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

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Message-ID: <5257BFA3.9090202@metafoo.de>
Date:	Fri, 11 Oct 2013 11:06:43 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Belisko Marek <marek.belisko@...il.com>
CC:	Tomi Valkeinen <tomi.valkeinen@...com>,
	"Dr. H. Nikolaus Schaller" <hns@...delico.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 10/11/2013 10:59 AM, Belisko Marek wrote:
> Hi Tomi,
> 
> On Fri, Oct 11, 2013 at 10:17 AM, Tomi Valkeinen <tomi.valkeinen@...com> wrote:
>> On 11/10/13 10:42, Dr. H. Nikolaus Schaller wrote:
>>
>>> I am not sure if there is a SPI driver for a McBSP port [1]? And to make that
>>> work (reliably) and tested it might need a lot of work for us. At least I think
>>> such a change (e.g. setting up clock polarity etc.) is not done in some minutes.
>>> And the only feedback we have from the panel is "does not work"/"works". I.e.
>>> if we are not lucky that it works immediately we have no real means to debug.
>>>
>>> IMHO it also gives more flexibility to board designers to choose GPIOs instead
>>> of enforcing some SPI interface by the driver (and encapsulate this arguable
>>> protocol in the driver). Maybe some board has 3 spare GPIOs but neither
>>> McBSPs nor McSPIs available.
>>
>> This has been an interesting thread, I've learnt a lot =).
>>
>> I still think the panel driver should not handle this, but there should
>> be a separate spi bitbang driver for it.
>>
>> I understand you're not enthusiastic going that way, as the current
>> version works for you. However, when using DT, we need to think how to
>> represent the hardware in the device tree data, and it has to be right
>> from the beginning.
>>
>> That's why I won't allow representing this panel as having 4 gpios in
>> the DT data, because that is not correct. The panel has 3 pins. But
>> then, the panel does allow reading, which could be implemented using 4
>> gpios as you have done. This data should be in the spi-bitbang data, and
>> the panel should just use the standard SPI framework.
> I disagree. There are different drivers which pass in platform data
> gpios (encoder-tfp410.c or encoder-tpd12s015.c)
> and those must be covered by DT then. I cannot see problem why to have
> for td028 panel 3 or 4 gpios defined in DT.

The problem is not representing it in the devicetree, but representing it
correctly. This is a SPI slave device, hence it should be presented in the
devicetree as a SPI slave device and not as a platform device with 4 GPIOs.

- Lars

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