drm/panel: add backlight support
Panels often support backlight as specified in a device tree. Update the drm_panel infrastructure to support this to simplify the drivers. With this the panel driver just needs to add the following to the probe() function: err = drm_panel_of_backlight(panel); if (err) return err; Then drm_panel will handle all the rest. There is one caveat with the backlight support. If drm_panel_(enable|disable) are called multiple times in a row then backlight_(enable|disable) will be called multiple times. The above will happen when a panel drivers unconditionally calls drm_panel_disable() in their shutdown() function, whan the panel is already disabled and then shutdown() is called. Reading the backlight code it seems safe to call the backlight_(enable|disable) several times. v3: - Improve comments, fix grammar (Laurent) - Do not fail in drm_panel_of_backlight() if no DT support (Laurent) - Log if backlight_(enable|disable) fails (Laurent) - Improve drm_panel_of_backlight() docs - Updated changelog with backlight analysis (triggered by Laurent) v2: - Drop test of CONFIG_DRM_PANEL in header-file (Laurent) - do not enable backlight if ->enable() returns an error Signed-off-by: Sam Ravnborg <sam@ravnborg.org> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Cc: Thierry Reding <thierry.reding@gmail.com> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> Cc: Maxime Ripard <maxime.ripard@bootlin.com> Cc: Sean Paul <sean@poorly.run> Cc: David Airlie <airlied@linux.ie> Cc: Daniel Vetter <daniel@ffwll.ch> Link: https://patchwork.freedesktop.org/patch/msgid/20191207140353.23967-3-sam@ravnborg.org
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