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From:   Dave Emett <david.emett@broadcom.com>
Date:   Tue, 4 Dec 2018 11:53:47 +0000
Message-ID: <CAApk1dV+AhYZ2vf+thJ9ZghGA8VNg-wc=490dpngRu5PS8Ev4w@mail.gmail.com>
Subject: Re: [PATCH v2 4/6] drm/v3d: Drop the wait for L2T flush to complete.
To:     Eric Anholt <eric@anholt.net>
Cc:     dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        Thomas Spurden <thomas.spurden@broadcom.com>
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On Mon, 3 Dec 2018 at 22:24, Eric Anholt <eric@anholt.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.
>
> v2: Leave a note about why we don't need to wait for completion.
>
> Signed-off-by: Eric Anholt <eric@anholt.net>

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

> Fixes: 57692c94dcbe ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
> Reviewed-by: Dave Emett <david.emett@broadcom.com> (v1, comment requested)
> ---
>  drivers/gpu/drm/v3d/v3d_gem.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/v3d/v3d_gem.c b/drivers/gpu/drm/v3d/v3d_gem.c
> index 01e879c71cad..c268c7c79566 100644
> --- a/drivers/gpu/drm/v3d/v3d_gem.c
> +++ b/drivers/gpu/drm/v3d/v3d_gem.c
> @@ -143,13 +143,13 @@ v3d_invalidate_l2(struct v3d_dev *v3d, int core)
>  static void
>  v3d_flush_l2t(struct v3d_dev *v3d, int core)
>  {
> +       /* While there is a busy bit (V3D_L2TCACTL_L2TFLS), we don't
> +        * need to wait for completion before dispatching the job --
> +        * L2T accesses will be stalled until the flush has completed.
> +        */
>         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
>