Hi,

On Mon, Nov 03, 2014 at 08:42:47PM +0200, Mika Westerberg 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.

alright, so how does the firmware know that the operation region is
ready and why can't that be deferred until pinctrl-baytrail (module or
built-in) has finished probing ? That would sort both issues, would it
not ?

> > > 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.

that's why I said you should default to Y if BAYTRAIL or make it
tristate and always default to M.

> There is a mechanism in ACPI to solve these issues, called _DEP, but it
> is still very much work in progress.

alright.

-- 
balbi