Re: [v3] drm/msm: Fix race condition in msm driver with async layer updates

[Rob Clark <robdclark@gmail.com>]

On Fri, Oct 16, 2020 at 7:11 AM Krishna Manikandan
<mkrishn@codeaurora.org> wrote:
>
> When there are back to back commits with async cursor update,
> there is a case where second commit can program the DPU hw
> blocks while first didn't complete flushing config to HW.
>
> Synchronize the compositions such that second commit waits
> until first commit flushes the composition.
>
> This change also introduces per crtc commit lock, such that
> commits on different crtcs are not blocked by each other.
>
> Changes in v2:
>         - Use an array of mutexes in kms to handle commit
>           lock per crtc. (Rob Clark)
>
> Changes in v3:
>         - Add wrapper functions to handle lock and unlock of
>           commit_lock for each crtc. (Rob Clark)
>
> Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>

Reviewed-by: Rob Clark <robdclark@gmail.com>

> ---
>  drivers/gpu/drm/msm/msm_atomic.c | 37 ++++++++++++++++++++++++-------------
>  drivers/gpu/drm/msm/msm_kms.h    |  6 ++++--
>  2 files changed, 28 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/msm_atomic.c b/drivers/gpu/drm/msm/msm_atomic.c
> index 561bfa4..575e9af 100644
> --- a/drivers/gpu/drm/msm/msm_atomic.c
> +++ b/drivers/gpu/drm/msm/msm_atomic.c
> @@ -55,16 +55,32 @@ static void vblank_put(struct msm_kms *kms, unsigned crtc_mask)
>         }
>  }
>
> +static void lock_crtcs(struct msm_kms *kms, unsigned int crtc_mask)
> +{
> +       struct drm_crtc *crtc;
> +
> +       for_each_crtc_mask(kms->dev, crtc, crtc_mask)
> +               mutex_lock(&kms->commit_lock[drm_crtc_index(crtc)]);
> +}
> +
> +static void unlock_crtcs(struct msm_kms *kms, unsigned int crtc_mask)
> +{
> +       struct drm_crtc *crtc;
> +
> +       for_each_crtc_mask(kms->dev, crtc, crtc_mask)
> +               mutex_unlock(&kms->commit_lock[drm_crtc_index(crtc)]);
> +}
> +
>  static void msm_atomic_async_commit(struct msm_kms *kms, int crtc_idx)
>  {
>         unsigned crtc_mask = BIT(crtc_idx);
>
>         trace_msm_atomic_async_commit_start(crtc_mask);
>
> -       mutex_lock(&kms->commit_lock);
> +       lock_crtcs(kms, crtc_mask);
>
>         if (!(kms->pending_crtc_mask & crtc_mask)) {
> -               mutex_unlock(&kms->commit_lock);
> +               unlock_crtcs(kms, crtc_mask);
>                 goto out;
>         }
>
> @@ -79,7 +95,6 @@ static void msm_atomic_async_commit(struct msm_kms *kms, int crtc_idx)
>          */
>         trace_msm_atomic_flush_commit(crtc_mask);
>         kms->funcs->flush_commit(kms, crtc_mask);
> -       mutex_unlock(&kms->commit_lock);
>
>         /*
>          * Wait for flush to complete:
> @@ -90,9 +105,8 @@ static void msm_atomic_async_commit(struct msm_kms *kms, int crtc_idx)
>
>         vblank_put(kms, crtc_mask);
>
> -       mutex_lock(&kms->commit_lock);
>         kms->funcs->complete_commit(kms, crtc_mask);
> -       mutex_unlock(&kms->commit_lock);
> +       unlock_crtcs(kms, crtc_mask);
>         kms->funcs->disable_commit(kms);
>
>  out:
> @@ -189,12 +203,11 @@ void msm_atomic_commit_tail(struct drm_atomic_state *state)
>          * Ensure any previous (potentially async) commit has
>          * completed:
>          */
> +       lock_crtcs(kms, crtc_mask);
>         trace_msm_atomic_wait_flush_start(crtc_mask);
>         kms->funcs->wait_flush(kms, crtc_mask);
>         trace_msm_atomic_wait_flush_finish(crtc_mask);
>
> -       mutex_lock(&kms->commit_lock);
> -
>         /*
>          * Now that there is no in-progress flush, prepare the
>          * current update:
> @@ -232,8 +245,7 @@ void msm_atomic_commit_tail(struct drm_atomic_state *state)
>                 }
>
>                 kms->funcs->disable_commit(kms);
> -               mutex_unlock(&kms->commit_lock);
> -
> +               unlock_crtcs(kms, crtc_mask);
>                 /*
>                  * At this point, from drm core's perspective, we
>                  * are done with the atomic update, so we can just
> @@ -260,8 +272,7 @@ void msm_atomic_commit_tail(struct drm_atomic_state *state)
>          */
>         trace_msm_atomic_flush_commit(crtc_mask);
>         kms->funcs->flush_commit(kms, crtc_mask);
> -       mutex_unlock(&kms->commit_lock);
> -
> +       unlock_crtcs(kms, crtc_mask);
>         /*
>          * Wait for flush to complete:
>          */
> @@ -271,9 +282,9 @@ void msm_atomic_commit_tail(struct drm_atomic_state *state)
>
>         vblank_put(kms, crtc_mask);
>
> -       mutex_lock(&kms->commit_lock);
> +       lock_crtcs(kms, crtc_mask);
>         kms->funcs->complete_commit(kms, crtc_mask);
> -       mutex_unlock(&kms->commit_lock);
> +       unlock_crtcs(kms, crtc_mask);
>         kms->funcs->disable_commit(kms);
>
>         drm_atomic_helper_commit_hw_done(state);
> diff --git a/drivers/gpu/drm/msm/msm_kms.h b/drivers/gpu/drm/msm/msm_kms.h
> index 1cbef6b..2049847 100644
> --- a/drivers/gpu/drm/msm/msm_kms.h
> +++ b/drivers/gpu/drm/msm/msm_kms.h
> @@ -155,7 +155,7 @@ struct msm_kms {
>          * For async commit, where ->flush_commit() and later happens
>          * from the crtc's pending_timer close to end of the frame:
>          */
> -       struct mutex commit_lock;
> +       struct mutex commit_lock[MAX_CRTCS];
>         unsigned pending_crtc_mask;
>         struct msm_pending_timer pending_timers[MAX_CRTCS];
>  };
> @@ -165,7 +165,9 @@ static inline void msm_kms_init(struct msm_kms *kms,
>  {
>         unsigned i;
>
> -       mutex_init(&kms->commit_lock);
> +       for (i = 0; i < ARRAY_SIZE(kms->commit_lock); i++)
> +               mutex_init(&kms->commit_lock[i]);
> +
>         kms->funcs = funcs;
>
>         for (i = 0; i < ARRAY_SIZE(kms->pending_timers); i++)
> --
> 2.7.4
>