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

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Date:	Thu, 13 Sep 2012 09:29:20 +0200
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Tomi Valkeinen <tomi.valkeinen@...com>
Cc:	Sascha Hauer <s.hauer@...gutronix.de>,
	Alex Courbot <acourbot@...dia.com>,
	Stephen Warren <swarren@...dia.com>,
	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, Sep 13, 2012 at 10:03:27AM +0300, Tomi Valkeinen wrote:
> On Thu, 2012-09-13 at 09:00 +0200, Sascha Hauer wrote:
> > On Thu, Sep 13, 2012 at 09:54:09AM +0300, Tomi Valkeinen wrote:
> > > On Thu, 2012-09-13 at 15:36 +0900, Alex Courbot wrote:
> > > > On Thursday 13 September 2012 14:22:57 Tomi Valkeinen wrote:
> > > >  
> > > > > 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).
> > > 
> > > Yes, that's one concern of mine... I already can imagine someone
> > > suggesting adding conditionals to the power sequence data. Perhaps also
> > > direct memory read/writes so you can twiddle registers directly. And so
> > > on. Where's the limit what it should contain? Can we soon write full
> > > drivers with the DT data? =)
> > 
> > I have this concern aswell, that's why I'm sceptical about this patch
> > set. But what are the alternatives? Adding power code to the drivers and
> > thus adding board specific code to them is backwards.
> 
> As was pointed out in earlier posts in this thread, these are almost
> always device specific, not board specific.
> 
> Do you have examples of board specific power sequences or such?

It is true that most (perhaps all) power sequences can be associated
with a specific device, but if we go and implement drivers for these
kinds of devices we will probably end up with loads of variations of
the same scheme.

Lets take display panels as an example. One of the devices that we build
has gone through two generations so far and both are slightly different
in how they control the panel backlight: one has an external backlight
controller, the other has the display controller built into the panel.
However, from the board's perspective the control of the backlight
doesn't change, because both devices get the same inputs (an enable pin
and a PWM) that map to the same pins on the SoC.

This may not be a very good example because the timing isn't relevant,
but the basic point is still valid: if we provide a driver for both
panel devices, the code will be exactly the same. So we end up having to
refactor to avoid code duplication and use the same driver for a number
of backlight/panel combinations. Which in itself isn't very bad, but it
also means that we'll probably get to see a large number of "generic"
drivers which aren't very generic after all.

Another problem, which also applies to the case of power-sequences, is
that often the panel and backlight are not the same device. So you could
have the same panel with any number of different backlight controllers
or vice-versa any number of different panels with the same backlight
controller.

Thierry

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