linux-kernel - Re: [PATCH 4/6] drm/v3d: Drop the wait for L2T flush to complete.

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Message-ID: <CAApk1dXC5knQ7_JuknCjm4vc2Y_OfLK0Y+ci52WjCN3sFnDHdA@mail.gmail.com>
Date:   Mon, 3 Dec 2018 17:59:05 +0000
From:   Dave Emett <david.emett@...adcom.com>
To:     Eric Anholt <eric@...olt.net>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Thomas Spurden <thomas.spurden@...adcom.com>
Subject: Re: [PATCH 4/6] drm/v3d: Drop the wait for L2T flush to complete.

I'm surprised this made such a big difference. It should not take a
long time to flush the L2T -- no memory access should be involved, it
should just be a matter of the L2T iterating over its lines clearing
the tags. This should take thousands of V3D cycles, not milliseconds.
So the 3-4ms stall seems worthy of more investigation to me.

A comment describing why no waits are necessary would be good.

On Sat, 1 Dec 2018 at 00:58, Eric Anholt <eric@...olt.net> wrote:
>
> According to Dave, once you've started an L2T flush, all L2T accesses
> will be blocked until the flush completes.  This fixes a consistent
> 3-4ms stall between the ioctl and running the job, and 3DMMES Taiji
> goes from 27fps to 110fps.
>
> Signed-off-by: Eric Anholt <eric@...olt.net>

Reviewed-by: Dave Emett <david.emett@...adcom.com>

> Fixes: 57692c94dcbe ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
> ---
>  drivers/gpu/drm/v3d/v3d_gem.c | 4 ----
>  1 file changed, 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/v3d/v3d_gem.c b/drivers/gpu/drm/v3d/v3d_gem.c
> index cc4d025b01e0..0bd6892e3044 100644
> --- a/drivers/gpu/drm/v3d/v3d_gem.c
> +++ b/drivers/gpu/drm/v3d/v3d_gem.c
> @@ -146,10 +146,6 @@ v3d_flush_l2t(struct v3d_dev *v3d, int core)
>         V3D_CORE_WRITE(core, V3D_CTL_L2TCACTL,
>                        V3D_L2TCACTL_L2TFLS |
>                        V3D_SET_FIELD(V3D_L2TCACTL_FLM_FLUSH, V3D_L2TCACTL_FLM));
> -       if (wait_for(!(V3D_CORE_READ(core, V3D_CTL_L2TCACTL) &
> -                      V3D_L2TCACTL_L2TFLS), 100)) {
> -               DRM_ERROR("Timeout waiting for L2T flush\n");
> -       }
>  }
>
>  /* Invalidates the slice caches.  These are read-only caches. */
> --
> 2.20.0.rc1
>