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

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Message-ID: <2689722.93BQTh4lSC@percival>
Date:	Thu, 13 Sep 2012 15:36:23 +0900
From:	Alex Courbot <acourbot@...dia.com>
To:	Tomi Valkeinen <tomi.valkeinen@...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 Thursday 13 September 2012 14:22:57 Tomi Valkeinen wrote:
> * PGP Signed by an unknown key
> 
> On Thu, 2012-09-13 at 15:08 +0900, Alex Courbot wrote:
> 
> > On Thursday 13 September 2012 13:45:39 Tomi Valkeinen wrote:
> > 
> > > > Old 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"?

Any crazy circuit design that would make the regular power sequence not usable 
on a specific board. Sorry, I don't have any concrete example in mind, the 
above is just speculation.

> 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.

Right. Note that this framework is supposed to be extended - I would like to 
at least add regulator voltage setting, and maybe even support for clocks and 
pinmux (but that might be out of place).

> 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.

You're definitely right about the powering issue being a device issue 99% of 
the time. For the rest I do not have enough insight to emit an opinion.

Alex.

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