linux-kernel - Re: [RFC PATCH] gpio: support for GPIO forwarding

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Date:	Wed, 14 Jan 2015 08:32:41 -0800
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Linus Walleij <linus.walleij@...aro.org>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Arnd Bergmann <arnd@...db.de>
CC:	Alexandre Courbot <gnurou@...il.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Ortiz, Samuel" <samuel.ortiz@...el.com>
Subject: Re: [RFC PATCH] gpio: support for GPIO forwarding

Ugh, Samuel actually Cc'd this time...

On 1/14/15 4:58 AM, Linus Walleij wrote:
> On Thu, Dec 18, 2014 at 9:23 AM, Heikki Krogerus
> <heikki.krogerus@...ux.intel.com> wrote:
> 
>> This makes it possible to assign GPIOs at runtime. The
>> motivation for it is because of need to forward GPIOs from
>> one device to an other. That feature may be useful for
>> example with some mfd devices, but initially it is needed
>> because on some Intel Braswell based ACPI platforms the GPIO
>> resources controlling signals of the USB PHY are given to
>> the controller device object for whatever reason, so the
>> driver of that controller needs be able to pass them to the
>> PHY device somehow.
>>
>> So basically this is meant to allow patching of bad (bad
>> from Linux kernels point of view) hardware descriptions
>> provided by system fw in the drivers.
>>
>> Signed-off-by: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
>> ---
>>
>> Hi,
>>
>> I'm sending this first as a RFC in case you guys have some better
>> idea how to solve our problem. I actually already have couple
>> platforms where the GPIO resources are given to the "wrong" device
>> objects now.
>>
>> Originally we were thinking about simply handling our problem with
>> hacks to the PHY drivers, basically also checking if the parent has
>> GPIOs. That would only work if the controller is always the parent,
>> which it's not, but even if it was, it would be too risky. The PHY
>> drivers don't know which controller they are attached to, so what is
>> to say that the GPIOs aren't really attached to the controller.
>>
>> So the safest way to handle our problem is to deal with it in quirks
>> in the controller drivers. Solving it by bouncing the GPIOs did not
>> feel ideal there doesn't seem to be any other way. The API is copied
>> from clkdev (clk_register_clkdev). In the end it's really only one
>> function for adding the lookups and one for removing them.
>>
>> The way it's implemented is by modifying the current style of handling
>> the lookups a little bit. The per device lookup table is basically
>> reverted back to the single linked-list format since it seems that
>> these lookups may have to be assigned from several sources.
> 
> Oh ain't that great.
> 
> So now we start patching around the kernel because the ACPI
> tables are erroneous for GPIOs. I'd like some feedback from
> Rafael &| Darren on this patch, i.e. if you two think this is a good
> way of accounting for bad GPIO descriptions in ACPI tables then
> ACK this patch.
> 
> I guess the other option would be to fix up the ACPI DSDT
> itself to put resources right, correct? Is this not possible?
> 
> Alexandre also need to ACK it before I dare do anything with
> it.
> 
> Do we want to use the same mechanism for augmenting
> bad device trees too?
> 
> What I like about it so far is the create/remove symmetry
> though.
> 
> Yours,
> Linus Walleij
> 

-- 
Darren Hart
Intel Open Source Technology Center
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