Re: [PATCH] drm/dp: For drm_panel_dp_aux_backlight(), init backlight as disabled

[Lyude Paul]

Reviewed-by: Lyude Paul <lyude@xxxxxxxxxx>

On Wed, 2021-07-14 at 10:17 -0700, Douglas Anderson wrote:
> Even after the DP AUX backlight on my board worked OK after applying
> the patch ("drm/panel-simple: Power the panel when probing DP AUX
> backlight") [1], I still noticed some strange timeouts being reported
> by ti_sn_aux_transfer(). Digging, I realized the problem was this:
> * Even though `enabled` in `struct dp_aux_backlight` was false, the
>   base backlight structure (`base` in that structure) thought that the
>   backlight was powered on.
> * If userspace wrote to sysfs in this state then we'd try to enable
>   the backlight.
> * Unfortunatley, enabling the backlight didn't work because the panel
>   itself wasn't powered.
> 
> We can only use the backlight if the panel is on and the panel is not
> officially on when we probe (it's temporarily just on enough for us to
> talk to it).
> 
> The important thing we want here is to get `BL_CORE_FBBLANK` set since
> userspace can't mess with that. This will keep us disabled until
> drm_panel enables us, which means that the panel is enabled
> first. Ideally we'd just set this in our `props` before calling
> devm_backlight_device_register() but the comments in the header file
> are pretty explicit that we're not supposed to much with the `state`
> ourselves. Because of this, there may be a small window where the
> backlight device is registered and someone could try to tweak with the
> backlight. This isn't likely to happen and even if it did, I don't
> believe this causes any huge problem.
> 
> [1]
> https://lore.kernel.org/lkml/20210714093334.1.Idb41f87e5abae4aee0705db7458b0097fc50e7ab@changeid/
> 
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> 
>  drivers/gpu/drm/drm_dp_helper.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c
> b/drivers/gpu/drm/drm_dp_helper.c
> index e8eec20ab364..b5f75ca05774 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -3568,6 +3568,8 @@ int drm_panel_dp_aux_backlight(struct drm_panel
> *panel, struct drm_dp_aux *aux)
>         if (IS_ERR(bl->base))
>                 return PTR_ERR(bl->base);
>  
> +       backlight_disable(bl->base);
> +
>         panel->backlight = bl->base;
>  
>         return 0;

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat