Re: [PATCH v2 1/2] drm/panel: lg-sw43408: do not assert reset across unprepare

[David Heidelberg <david@ixit.cz>]

On 10/08/2026 11:55, 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")
> Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
> Signed-off-by: David Heidelberg <david@ixit.cz>
> ---
>   drivers/gpu/drm/panel/panel-lg-sw43408.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 

Good news is, that this patch really doesn't make panel power-off fatal for the 
phone, but recent testing got me into state where I observed:

10 hours, display on, phone operating normally.

display on (various time), display off, display on OK, but sometimes

display on, display off (for longer time), after pressing power button (to wake 
up) the phone reboot into bootloader.

Nothing printed into dmesg (I'll try enable drm-debug, but not having high hopes.

So far, this is still heavy improvement comparing to instant crash, but not the 
perfect solution either.

David

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