linux-kernel - Re: [PATCH v6 1/4] Runtime Interpreted Power Sequences

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Date:	Thu, 13 Sep 2012 09:22:57 +0300
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Alex Courbot <acourbot@...dia.com>
Cc:	Stephen Warren <swarren@...dia.com>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	Simon Glass <sjg@...omium.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Anton Vorontsov <cbou@...l.ru>,
	David Woodhouse <dwmw2@...radead.org>,
	Arnd Bergmann <arnd@...db.de>,
	Leela Krishna Amudala <l.krishna@...sung.com>,
	"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>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v6 1/4] Runtime Interpreted Power Sequences

On Thu, 2012-09-13 at 15:08 +0900, Alex Courbot wrote:
> On Thursday 13 September 2012 13:45:39 Tomi Valkeinen wrote:
> > * PGP Signed by an unknown key
> > 
> > On Wed, 2012-09-12 at 18:57 +0900, 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.
> > 
> > 
> > The sequences are not board-specific, they are device (backlight, etc.)
> > specific. The sequences have been handled in board-specific hook
> > functions so far because there hasn't been proper drivers for the
> > devices.
> > 
> > If I were to take the same panel (and backlight) you have and install it
> > on my board, I would need the same power sequence.
> 
> You could also have power sequences that control a set of GPIOs for an 
> external interface (and would then be more board-specific), but you are right 

What do you mean with "external interface"?

But it's true that there can always be interesting board specific
hardware designs, and they truly are board specific. In my experience
these are quite rare, though, but perhaps not so rare that we wouldn't
need to care about them.

However, I fear these board specific things may be quite a bit anything,
so it may well be pwm, gpios and regulators are not enough for them. For
example, there could be an FPGA on the board which requires some
configuration to accomplish the task at hand. It could be rather
difficult to handle it with a generic power sequence.

So I guess what I'm saying is that mostly these issues are device
specific, and when they are not, they may be rather complex/strange and
require c code.

 Tomi


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