Re: Question: GPIO driver how to get irq_base

[Grant Likely <grant.likely@secretlab.ca>]

On Thu, Apr 21, 2011 at 04:08:11PM +0200, Lars-Peter Clausen wrote:
> On 04/21/2011 03:45 PM, Grant Likely wrote:
> > On Thu, Apr 21, 2011 at 10:09:14AM +0200, Lars-Peter Clausen wrote:
> >> [...]
> >>
> >> In my opinion the best option for expander chips is to allocate a new irq_desc
> >> range by calling irq_alloc_descs. It will return the first irq number in the
> >> newly allocated range, which will be your irq_base.
> > 
> > Yes, irq_alloc_descs() will give you a range of irqs that the gpio
> > expander can use.  In many drivers, the irq_base is hard coded in the
> > board file and passed in via pdata, but as much as possible I
> > recommend avoiding that and letting Linux dynamically allocate the
> > irq_base for you.  Setting gpio_base to -1 will do this.
> > 
> > The ->to_irq() hook in your driver can translate
> > from a gpio number to an irq number for a specific gpio.
> > 
> >> But be aware that this will require SPARSE_IRQ to work.
> > 
> > Why?
> > 
> 
> Because in a non-SPARSE_IRQ setup all irq_descs will be allocated by default.
> See early_irq_init for the non-SPARSE_IRQ case in kernel/irq/irq_desc.c.

They'll be allocated, but not all need to be assigned.

> 
> Though platform code could make irq_descs explicitly available byfirst setting
> NR_IRQS to a larger number then it actually requires and then call
> irq_free_descs for those irqs which are not used by platform code, but using
> SPARSE_IRQ is in my opinion the better alternative in this case.

On the powerpc platforms, pretty much all irqs are dynamically
allocated and SPARSE_IRQ can be either on or off.  So, yes, you need
to make sure NR_IRQs is large enough if you don't want to use SPARSE,
but SPARSE is not a hard requirement.

g.