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

Hi,

On Tue, Nov 04, 2014 at 09:51:35AM +0200, Mika Westerberg wrote:

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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.

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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 firmware checks the dependent object for AVBL or similar variable.
If it is != 0 it assumes that the operation region is there. The problem
is how can you tell the firmware that it should somehow try again?

Here _DEP is the right solution as it forces the dependencies to be
loaded first.

alright.

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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.

So you are saying it is better to add config option BAYTRAIL and then
make the driver Y? Well, now generic distros need to select that as Y
which doesn't solve anything.

ok, so there's no differentiation for when you're building a baytrail
kernel, that's fine. Then it gets really difficult to do anything. I
guess you really need to to wait for _DEP.

-- 
balbi