Re: [v5, PATCH] drm/mediatek: add dma buffer control for drm plane disable

[CK Hu (胡俊光)]

Hi, Maxime:

On Wed, 2023-07-05 at 09:48 +0200, Maxime Ripard wrote:
> Hi,
> 
> On Wed, Jul 05, 2023 at 02:07:18PM +0800, Yongqiang Niu wrote:
> > dma buffer release before overlay disable, that will cause
> > m4u translation fault warning.
> > 
> > add dma buffer control flow in mediatek driver:
> > get dma buffer when drm plane disable
> > put dma buffer when overlay really disable
> > 
> > Fixes: 41016fe1028e ("drm: Rename plane->state variables in atomic
> > update and disable")
> > Signed-off-by: Yongqiang Niu <yongqiang.niu@xxxxxxxxxxxx>
> 
> I think we need more details in the commit message about what the
> issue
> is exactly and how it's fixed.
> 
> This definitely feels like it's not something drivers should have to
> do.

Because MediaTek display hardware has no internal SRAM which could
store the on screen buffer, plane should be updated in vblank period to
prevent tearing effect. In MediaTek plane disable callback function, if
we wait vblank period to disable plane, the software would be blocked
for a long time. If there are four planes to be disabled, the total
blocking time would be almost 4 vsync time. So this patch is just a
work around to use get_dma_buf() to prevent buffer to be freed before
vblank period and not block the software. For the behavior DRM core
free plane buffer just after plane disable callback function return,
I'm not sure it's DRM core's bug or it's DRM core's correct behavior.
If this is correct behavior, it means that DRM core assume that
hardware should have internal SRAM but MediaTek display hardware
violate this assumption. If so, I think I could just accept this work
around patch.

Regards,
CK


> 
> Maxime