Re: [PATCH] drm/mediatek: Set sensible cursor width/height values to fix crash

["Shawn Sung (宋孝謙)" <Shawn.Sung@mediatek.com>]

On Thu, 2024-07-18 at 16:49 +0800, Chen-Yu Tsai wrote:
>    
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  (CC-ed Fei Shao)
> 
> On Thu, Jul 18, 2024 at 4:24 PM AngeloGioacchino Del Regno
> <angelogioacchino.delregno@collabora.com>
 wrote:
> >
> > Hardware-speaking, there is no feature-reduced cursor specific
> > plane, so this driver reserves the last all Overlay plane as a
> > Cursor plane, but sets the maximum cursor width/height to the
> > maximum value that the full overlay plane can use.
> >
> > While this could be ok, it raises issues with common userspace
> > using libdrm (especially Mutter, but other compositors too) which
> > will crash upon performing allocations and/or using said cursor
> > plane.
> >
> > Reduce the maximum width/height for the cursor to 512x512 pixels,
> > value taken from IGT's maximum cursor size test, which succeeds.
> >
> > Fixes: a4c9410b31ca ("drm/mediatek: Set DRM mode configs
> accordingly")
> > Signed-off-by: AngeloGioacchino Del Regno <
> angelogioacchino.delregno@collabora.com>
> > ---
> >  drivers/gpu/drm/mediatek/mtk_drm_drv.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> > index 6f0b415a978d..b96763664c4f 100644
> > --- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> > +++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> > @@ -540,8 +540,8 @@ static int mtk_drm_kms_init(struct drm_device
> *drm)
> >         }
> >
> >         /* IGT will check if the cursor size is configured */
> > -       drm->mode_config.cursor_width = drm->mode_config.max_width;
> > -       drm->mode_config.cursor_height = drm-
> >mode_config.max_height;
> > +       drm->mode_config.cursor_width = 512;
> > +       drm->mode_config.cursor_height = 512;
> 
> Fei already did the same (?) workaround downstream just recently.
> 
> OOTH, Intel recently added a feature for enumerating "suggested"
> cursor sizes. See
https://patchwork.freedesktop.org/patch/583299/
> 
> Not sure if other compositors will end up using it or not.
> 
> ChenYu
> 
> >         /* Use OVL device for all DMA memory allocations */
> >         crtc = drm_crtc_from_index(drm, 0);
> > --
> > 2.45.2
> >

Just for more information, as the driver is describing the hardware limitation, and the comments for cursor_width/cursor_height in struct
drm_mode_config indicates "hint to userspace for 'max' cursor width/height". After few discussions with the reviewer, we eventually set it to the maximum size the Overlay supports (d46f652fc9fde61d7e88bfeace62f27847d55d40.camel@mediatek.com>).

Not sure if it makes sense for these userspace compositors to always
use the maximum size of the cursor plane as the cursor size.

Regards,
Shawn