Re: [PATCH 1/2] drm/panel: lg-sw43408: do not assert reset across unprepare

[Neil Armstrong <neil.armstrong@linaro.org>]

On 8/9/26 02:04, David Heidelberg via B4 Relay wrote:
> From: David Heidelberg <david@ixit.cz>
> 
> Asserting the reset GPIO in unprepare() and leaving it asserted puts
> the DDIC into a state it never recovers from: after the following
> prepare() the panel answers all DCS commands, reports display-on with
> clean self-diagnostics, and its configuration registers read back
> identical to a working panel, but it never lights up again.
> 
> On the Pixel 3, vddi is shared with the touchscreen controller and
> remains powered while the panel is off, so with this driver the DDIC
> sits in reset with its logic rail up for the whole blank period - a
> state the vendor stack never creates: the vendor firmware only toggles
> reset as part of a full power-on sequence, and the vendor OS never
> power-cycles the panel at all (LP mode is used instead).
> 
> Leave reset alone in unprepare(); prepare() performs the regular reset
> sequence anyway.
> 
> Without this, the Pixel 3 display never comes back after the first
> blank/unblank cycle.
> 
> Since there isn't any other panel or device using this DDIC, this
> approach seems resoanble to address the issue.
> 
> Fixes: 069a6c0e94f9 ("drm: panel: Add LG sw43408 panel driver")
> Signed-off-by: David Heidelberg <david@ixit.cz>
> ---
>   drivers/gpu/drm/panel/panel-lg-sw43408.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panel/panel-lg-sw43408.c b/drivers/gpu/drm/panel/panel-lg-sw43408.c
> index 293826c5006b9..0bf8e08f10b9c 100644
> --- a/drivers/gpu/drm/panel/panel-lg-sw43408.c
> +++ b/drivers/gpu/drm/panel/panel-lg-sw43408.c
> @@ -50,18 +50,24 @@ static int sw43408_unprepare(struct drm_panel *panel)
>   	int ret;
>   
>   	mipi_dsi_dcs_set_display_off_multi(&ctx);
>   
>   	mipi_dsi_dcs_enter_sleep_mode_multi(&ctx);
>   
>   	mipi_dsi_msleep(&ctx, 100);
>   
> -	gpiod_set_value(sw43408->reset_gpio, 1);
> -
> +	/*
> +	 * Keep reset deasserted: vddi may stay powered (it is shared with
> +	 * the touchscreen on Pixel 3) and holding the DDIC in reset while
> +	 * its logic rail is up puts it into a state that no init sequence
> +	 * recovers from - the panel keeps answering DCS commands and
> +	 * reports display-on, but never lights up again. The vendor stack
> +	 * only ever toggles reset as part of a powered-up init sequence.
> +	 */
>   	ret = regulator_bulk_disable(ARRAY_SIZE(sw43408_supplies), sw43408->supplies);

Asserting the reset then shutting down half the rails up may force the DDIC to enter
a special low power mode with a bad sequence and then would not come out.

But anyway reset has a sense only if all rails are up and we want to get a proper
initial state, if somehow we want to handle those special low power mode to speed
up suspend and resume we will need to understand how to get in and get out.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>

Thanks,
Neil

>   
>   	return ret ? : ctx.accum_err;
>   }
>   
>   static int sw43408_program(struct drm_panel *panel)
>   {
>   	struct sw43408_panel *sw43408 = to_panel_info(panel);
>