linux-kernel - Re: Adding interrupt support to gpio-ich driver (possibly via SCI)

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Message-ID: <20140408033114.GA28159@srcf.ucam.org>
Date:	Tue, 8 Apr 2014 04:31:14 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	Peter Tyser <ptyser@...-inc.com>,
	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: Re: Adding interrupt support to gpio-ich driver (possibly via SCI)

On Mon, Apr 07, 2014 at 08:21:00PM -0700, Guenter Roeck wrote:
> On 04/07/2014 07:48 PM, Matthew Garrett wrote:
> >You shouldn't need to install an SCI handler - the way the hardware will
> >generate an SCI is to raise a GPE. If you know which GPE the device
> >raises (my recollection is that for most Intel chipsets it's GPIO number
> >+ 0x10) then you can just call acpi_install_gpe_handler(). The problem
> 
> Sounds good. Do you by any chance have a pointer to some documentation
> explaining this in some more detail ?

The SCI is just IRQ 9 - it tells the OS that there's a firmware event, 
but in itself doesn't say what that event was. This is handled by the 
platform setting bits in the GPE*_STS registers. The ACPI code reads 
that and then dispatches the event to the appropriate handler. This will 
typically be some ACPI code (declared by _Lxx and _Exx methods in the 
ACPI tables - xx corresponds to the GPE number, L and E whether it's 
level or edge triggered), but in some cases you want to install a 
hardcoded event handler.

I've only got the 5-series docs to hand, and I can't remember whether 
that's Panther Point, but you want to look at the definition of GPE0_STS 
to figure out which hardware events cause which GPEs. GPEs 16 to 31 
appear to correspond to GPIO 0 to 15, which is easy enough to handle.

> >is that the firmware may well already be using some of those GPIOs, and
> >there's no easy way to tell. Checking the interrupt configuration isn't
> >sufficient, since some of them may just be used as outputs.
> >
> The gpio-ich driver already has some magic to detect that condition - I
> noticed that I can not request all GPIO pins on all hardware. Either case,
> the gpio pins I am interested in are well defined on the hardware I am
> dealing with, so I can be sure I won't step on some unexpected use.

Ok. As long as you don't reprogram anything by default, I think this 
should be fine.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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