Re: [PATCH] drm/panel: sony-acx565akm: Fix race condition in probe

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Sebastian,

Thank you for the patch.

On Fri, Nov 27, 2020 at 09:04:29PM +0100, Sebastian Reichel wrote:
> The probe routine acquires the reset GPIO using GPIOD_OUT_LOW. Directly
> afterwards it calls acx565akm_detect(), which sets the GPIO value to
> HIGH. If the bootloader initialized the GPIO to HIGH before the probe
> routine was called, there is only a very short time period of a few
> instructions where the reset signal is LOW. Exact time depends on
> compiler optimizations, kernel configuration and alignment of the stars,
> but I expect it to be always way less than 10us. There are no public
> datasheets for the panel, but acx565akm_power_on() has a comment with
> timings and reset period should be at least 10us. So this potentially
> brings the panel into a half-reset state.

Good catch.

Looks like we got the reset polarity wrong in the driver though.
GPIOD_OUT_LOW should mean de-asserted, but the driver expects it to mean
low level. We can't fix that as it would require changing the device
tree :-(

> The result is, that panel may not work after boot and can get into a
> working state by re-enabling it (e.g. by blanking + unblanking), since
> that does a clean reset cycle. This bug has recently been hit by Ivaylo
> Dimitrov, but there are some older reports which are probably the same
> bug. At least Tony Lindgren, Peter Ujfalusi and Jarkko Nikula have
> experienced it in 2017 describing the blank/unblank procedure as
> possible workaround.
> 
> Note, that the bug really goes back in time. It has originally been
> introduced in the predecessor of the omapfb driver in 3c45d05be382
> ("OMAPDSS: acx565akm panel: handle gpios in panel driver") in 2012.
> That driver eventually got replaced by a newer one, which had the bug
> from the beginning in 84192742d9c2 ("OMAPDSS: Add Sony ACX565AKM panel
> driver") and still exists in fbdev world. That driver has later been
> copied to omapdrm and then was used as a basis for this driver. Last
> but not least the omapdrm specific driver has been removed in
> 45f16c82db7e ("drm/omap: displays: Remove unused panel drivers").
> 
> Reported-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
> Reported-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
> Reported-by: Tony Lindgren <tony@atomide.com>
> Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
> Reported-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
> Cc: Merlijn Wajer <merlijn@wizzup.org>
> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
> Fixes: 1c8fc3f0c5d2 ("drm/panel: Add driver for the Sony ACX565AKM panel")
> Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
> ---
>  drivers/gpu/drm/panel/panel-sony-acx565akm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/panel/panel-sony-acx565akm.c b/drivers/gpu/drm/panel/panel-sony-acx565akm.c
> index e95fdfb16b6c..ba0b3ead150f 100644
> --- a/drivers/gpu/drm/panel/panel-sony-acx565akm.c
> +++ b/drivers/gpu/drm/panel/panel-sony-acx565akm.c
> @@ -629,7 +629,7 @@ static int acx565akm_probe(struct spi_device *spi)
>  	lcd->spi = spi;
>  	mutex_init(&lcd->mutex);
>  
> -	lcd->reset_gpio = devm_gpiod_get(&spi->dev, "reset", GPIOD_OUT_LOW);
> +	lcd->reset_gpio = devm_gpiod_get(&spi->dev, "reset", GPIOD_OUT_HIGH);

Wouldn't it be better to instead add a delay here (or in
acx565akm_detect()) ? If the panel is in a wrong state at boot time, a
real reset can help.

>  	if (IS_ERR(lcd->reset_gpio)) {
>  		dev_err(&spi->dev, "failed to get reset GPIO\n");
>  		return PTR_ERR(lcd->reset_gpio);

-- 
Regards,

Laurent Pinchart