linux-kernel - Re: [PATCH 0/3] GPIO support for BRCMSTB

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Date:	Wed, 13 May 2015 10:59:30 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Gregory Fong <gregory.0xf0@...il.com>
Cc:	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	Alexandre Courbot <gnurou@...il.com>,
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
	Brian Norris <computersforpeace@...il.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCH 0/3] GPIO support for BRCMSTB

On Tue, May 12, 2015 at 9:38 PM, Gregory Fong <gregory.0xf0@...il.com> wrote:
> On Tue, May 12, 2015 at 3:59 AM, Linus Walleij <linus.walleij@...aro.org> wrote:
>> On Wed, May 6, 2015 at 10:37 AM, Gregory Fong <gregory.0xf0@...il.com> wrote:
>>
>>> There is only one IRQ for each GIO IP block (i.e. several register banks share
>>> an IRQ).  After briefly looking into the generic IRQ chip implementation, it
>>> seemed like in this case that using it would result in the driver being more
>>> complex than necessary because AFAICT it expects a 1:1 mapping of
>>> irq_chip_generic to gpio_chip.  It seemed like less of a pain to have a single
>>> irq_chip since we have a single IRQ for all register banks (multiple
>>> gpio_chips).  I might be missing something, maybe using a shared IRQ across
>>> multiple irq_chips is easier than I think?  Suggestions welcome.
>>
>> What is needed is a 1:1 mapping between GPIO offsets and IRQ
>> offsets.
>>
>> If you just number your GPIOs 0...n and your IRQs 0...n
>> it should work just fine with one irqchip for all banks.
>>
>> What screws things up is likely that the hardware supports
>> 32 lines per bank and not all are used.
>>
>> I suggest you enable 32 line and 32 IRQs per bank,
>> so that hwirq maps nicely 1:1 on the GPIO offsets,
>> then just use the width thing to NACK operations on
>> GPIO lines you are not using. This way you can also
>> decode and warn on spurious IRQs on the unused lines.
>
> For having 32 lines per bank, the big problem here is the upper limit
> of 256 GPIOs.

Which arch is this?
Usually this limit comes from
arch/*/include/asm/gpio.h

For ARM that was bumped to 512 a while back. It is also possible
to define a custom value for your system by defining
ARCH_NR_GPIOS

> Anyway, I don't think I understand IRQ domains and irq_chip_generic
> very well.  One possibility _might_ be to use multiple irq_chips.

That is probably not possible if there is just one IRQ for all
banks.

The task of the irqdomain is a 1-to-1 translation from one
hardware numberspace to the Linux IRQ number space.

In your case the hardware IRQ (hwirq) numberspace
should be:

bank0: 0..31
bank1: 32..63
....
bankn: 32*n..32*n+31

I think the gpiolib irqchip code can translate that properly
as it is just a simple 0...x mapping, the irq handler need
some magic to loop over all banks from 0..n though.

Yours,
Linus Walleij
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