Message-ID: <20120913082659.GA12259@avionic-0098.mockup.avionic-design.de>
Date:	Thu, 13 Sep 2012 10:26:59 +0200
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Alex Courbot <acourbot@...dia.com>
Cc:	Sascha Hauer <s.hauer@...gutronix.de>,
	Tomi Valkeinen <tomi.valkeinen@...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 05:21:10PM +0900, Alex Courbot wrote:
> On Thursday 13 September 2012 15:50:37 Sascha Hauer wrote:
> > 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.
> 
> Actually the sequence I submitted in this patchset only takes care of the 
> backlight device (the panel - or LCD - should have its own). The regulator 
> controls the power supply, the PWM the intensity, and on top of that it also 
> has an enable GPIO. These 3 resources are exclusively for the LED - the LCD 
> uses other ones. So as of now it seems that the LCD/backlight separation is 
> effective and the resources needed are not so uniform across backlights (not 
> even mentioning the delays).
> 
> The LCD's power sequence is even weirder - VDD must take at least 0.5ms for 
> going from 10% to 90% of its power, you must wait 400ms after switching it off 
> before switching it on again, and you should also transmit data for 200ms 
> before switching the backlight's LED on (using its own sequence). That last 
> point is interesting since it somehow makes the LCD and LED dependent on each 
> other - on an unrelated note, this might be something to consider in Laurent's 
> proposal for a panel framework.

Maybe this could be solved by adding a backlight resource type and
embedding a reference to the backlight within the panel's power
sequence?

Thierry

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