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

[Jerome Brunet]

On Thu, 2017-06-29 at 17:53 +0100, Marc Zyngier wrote:
> On 29/06/17 15:57, Jerome Brunet wrote:
> > On Thu, 2017-06-29 at 16:14 +0200, Linus Walleij wrote:
> > > On Tue, Jun 27, 2017 at 8:25 PM, Jerome Brunet <jbrunet at baylibre.com>
> > > wrote:
> > > 
> > > > At the time Linus strongly rejected the idea of
> > > > calling  irq_create_mapping
> > > > (or
> > > > any sleeping functions) in gpio_to_irq: please see the reply from Oct
> > > > 26,
> > > > 2016
> > > > (sorry for quoting such an old discussion but this is really the
> > > > starting
> > > > point)
> > > > 
> > > > * Me: There is really a *lot* of gpio drivers which use
> > > > irq_create_mapping
> > > > in
> > > > the to_irq callback, are these all wrong ?
> > > > * Linus: Yes they are all wrong. They should all be using
> > > > irq_find_mapping().
> > > > 
> > > > * Me: If this should not be used, what should we all do instead ?
> > > > * Linus: Call irq_create_mapping() in some other place.
> > > > 
> > > > gpio_prepare_irq is a proposition for this 'other place'.
> > > 
> > > There is a misunderstanding here.
> > > 
> > > I wrote (at the time):
> > > 
> > > > Yes, but you want to call irq_create_mapping() in slowpath (irq setup)
> > > > and irq_find_mapping() in fastpath (irq handler). Else the first IRQ
> > > > may result in unwelcomed surprises.
> > > 
> > > This does not mean that irq_create_mapping() cannot be called
> > > in irq context because I think it can.
> > > 
> > > What it means is that I think that is suboptimal from a performance
> > > point of view if called from gpio_to_irq(), because nominally, at this
> > > point, the mapping should already exist, since the GPIO and IRQ
> > > frameworks are orthogonal.
> > 
> > Agreed. It should. In such case, irq_create_mapping is just a call to
> > irq_find_mapping which is fine. If, for whatever reason this is not the
> > case,
> > this is going to call sleeping function in irq context. It should not happen
> > but
> > it is not entirely impossible ...
> > 
> > > 
> > > But that may not apply to the case with many-to-few interrupt
> > > mappings... so I think I was in some 1-to-1-mapping context
> > > when I wrote this. Sorry :(
> > > 
> > > So I changed my mind, it is fine for this type of driver to call
> > > irq_create_mapping() in gpio_to_irq(). Preferably with some comment
> > > around the call.
> > 
> > What about disposing of the mapping ? there still is no counter part
> > function to
> > gpio_to_irq. It seems weird to leave them lying around, don't you think ?
> > 
> > Here is a real use we will be having a meson:
> > * MMC driver load and call gpio_to_irq on its cd pin
> >   This is going to create a mapping 
> > * MMC driver request an irq with IRQ_EDGE_BOTH trigger (which we don't/can't
> >   support at the moment). request_irq fails.
> > * MMC defaults to polling the GPIO
> > 
> > Remember that we have only 8 possibles mappings. Now there is one (useless)
> > mapping lying around which can't get rid of.
> 
> Which should be dropped by a dispose_mapping() call in the MMC driver
> (or ideally some gpio-specific wrapper around this function).

Actually you must go through a gpio-specific function.

Not all gpio drivers use irq domains and mapping unfortunately (legacy...)  
Only the gpio driver knows how this "mapping" (between the gpio number and the
virq) was done.

The proposition at the start of this is my idea for this gpio-specific function.
I'm open to other ideas, of course :)

> 
> > If there is more drivers doing this sort of tricks, we are going to exhaust
> > the
> > ressource pretty quickly.
> 
> We can fix those drivers as we go. It's not like there's a huge variety
> of potential HW on this particular platform of yours.
> 

Those drivers are consumer of gpios. They should not have platform specific
fixes/quirks , right? Like the mmc framework, or the gpio-button driver...

In addition, this problem of un-disposed mappings may also affect any of the 26
driver which create mapping in gpio_to_irq, so it is not specific to a single
platform.

> Thanks,
> 
> 	M.