linux-kernel - Re: [RFC][PATCH v3 1/3] runtime interpreted power sequences

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Date:	Tue, 31 Jul 2012 19:15:57 +0900
From:	Alex Courbot <acourbot@...dia.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Rob Herring <robherring2@...il.com>,
	Stephen Warren <swarren@...dia.com>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	Simon Glass <sjg@...omium.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>
Subject: Re: [RFC][PATCH v3 1/3] runtime interpreted power sequences

On 07/31/2012 07:26 AM, Stephen Warren wrote:
> On 07/30/2012 09:44 AM, Rob Herring wrote:
>> On 07/27/2012 07:05 AM, Alexandre Courbot wrote:
>>> Some device drivers (panel backlights especially) need to follow precise
>>> sequences for powering on and off, involving gpios, regulators, PWMs
>>> with a precise powering order and delays to respect between each steps.
>>> These sequences are board-specific, and do not belong to a particular
>>> driver - therefore they have been performed by board-specific hook
>>> functions to far.
>>>
>>> With the advent of the device tree and of ARM kernels that are not
>>> board-tied, we cannot rely on these board-specific hooks anymore but
>>> need a way to implement these sequences in a portable manner. This patch
>>> introduces a simple interpreter that can execute such power sequences
>>> encoded either as platform data or within the device tree.
>>>
>>
>> Why not? We'll always have some amount of board code. The key is to
>> limit parts that are just data. I'm not sure this is something that
>> should be in devicetree.
>>
>> Perhaps what is needed is a better way to hook into the driver like
>> notifiers?
>
> I would answer that by asking the reverse question - why should we have
> to put some data in DT, and some data into board files still?
>
> I'd certainly argue that the sequence of which GPIOs/regulators/PWMs to
> manipulate is just data.
>
> To be honest, if we're going to have to put some parts of a board's
> configuration into board files anyway, then the entirety of DT seems
> useless; I'd far rather see all the configuration in one cohesive place
> than arbitrarily split into two/n different locations - that would make
> everything harder to maintain.

Also, having these sequences into the DT would allow an older kernel to 
boot on and correctly initialize a newer board with - which is also part 
of the DT's purpose if I am not mistaken.

Alex.

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