Re: [PATCH] pinctrl: baytrail: show output gpio state correctly on Intel Baytrail

[David Cohen]

On Tue, Nov 04, 2014 at 09:59:36AM +0200, Mika Westerberg wrote:
> On Mon, Nov 03, 2014 at 02:19:03PM -0800, David Cohen wrote:
> > Hi Mika,
> > 
> > Thanks for your feedbacks :)
> > 
> > On Mon, Nov 03, 2014 at 08:42:47PM +0200, Mika Westerberg wrote:
> > > On Mon, Nov 03, 2014 at 09:50:11AM -0600, Felipe Balbi wrote:
> > > > Hi,
> > > > 
> > > > On Mon, Nov 03, 2014 at 05:42:07PM +0200, Mika Westerberg wrote:
> > > > > On Mon, Nov 03, 2014 at 05:27:43PM +0200, Mika Westerberg wrote:
> > > > > > On Mon, Nov 03, 2014 at 09:00:48AM -0600, Felipe Balbi wrote:
> > > > > > > On Mon, Nov 03, 2014 at 11:24:02AM +0200, Mika Westerberg wrote:
> > > > > > > > On Fri, Oct 31, 2014 at 11:45:09AM -0700, David Cohen wrote:
> > > > > > > > > > I think adding the module exit + allowing this driver to be a module
> > > > > > > > > > would be a good approach. Then we don't need to force generic x86 kernel
> > > > > > > > > > binaries to always have this driver. Unless Mathias or Mika knows a
> > > > > > > > > > constraint to force this driver to be builtin only.
> > > > > > > > > 
> > > > > > > > > It helps if I CC them when asking for feedback :)
> > > > > > > > > 
> > > > > > > > > Mathias, Mika, do you know any constraint that forces pinctrl-baytrail
> > > > > > > > > to be bool?
> > > > > > > > 
> > > > > > > > The only constraint that has been keeping this driver as bool is that
> > > > > > > > some machines like, Asus T100, uses ACPI GPIO operation regions for
> > > > > > > > toggling GPIOs to get things like sensor hub powered on. The GPIO
> > > > > > > > operation region code does not yet handle -EPROBE_DEFER so only way to
> > > > > > > > ensure that the operation region is there is to have the driver compiled
> > > > > > > > in to the kernel.
> > > > > > > 
> > > > > > > But that's not enough excuse to have every single x86 in the market
> > > > > > > shipping with this driver. Think about a distro kernel, most likely this
> > > > > > > gets enabled and it's wrong in 80% of the cases.
> > > > > > 
> > > > > > True, but see below.
> > > > > > 
> > > > > > > It would be nicer to add EPROBE_DEFER support, convert this into
> > > > > > > tristate and have default = M if BAYTRAIL, or something.
> > > > > > 
> > > > > > If it were simple as that we would have done that already. Please check
> > > > > > drivers/gpio/gpiolib-acpi.c:acpi_gpio_adr_space_handler() and tell me
> > > > > > how we can do that.
> > > > > 
> > > > > Actually the above is not the problem because we already have registered
> > > > > the GPIO chip and hence we have the GPIO available to the firmware code.
> > > > 
> > > > what happens before you registered the gpio chip ? It takes some time
> > > > from head.S to gpiochip_irqchip_add(). Anywhere between that time,
> > > > firmware could try to access gpios and the same problem would occur.
> > > 
> > > The operation region is not ready and the firmware does not try to use
> > > it. However, the subsys_initcall() is there just to be sure that the
> > > GPIO driver gets loaded before anything that is going to use GPIOs from
> > > firmware.
> > 
> > That sounds hackish and dangerous. There are shared registers between
> > different GPIOs.
> 
> For one it certainly is not hackish and dangerous. It is well defined
> in the ACPI specification as interfaces between firmware and OS.

It looks we have an implicit dependency to GPIO driver in Bay Trail, and
having this window until load the module is not acceptable to fulfill
this implicit dependency.

But IMHO all dependency to a driver should be explicitly described
(e.g. on Kconfigs, or maybe failing probe). With current situation if we
do not select pinctrl_baytrail, instead of affecting just the drivers
that explicitly depend on that, it affects others which we are unable to
easily identify.

Maybe I don't like or fully agreee/understand ACPI, but I'd call that
dangerous.

> 
> > > > > The real problem is that if the ACPI GPIO operation handler is not there
> > > > > at the time firmware decides to do something it will just skip things
> > > > > that depend on the operation region. So if it has a GPIO that is used to
> > > > > turn on sensor hub or touch panel or whatever, this will not be done and
> > > > > it results that the device in question might not work properly.
> > > > 
> > > > that's an issue that needs solving, but forcing every x86 kernel to ship
> > > > with this driver, is not a proper solution.
> > > 
> > > I would rather have the driver build in to the kernel now (and btw it
> > > has been already in mainline quite some time so I suspect many distros
> > > have already enabled it), than turning it module and render some devices
> > > that have been working previously, fail suddenly.
> > 
> > Hm.... it sounds like we're saying: we got it working somehow and from
> > now on we'll avoid change it to not create regression.
> 
> That's exactly what I'm saying :-)

In this case, I think we need to carefully document our concerns about
the driver to help others not involved in original implementation to
easily contribute to it.

> 
> > Correct if I'm wrong, but there's nothing really preventing this driver
> > to be a module beside the buggy ACPI interface that allows kernel and fw
> > to play freely (and silently) with same piece of hw.
> 
> Not a buggy ACPI interface. Lack of implementation from Linux side.
> 
> > IMHO we could allow this driver be M, but default to Y and an
> > explanation to set to Y in case FW needs to use GPIO before module gets
> > loaded. Since we've development board (Minnow Max), we should let kernel
> > more free for other developers as well.
> 
> All I'm saying both to you guys that this is the reasoning we have the
> driver as a bool. Which is what you asked from me in the first place.

I apology for my frustration :)
You've been more than helpful clarifying our questions.

> 
> Since you both insist to turn it module, please send a patch with the
> corresponding changelog telling why you think it does not need to build
> in and let's see if we are getting any regressions. If yes, then we just
> revert the patch. Does that work for you?

I'm in favor of not hardcoding kernel features based on possible hidden
things that may happen on closed source FWs.
Having the module option does help further development of this driver
and others that depend on it. A clear explanation on Kconfig why we
recommend Y for production and why M is still experimental should be
enough to guide ppl working on distributions and at same time help
kernel developers involved with the driver.

Br, David
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