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

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Message-ID: <20120913075037.GG6180@pengutronix.de>
Date:	Thu, 13 Sep 2012 09:50:37 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Thierry Reding <thierry.reding@...onic-design.de>
Cc:	Tomi Valkeinen <tomi.valkeinen@...com>,
	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 09:29:20AM +0200, Thierry Reding wrote:
> 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.

Maybe that is the problem that needs to be addressed? They *are* not the
same device, still they are handled in a single platform callback (or
now power sequence). Maybe the amount of combinations dastrically go
down if we really make them two devices.

Most of our panels have:

- A regulator (or gpio) for turning them on

And the backlights have:

- A regulator (or gpio) for turning them on
- A PWM for controlling brightness.

The power sequence for the above is clear: Turn on the panel the panel,
wait until it stabilized and afterwards turn on the backlight.

Sascha

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