Re: [Freedreno] [PATCH] drm/msm/dsi: Delay drm_panel_enable() until dsi_mgr_bridge_enable()

[Jeffrey Hugo <jeffrey.l.hugo@gmail.com>]

On Fri, Nov 8, 2019 at 2:29 PM Stephan Gerhold <stephan@gerhold.net> 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@gmail.com>
> Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
> ---
> 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.