linux-kernel - Re: [PATCH 1/4] ARM: dts: omap3-n900: fix LCD reset line polarity

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Date:   Tue, 11 Oct 2022 17:47:39 +0300
From:   Tony Lindgren <tony@...mide.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Sebastian Reichel <sre@...nel.org>,
        Benoît Cousson <bcousson@...libre.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-omap@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Subject: Re: [PATCH 1/4] ARM: dts: omap3-n900: fix LCD reset line polarity

* Dmitry Torokhov <dmitry.torokhov@...il.com> [221011 14:30]:
> On Tue, Oct 11, 2022 at 05:30:12PM +0300, Tony Lindgren wrote:
> > * Dmitry Torokhov <dmitry.torokhov@...il.com> [221011 13:57]:
> > > Hi Sebastian,
> > > 
> > > On Tue, Oct 11, 2022 at 02:37:26PM +0200, Sebastian Reichel wrote:
> > > > Hi,
> > > > 
> > > > On Tue, Oct 11, 2022 at 08:45:54AM +0300, Tony Lindgren wrote:
> > > > > * Dmitry Torokhov <dmitry.torokhov@...il.com> [221004 21:26]:
> > > > > > The LCD driver (panel-sony-acx565akm), when probing, starts with line
> > > > > > driven low, and then toggles it to high and keeps it there. Also, the
> > > > > > line is driven low when powering off the device, and ls released when
> > > > > > powering it back on. This means that the reset line should be described
> > > > > > as "active low" in DTS. This will be important when the driver is
> > > > > > converted to gpiod API which respects the polarity declared in DTS.
> > > > > 
> > > > > We should ensure these patches get merged together with the driver
> > > > > change to avoid breaking LCD for booting. Probably no need to have
> > > > > the driver quirk handling for inverted polartity in this case.
> > > > > 
> > > > > It's probably easiest to have an immutable branch for the driver
> > > > > changes I can base the dts changes on. Or I can ack the dts changes
> > > > > if they get merged with the driver.
> > > > 
> > > > Both drivers are already using gpiod API:
> > > > 
> > > > drivers/gpu/drm/panel/panel-sony-acx565akm.c
> > > > drivers/gpu/drm/panel/panel-dsi-cm.c
> > > 
> > > I was looking at
> > > 
> > > drivers/video/fbdev/omap2/omapfb/displays/panel-sony-acx565akm.c
> > > drivers/video/fbdev/omap2/omapfb/displays/panel-dsi-cm.c
> > 
> > Ah OK that explains :)
> > 
> > > which are not using gpiod. Should they be retired?
> > 
> > Yes we should just get rid of them with omapdrm working just fine.
> 
> Will you be submitting such patches? I'd like to get rid of
> of_get_named_gpio() and friends if I can...

Adding Tomi to Cc, my guess is he already has such patches and knows
better which ones can go :)

> > > > So this just breaks things.
> > > 
> > > I missed the drivers in drivers/gpu/... and I see that they essentially
> > > abuse gpiod API as gpiod_set_value() operates on logical level
> > > (active/inactive) and not absolute (high/low). They should either use
> > > the gpiod_*_raw() variants, or they should be adjusted to do the proper
> > > thing together with the accompanying DTS change.
> > > 
> > > What are your preferences?
> > 
> > Seems like high/low at the connected device end is what we should use,
> > right? Otherwise things will misbehave if the panel is connected to
> > some other SoC possibly.
> 
> It is exactly because of this case the driver should use active/inactive
> and follow polarity described in DTS. If the driver does:
> 
> 	gpiod_set_value_cansleep(d->reset, 1);
> 
> then if DTS is saying that the reset line is active low, under the wraps
> the line will be driven to "0", but if DTS is saying that the line is
> active high, then the very same call will drive the line to "1".
> 
> This allows accommodating different designs without having to change the
> driver code.

OK

Thanks,

Tony