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SubjectRe: [PATCH v6 1/4] Runtime Interpreted Power Sequences
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On Thu, 2012-09-13 at 09:29 +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.

We had something a bit similar in Nokia. First versions had an
"independent" backlight controlled via pwm. Later versions had a
backlight that is controlled by the panel IP, so it was changed by
sending DSI commands to the panel.

> 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.

Yes, I think the backlight and the panel should be considered separate
devices. Just like, say, a touch screen and a panel may happen to be in
the same display module, a backlight and a panel can be in the same
display module. They are still separate, independent things, although
they are, of course, used together.

Tomi

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