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.Idb41f87e5abae4aee0705db7458b0...
Signed-off-by: Douglas Anderson dianders@chromium.org ---
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;