linux-kernel - Re: [PATCH] omapdss: Add new panel driver for Topolly td028ttec1 LCD.

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Date:	Fri, 11 Oct 2013 13:09:25 +0300
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	"Dr. H. Nikolaus Schaller" <hns@...delico.com>,
	Lars-Peter Clausen <lars@...afoo.de>
CC:	Belisko Marek <marek.belisko@...il.com>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	<linux-fbdev@...r.kernel.org>
Subject: Re: [PATCH] omapdss: Add new panel driver for Topolly td028ttec1
 LCD.

On 11/10/13 12:50, Dr. H. Nikolaus Schaller wrote:

> Hm. Is this a SPI or does it just look like one? Or is it some - otherwise
> unknown - "3 wire serial interface". Or is it a "3(+1) GPIO slave device"?
> I am still not sure about this.

Lars-Peter said "Back in the OpenMoko days we used the panel in normal
4-wire SPI mode with the GPIO bitbang SPI master."

I don't know much about SPI, so I can't answer to that. If the serial
bus is indeed not any kind of more or less standard SPI version, but
really a custom bus for this controller, then the case is a bit unclear.

> If we really want to do it correctly, we may have to write a driver for that
> special serial protocol as well. If it turns out that we can't mis-use and tweak
> it into a standard SPI driver with bit-bang backend.
> 
> I simply fear that we get dependencies with the SPI subsystem and have
> to test, debug and maintain it. Maintaining the GPIO thing we currently have
> is easy.
> 
> What would be against taking the GPIO approach first and then upgrade as soon
> as someone raises his/her finger that he/she wants to really interface this display
> differently and is not happy with the 3/4 GPIOs? Either they come up with a patch
> or contact the driver author (=me).

I don't have anything against that as long as we use only platform data.

But DT data is not an in-kernel API, it's an external API. Once we
define that the DT data for this panel is something, that's it, we
should stick to it. Of course, we can build compatibility layers for old
DT data, but I would avoid that if at all possible.

If we now create the DT data with gpios, and the panel as platform
device, it'd be rather nasty change to make it a child of an spi bus. (I
presume, I have never made such a change).

And, as the gpios and platform device approach is clearly wrong way to
describe the hardware, I'm quite against using that description in the
DT data.

That said, one option is to describe the hardware correctly in the DT
data, but have a platform device for the panel, with panel driver doing
the bitbanging. In that case it is possible to update the system to use
SPI framework if needed, without changing the DT data. However, I'm not
sure how easy that would be.

 Tomi

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