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

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Date:   Fri, 23 Oct 2020 20:20:02 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org
Cc:     Rob Clark <robdclark@...omium.org>,
        David Airlie <airlied@...ux.ie>, linux-arm-msm@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>,
        Sean Paul <sean@...rly.run>,
        "Kristian H . Kristensen" <hoegsberg@...gle.com>,
        freedreno@...ts.freedesktop.org
Subject: Re: [PATCH v4 23/23] drm/msm: Don't implicit-sync if only a single
 ring

On Fr, 2020-10-23 at 09:51 -0700, Rob Clark wrote:
> From: Rob Clark <robdclark@...omium.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@...omium.org>
> Reviewed-by: Kristian H. Kristensen <hoegsberg@...gle.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;
>