How to use linear mapping without DT?

[Shawn Guo]

On Thu, Jun 14, 2012 at 11:54:36AM +0200, ludovic.desroches wrote:
> Hello,
> 
> In fact, my question depends on sparse irq, irq domain and non DT.
> 
> I have enabled sparse_irq use for at91 sub architecture. I don't
> know if it is the good solution... I won't like to use n_irqs in my
> machine descriptor. I think the goal of sparse irq is to allocate
> the irq descriptor only when the irq is used. It's better than
> allocating the range that can be managed by the controller which
> should be indicated by nr_irqs.
> 
> In this case using the legacy irq domain is not the right solution
> since I will have to allocate the range of irq managed by the
> controller. Moving to linear irq domain should be better. Moreover,
> I have seen that Grant may want to remove it.
> 
> So, how to move to linear irq domain? In DT case, all the interrupts
> into the DT are mapped so the irq descriptor is allocated and my
> device will have the right virq number. In non DT case, my device
> has an irq number corresponding to a fixed value (which is in fact
> the hwirq number) but probably not to any existing virq.
> 
> So how to get a valid virq? Do I have to call irq_create_mapping
> from my driver? It's probably not the right way... Should I keep
> legacy domain and set nr_irqs?
> 
Short answer - Don't use linear but legacy domain for non-DT. 

I was getting punched on the head for thinking about using legacy
domain for non-DT.[1]  Also Documentation/IRQ-domain.txt suggests
that as below.

| The Legacy mapping is a special case for drivers that already have a
| range of irq_descs allocated for the hwirqs.  It is used when the
| driver cannot be immediately converted to use the linear mapping.  For
| example, many embedded system board support files use a set of #defines
| for IRQ numbers that are passed to struct device registrations.  In that
| case the Linux IRQ numbers cannot be dynamically assigned and the legacy
| mapping should be used.

So as long as you need to support non-DT users, you will need to use
legacy domain.  But that does not necessarily mean you need to set
.nr_irqs in mach_desc for enabling spare-irq.  If you have all your
irqchips call irq_alloc_descs to allocate their irqdescs, you can enable
spare-irq without the need of setting .nr_irq.  I posted the series
"[PATCH 00/16] Enable SPARSE_IRQ support for imx" yesterday doing that
for imx.

Regards,
Shawn

[1] http://thread.gmane.org/gmane.linux.ports.arm.kernel/151860/focus=152072