Re: [PATCH v4 23/23] drm/msm: Don't implicit-sync if only a single ring

Lucas Stach
2020-10-23T18:20:10Z

On Fr, 2020-10-23 at 09:51 -0700, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
>
> If there is only a single ring (no-preemption), everything is FIFO order
> and there is no need to implicit-sync.
>
> Mesa should probably just always use MSM_SUBMIT_NO_IMPLICIT, as behavior
> is undefined when fences are not used to synchronize buffer usage across
> contexts (which is the only case where multiple different priority rings
> could come into play).

Really, doesn't this break cross-device implicit sync? Okay, you may
not have many peripherals that rely on implicit sync on devices where
Adreno is usually found, but it seems rather heavy-handed.

Wouldn't it be better to only ignore fences from your own ring context
in the implicit sync, like we do in the common DRM scheduler
(drm_sched_dependency_optimized)?

Regards,
Lucas

> Signed-off-by: Rob Clark <robdclark@chromium.org>
> Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
> ---
> drivers/gpu/drm/msm/msm_gem_submit.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/msm_gem_submit.c b/drivers/gpu/drm/msm/msm_gem_submit.c
> index d04c349d8112..b6babc7f9bb8 100644
> --- a/drivers/gpu/drm/msm/msm_gem_submit.c
> +++ b/drivers/gpu/drm/msm/msm_gem_submit.c
> @@ -283,7 +283,7 @@ static int submit_lock_objects(struct msm_gem_submit *submit)
> return ret;
> }
>
> -static int submit_fence_sync(struct msm_gem_submit *submit, bool no_implicit)
> +static int submit_fence_sync(struct msm_gem_submit *submit, bool implicit_sync)
> {
> int i, ret = 0;
>
> @@ -303,7 +303,7 @@ static int submit_fence_sync(struct msm_gem_submit *submit, bool no_implicit)
> return ret;
> }
>
> - if (no_implicit)
> + if (!implicit_sync)
> continue;
>
> ret = msm_gem_sync_object(&msm_obj->base, submit->ring->fctx,
> @@ -774,7 +774,8 @@ int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
> if (ret)
> goto out;
>
> - ret = submit_fence_sync(submit, !!(args->flags & MSM_SUBMIT_NO_IMPLICIT));
> + ret = submit_fence_sync(submit, (gpu->nr_rings > 1) &&
> + !(args->flags & MSM_SUBMIT_NO_IMPLICIT));
> if (ret)
> goto out;
>