baa64f93 ("drm/scheduler: Change scheduled fence track v2")

Looks correct to me.

Reviewed-by: Alex Deucher <alexander.deucher at amd.com>

> ---
>  drivers/dma-buf/dma-fence-unwrap.c     | 13 ++++---------
>  drivers/dma-buf/sync_file.c            |  9 +++------
>  drivers/gpu/drm/scheduler/sched_main.c |  2 +-
>  include/linux/dma-fence.h              | 19 +++++++++++++++++++
>  4 files changed, 27 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-fence-unwrap.c b/drivers/dma-buf/dma-fence-unwrap.c
> index c625bb2b5d56..628af51c81af 100644
> --- a/drivers/dma-buf/dma-fence-unwrap.c
> +++ b/drivers/dma-buf/dma-fence-unwrap.c
> @@ -76,16 +76,11 @@ struct dma_fence *__dma_fence_unwrap_merge(unsigned int num_fences,
>                 dma_fence_unwrap_for_each(tmp, &iter[i], fences[i]) {
>                         if (!dma_fence_is_signaled(tmp)) {
>                                 ++count;
> -                       } else if (test_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT,
> -                                           &tmp->flags)) {
> -                               if (ktime_after(tmp->timestamp, timestamp))
> -                                       timestamp = tmp->timestamp;
>                         } else {
> -                               /*
> -                                * Use the current time if the fence is
> -                                * currently signaling.
> -                                */
> -                               timestamp = ktime_get();
> +                               ktime_t t = dma_fence_timestamp(tmp);
> +
> +                               if (ktime_after(t, timestamp))
> +                                       timestamp = t;
>                         }
>                 }
>         }
> diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
> index af57799c86ce..2e9a316c596a 100644
> --- a/drivers/dma-buf/sync_file.c
> +++ b/drivers/dma-buf/sync_file.c
> @@ -268,13 +268,10 @@ static int sync_fill_fence_info(struct dma_fence *fence,
>                 sizeof(info->driver_name));
>
>         info->status = dma_fence_get_status(fence);
> -       while (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags) &&
> -              !test_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags))
> -               cpu_relax();
>         info->timestamp_ns =
> -               test_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags) ?
> -               ktime_to_ns(fence->timestamp) :
> -               ktime_set(0, 0);
> +               dma_fence_is_signaled(fence) ?
> +                       ktime_to_ns(dma_fence_timestamp(fence)) :
> +                       ktime_set(0, 0);
>
>         return info->status;
>  }
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 506371c42745..5a3a622fc672 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -929,7 +929,7 @@ drm_sched_get_cleanup_job(struct drm_gpu_scheduler *sched)
>
>                 if (next) {
>                         next->s_fence->scheduled.timestamp =
> -                               job->s_fence->finished.timestamp;
> +                               dma_fence_timestamp(&job->s_fence->finished);
>                         /* start TO timer for next job */
>                         drm_sched_start_timeout(sched);
>                 }
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index 0d678e9a7b24..ebe78bd3d121 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -568,6 +568,25 @@ static inline void dma_fence_set_error(struct dma_fence *fence,
>         fence->error = error;
>  }
>
> +/**
> + * dma_fence_timestamp - helper to get the completion timestamp of a fence
> + * @fence: fence to get the timestamp from.
> + *
> + * After a fence is signaled the timestamp is updated with the signaling time,
> + * but setting the timestamp can race with tasks waiting for the signaling. This
> + * helper busy waits for the correct timestamp to appear.
> + */
> +static inline ktime_t dma_fence_timestamp(struct dma_fence *fence)
> +{
> +       if (WARN_ON(!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)))
> +               return ktime_get();
> +
> +       while (!test_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags))
> +               cpu_relax();
> +
> +       return fence->timestamp;
> +}
> +
>  signed long dma_fence_wait_timeout(struct dma_fence *,
>                                    bool intr, signed long timeout);
>  signed long dma_fence_wait_any_timeout(struct dma_fence **fences,
> --
> 2.34.1
>