linux-kernel - Re: [PATCH 0/2] drm/panel: Add support for the Sitronix ST7789V

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Message-ID: <20170207164621.GB28522@ulmo.ba.sec>
Date:   Tue, 7 Feb 2017 17:46:21 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:     Noralf Trønnes <noralf@...nnes.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 0/2] drm/panel: Add support for the Sitronix ST7789V

On Tue, Feb 07, 2017 at 04:34:44PM +0100, Maxime Ripard wrote:
> On Mon, Feb 06, 2017 at 12:29:31PM +0100, Noralf Trønnes wrote:
> > 
> > Den 06.02.2017 11.39, skrev Maxime Ripard:
> > > Hi Noralf,
> > > 
> > > On Fri, Feb 03, 2017 at 07:48:51PM +0100, Noralf Trønnes wrote:
> > > > Den 03.02.2017 10.59, skrev Maxime Ripard:
> > > > > Hi,
> > > > > 
> > > > > Here is an attempt at supporting the ST7789V LCD controller from Sitronix.
> > > > What happens if there's another panel driven by ST7789V that needs
> > > > a different controller initialization?
> > > You know those panels / controllers much better than I do, but why
> > > would that be the case?
> > > 
> > > > Maybe it's better to name it after the panel, not the controller.
> > > I guess you could also use that panel directly without the controller?
> > 
> > A controller can drive many different panels that can require different
> > initializations. I faced that with staging/fbtft, when I wrote
> > controller drivers having initialization code, and then came across
> > displays with the same controller but with a different initialization.
> > 
> > Trying to write controller drivers for these controllers is very
> > difficult with all the possible permutations. On top of that we have
> > those undocumented commands/registers.
> > 
> > Some panels come with embedded controllers, in which case it makes
> > sense to write a driver for the panel.
> > 
> > But if the panel and controller are separate, then I don't know. Maybe
> > the chance of coming across two uncompatible ST7789V and panel
> > combinations in drm/panel is extremly low.
> 
> Hmm, I see. If we ever come across that case, I guess we could just
> add new optional properties to override the current sequence.

I think if that ever happens it's probably best to split out ST7789V
code into a helper library that has functions which take parameters.
Then we can simply call those functions with parameters specific to
a panel from a panel-specific driver.

Trying to fit all that into device tree properties is likely going to
end up being really messy.

Thierry

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