Re: [PATCH 6/7] gpio: brcmstb: consolidate interrupt domains

[Doug Berger <opendmb@xxxxxxxxx>]

On 10/03/2017 08:03 PM, Gregory Fong wrote:
> Hi Doug,
> 
> On Fri, Sep 29, 2017 at 8:40 PM, Doug Berger <opendmb@xxxxxxxxx> wrote:
>> The GPIOLIB IRQ chip helpers were very appealing, but badly broke
>> the 1:1 mapping between a GPIO controller's device_node and its
>> interrupt domain.
> 
> Out of curiosity, what sort of problems have you seen from this?
> 

As you know, the BRCMSTB devices conceptually distinguish between an
always-on GPIO device and a regular GPIO device that each can have many
more than 32 General Purpose I/Os. The driver supports these by dividing
the GPIO across a number of banks each of which is implemented as a
separate gpiochip as an implementation convenience. The main issue is
that each gpiochip that uses the GPIOLIB IRQ chip helpers creates its
own irq domain even though they are associated with the same device and
device-tree node.

When another device-tree node references a GPIO device as its interrupt
parent, the irq_create_of_mapping() function looks for the irq domain of
the GPIO device and since all bank irq domains reference the same GPIO
device node it always resolves to the irq domain of the first bank
regardless of which bank the number of the GPIO should resolve. This
domain can only map hwirq numbers 0-31 so interrupts on GPIO above that
can't be mapped by the device-tree.

>>
>> This commit consolidates the per bank irq domains to a version
>> where we have one larger interrupt domain per GPIO controller
>> instance spanning multiple GPIO banks.
> 
> This works (and is reminiscent to my initially submitted
> implementation at [1]), but I think it might make sense to keep as-is
> (using the gpiolib irqchip helpers), and instead allocate an irqchip
> fwnode per bank and use to_of_node() to set it as the of_node for the
> gpiochip before calling gpiochip_irqchip_add().  OTOH, that capability
> might go away...
> 
> Linus, can you comment on the FIXME in gpiochip_irqchip_add_key() that
> says "get rid of this and use gpiochip->parent->of_node everywhere"?
> It seems like it would still be beneficial to be able to override the
> associated node for a gpiochip, since that's what's used for the
> irqdomain, but if that's going away, obviously we don't want to start
> using that now.
> 

Yes, this is effectively a reversion to an earlier implementation. I
produced an implementation based on the generic irqchip libraries, but
that was stripped from this submission when I discovered that no support
exists within the generic irqchip libraries for removal of domain
generic chips and we wanted to preserve the module support of this driver.

It is conceivable that the current GPIO device-tree nodes could be
broken down into separate devices per bank, but it is believed that this
would only confuse things for users of the device as the concept
diverges from the concept expressed in device documentation.

> Thanks,
> Gregory
> 
> [1] https://patchwork.kernel.org/patch/6347811/
> 

Thanks for the review,
    Doug
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