On Fri, Nov 8, 2019 at 2:29 PM Stephan Gerhold <stephan@xxxxxxxxxxx> wrote: > > At the moment, the MSM DSI driver calls drm_panel_enable() rather early > from the DSI bridge pre_enable() function. At this point, the encoder > (e.g. MDP5) is not enabled, so we have not started transmitting > video data. > > However, the drm_panel_funcs documentation states that enable() > should be called on the panel *after* video data is being transmitted: > > The .prepare() function is typically called before the display controller > starts to transmit video data. [...] After the display controller has > started transmitting video data, it's safe to call the .enable() function. > This will typically enable the backlight to make the image on screen visible. > > Calling drm_panel_enable() too early causes problems for some panels: > The TFT LCD panel used in the Samsung Galaxy Tab A 9.7 (2015) (APQ8016) > uses the MIPI_DCS_SET_DISPLAY_BRIGHTNESS command to control > backlight/brightness of the screen. The enable sequence is therefore: > > drm_panel_enable() > drm_panel_funcs.enable(): > backlight_enable() > backlight_ops.update_status(): > mipi_dsi_dcs_set_display_brightness(dsi, bl->props.brightness); > > The panel seems to silently ignore the MIPI_DCS_SET_DISPLAY_BRIGHTNESS > command if it is sent too early. This prevents setting the initial brightness, > causing the display to be enabled with minimum brightness instead. > Adding various delays in the panel initialization code does not result > in any difference. > > On the other hand, moving drm_panel_enable() to dsi_mgr_bridge_enable() > fixes the problem, indicating that the panel requires the video stream > to be active before the brightness command is accepted. > > Therefore: Move drm_panel_enable() to dsi_mgr_bridge_enable() to > delay calling it until video data is being transmitted. > > Move drm_panel_disable() to dsi_mgr_bridge_disable() for similar reasons. > (This is not strictly required for the panel affected above...) > > Tested-by: Jasper Korten <jja2000@xxxxxxxxx> > Signed-off-by: Stephan Gerhold <stephan@xxxxxxxxxxx> > --- > Since this is a core change I thought it would be better to send this > early. I believe Jasper still wants to finish some other changes before > submitting the initial device tree for the Samsung Galaxy Tab A 9.7 (2015). ;) > > I also tested it on msm8916-samsung-a5u-eur, its display is working fine > with or without this patch. Nack, please. I was curious so I threw this on the Lenovo Miix 630 laptop. I don't get a display back with this patch. I'll try to figure out why, but currently I can't get into the machine.
