[RFC] gpio: about the need to manage irq mapping dynamically.

[Jerome Brunet]

On Tue, 2017-06-20 at 18:37 +0200, Linus Walleij wrote:
> On Tue, Jun 20, 2017 at 12:26 PM, Jerome Brunet <jbrunet at baylibre.com> wrote:
> 
> So I finally understood that what you want is to handle those cases
> where gpio_to_irq() is currently in use, and not expand the use of that
> function. And that is good.

Indeed

> 
> > On Tue, 2017-06-20 at 10:39 +0200, Linus Walleij wrote:
> > > > The fact that 26 gpio drivers [3] create mapping in the
> > > > gpio_to_irq callback shows that the problem of managing the irq mapping
> > > > is
> > > > not
> > > > specific to Amlogic and that an evolution might be useful.
> > > 
> > > Calling irq_create_mapping() in gpio_to_irq() is not a problem if
> > > the mapping is free:ed elsewhere. But yeah, there are lots of old
> > > drivers with old and erroneous solutions to this problem.
> > 
> > A lot of these drivers are old indeed, and dealing with all this history
> > must be
> > quite a challenge ;)
> > However, some are fairly recent.
> 
> Unfortunately the API has merits. Like for keys: OK there is a key
> for an GPIO line, and then it asks "can I have an IRQ for that"?
> There are cases where the IRQ is really optional, not a required
> feature.
> 

I think you misunderstood me. I have nothing against the API. I absolutely want
to keep it. I'd like to extend it.

> > So there is 2 use cases:
> > * What you described above: This is indeed completely orthogonal to gpio
> > framework. It will be using the irqchip callbacks only. In this particular
> > case,
> > it is the device tree platform code which will handle the mapping creation
> > for
> > us.
> 
> OK
> 
> So the problem is really to use drivers that cannot do all their
> mappings at probe
> time with drivers that use gpio_to_irq(), right?

Yep

> 
> So, if the idea is to patch *ALL* drivers using gpio_to_irq(), I think
> for each case
> it should be investigated whether that should really be using gpio_to_irq()
> but I suspect they are all pretty solid.
> 
> Well you're probably right, the API has to change for the case where the
> number
> of IRQs are limited and cannot just be handled out to the left and right, but
> we
> should really replace *ALL* occurences of gpio_to_irq() with a pair of
> gpio_irq_prepare() that also returns the valid IRQ if found, and
> gpio_irq_unprepare()
> that removes it.
> 
> For the transitional period, gpio_to_irq() should *FAIL* if gpio_irq_prepare()
> was called first for the same GPIO line.
> 
> Eventually gpio_to_irq() should be DELETED and replaced in full with
> the prepare/unprepare calls.

Woahh, that's not what I meant. gpio_to_irq should stay. Getting rid of it would
be a mess and it is a useful call.

The gpio_irq_prepare is meant so that the consumer can tell the gpio driver it
will want to get irq from a particular gpio at some point.

IOW, it's the consumer saying to the gpio driver "please do whatever you need to
do, if anything, so this gpio can generate an interrupt"

This is a much simpler change. Using devm, all we need is to put a
devm_gpio_irq_prepare(<gpio_num>) in the probe of the drivers using gpio_to_irq.

Mandating call to gpio_irq_prepare before any call to gpio_to_irq will be fairly
easy.

> 
> And I would like strong confidence that the change will be carried all the way
> through, not half-done and left to me to complete. I already have too many
> problems of that type.


Fair enough.

> 
> Yours,
> Linus Walleij