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

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Date:	Tue, 12 May 2015 12:38:43 -0700
From:	Gregory Fong <gregory.0xf0@...il.com>
To:	Linus Walleij <linus.walleij@...aro.org>
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 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.  We would hit that limit on SoCs that already exist with
the SoC GPIOs alone, let alone any GPIO extenders.  I'm really not
sure what the right way would be to deal with that.

Anyway, I don't think I understand IRQ domains and irq_chip_generic
very well.  One possibility _might_ be to use multiple irq_chips.  But
from what I do understand, if there's only a single parent IRQ used by
all of the GPIO banks, in order to use an irq_chip per bank, this
would have to stop using the chained irq logic because the parent IRQ
is shared across banks, and that implementation seems unnecessarily
confusing.  Any other ideas?

Best regards,
Gregory
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