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

[Sam Ravnborg]

Hi Sebastian,
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.
> 
> 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 at bitmer.com>
> Reported-by: Peter Ujfalusi <peter.ujfalusi at ti.com>
> Reported-by: Tony Lindgren <tony at atomide.com>
> Reported-by: Aaro Koskinen <aaro.koskinen at iki.fi>
> Reported-by: Ivaylo Dimitrov <ivo.g.dimitrov.75 at gmail.com>
> Cc: Merlijn Wajer <merlijn at wizzup.org>
> Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> Cc: Tomi Valkeinen <tomi.valkeinen at ti.com>
> Fixes: 1c8fc3f0c5d2 ("drm/panel: Add driver for the Sony ACX565AKM panel")
> Signed-off-by: Sebastian Reichel <sebastian.reichel at collabora.com>


Fixed up the commit references, added Tested-by (impressive list) and
committed to drm-misc-fixes.

Commit references shall look like this:

    commit 84192742d9c2 ("OMAPDSS: Add Sony ACX565AKM panel driver")

The word commit is required according to dim (the tool we use for
drm-misc maintenance).

	Sam