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

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Date:	Tue, 4 Nov 2014 08:44:21 -0600
From:	Felipe Balbi <balbi@...com>
To:	Mika Westerberg <mika.westerberg@...ux.intel.com>
CC:	Felipe Balbi <balbi@...com>,
	David Cohen <david.a.cohen@...ux.intel.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	stable <stable@...r.kernel.org>, <mathias.nyman@...ux.intel.com>
Subject: 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:
> > > > > > > > > > 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 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.

> > > > > 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

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