Re: Question: GPIO driver how to get irq_base

[Lars-Peter Clausen <lars@metafoo.de>]

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.

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.

- Lars