linux-kernel - Re: [PATCH 3/9] drm/panel: simple: make it possible to override LCD bus format

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Date:   Tue, 17 Oct 2017 14:44:23 +0200
From:   Lothar Waßmann <LW@...O-electronics.de>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     David Airlie <airlied@...ux.ie>,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/9] drm/panel: simple: make it possible to override LCD
 bus format

Hi,

On Tue, 17 Oct 2017 14:12:40 +0200 Thierry Reding wrote:
> On Wed, Oct 11, 2017 at 01:23:35PM +0200, Lothar Waßmann wrote:
> > The baseboards for the Ka-Ro electronics series of i.MX modules
> > use a 24bit LCD interface, no matter what LCD bus width the SoC on the
> > module provides and what the LCD panel expects. LCDs with 6bit per color
> > will ignore the 2 LSBs of each color lane, and modules using a SoC
> > that provides only 6bit per color, drive the display information on the
> > 6 MSBs of each color lane and tie the 2 LSBs of each color lane to GND.
> > 
> > Thus, no matter what combination of LCD and SoC is used, the LCD port
> > can be used without shuffling bit lanes by always configuring the LCD
> > output to 24bit mode.
> > 
> > Add a function to handle certain quirks of the LCD interface to the
> > panel driver to be able to override the bus format specified in a
> > panel's display_mode.
> 
> I think the above paragraph clearly indicates that this is the wrong
> place to workaround this. You say yourself that the LCD interface has
> quirks that need to be handled, so why do you want to force this
> handling into the panel driver?
> 
The quirk is in the interfacing of the SoM's LCD output to the LCD
panel. Thus it can be handled in either place.

> The panel remains the same, no matter what interface you connect it to.
> 
Because that's just ONE place to change, no matter what LCD driver is
being used.


Lothar Waßmann