Re: [PATCH] drm/loongson: Fix framebuffer selection in async cursor updates

[wuqianhai]

Reviewed-by: Qianhai Wu <wuqianhai@xxxxxxxxxxx> 
 
 

 
 
 

     The fix is correct. 
 
 

 
 
 

     Thanks, 
 
 

     Qianhai Wu

在 2026/10/9 16:33, Icenowy Zheng 写道:
> (Cc'ing drm-misc maintainers)
>
> 在 2026-10-03六的 09:00 +0000，Evanshenf写道：
> > An asynchronous cursor update swaps the new framebuffer into the live
> > plane state. However, the driver reads both old_fb and new_fb from
> > the
> > live state before this swap. When an enabled cursor changes buffers,
> > the
> > address-update condition is therefore false and the hardware
> > continues
> > scanning the previous buffer.
> >
> > Read new_fb from the new plane state before swapping the framebuffer
> > references. This updates the hardware address when the framebuffer
> > changes while retaining the existing behavior for cursor movement.
> >
> > Verified on a Loongson 3A5000 with LS7A2000 using 32x32 and 64x64
> > legacy
> > cursor updates. Callback counters and read-only register observations
> > confirmed the asynchronous path, the stale address before the change,
> > and the new address afterward. Same-framebuffer movement and cursor
> > disable/re-enable also passed, with the primary framebuffer and mode
> > unchanged.
> >
> > AI assistance was used to investigate the existing report, prepare
> > the
> > fix and test tools, run the build and hardware comparison, and draft
> > this message.
> >
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Closes:
> > https://lore.kernel.org/r/202311152357.4KFS0M8f-lkp@xxxxxxxxx/
> > Fixes: f39db26c5428 ("drm: Add kms driver for loongson display
> > controller")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Assisted-by: LLM
> > Signed-off-by: Evanshenf <archwse@xxxxxxxxx>
> > ---
> >   drivers/gpu/drm/loongson/lsdc_plane.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/loongson/lsdc_plane.c
> > b/drivers/gpu/drm/loongson/lsdc_plane.c
> > index bcc0ffa..7b9c65c 100644
> > --- a/drivers/gpu/drm/loongson/lsdc_plane.c
> > +++ b/drivers/gpu/drm/loongson/lsdc_plane.c
> > @@ -230,7 +230,7 @@ static void
> > lsdc_cursor_plane_atomic_async_update(struct drm_plane *plane,
> >   
> >   	new_state = drm_atomic_get_new_plane_state(state, plane);
> >   
> > -	new_fb = plane->state->fb;
> > +	new_fb = new_state->fb;
>
> This change looks okay, although I don't know whether the
> atomic_async_update() hook should be implemented at all now.
>
> ```
> Reviewed-by: Icenowy Zheng <zhengxingda@xxxxxxxxxxx>
> ```
>
> However, please wait for further clarification of the function itself
> from drm-misc maintainers.
>
> Thanks,
> Icenowy
>
> >   
> >   	plane->state->crtc_x = new_state->crtc_x;
> >   	plane->state->crtc_y = new_state->crtc_y;
> >
> > base-commit: bca45af5998a05f34b13a2ef11e639bac9c62643