Re: Question: GPIO driver how to get irq_base

[Grant Likely]

On Thu, Apr 21, 2011 at 10:09:14AM +0200, Lars-Peter Clausen wrote:
> On 04/21/2011 09:32 AM, Tomoya MORINAGA wrote:
> > Hi GPIO maintainer,
> > 
> > Need your help.
> > I want to add interrupt function to gpio/pch_gpio.c
> > 
> > According to other upstreamed GPIO driver, (e.g. pl061, langwell, etc...)
> > 2 IRQs(irq, irq_base) are used.

irq and irq_base are two different things.  irq is the irq number that
the gpio expander is wired up to.  irq_base is the starting range for
any irqs generated by the gpio expender.  Essentially, turning the
gpio controller into an irq controller.

> > 
> > I can get "irq" from (struct pci_dev *)pdev->irq.
> > However I can't understand where can I get the "irq_base".
> > Let me know how to get.
> > 
> > Thanks,
> > -----------------------------------------
> > Tomoya MORINAGA
> > OKI SEMICONDUCTOR CO., LTD.
> 
> 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?

g.
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