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

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Message-ID: <20141103152743.GB1618@lahna.fi.intel.com>
Date:	Mon, 3 Nov 2014 17:27:43 +0200
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Felipe Balbi <balbi@...com>
Cc:	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

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.

The problem is that it is *firmware* code that decides to use the GPIO
at some random point in time and we have no way to tell it to retry
later when the GPIO is available.
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