Subject: Re: [PATCH] irq: move some interrupt arch_* functions into struct irq_chip.

From: [email-masked] (Eric W. Biederman)
Date: Wed, 10 Mar 2010 09:42:08 -0800

Ian Campbell <[email-masked]> writes:

> On Wed, 2010-03-10 at 12:06 +0000, Yinghai Lu wrote:
>> On Wed, Mar 10, 2010 at 2:55 AM,  <[email-masked]> wrote:
>> > From: Ian Campbell <[email-masked]>
>> >
>> > Move arch_init_copy_chip_data and arch_free_chip_data into function
>> > pointers in struct irq_chip since they operate on irq_desc->chip_data.
>> >
>> > arch_init_chip_data cannot be moved into struct irq_chip at this time
>> > because irq_desc->chip is not known at the time the irq_desc is
>> > setup. For now rename arch_init_chip_data to arch_init_irq_desc (for
>> > PowerPC, the only other user, whose usage better matches the new name)
>> > and on x86 convert arch_init_chip_data to ioapic_init_chip_data and
>> > call this whenever the IO APIC code allocates a new IRQ.
>> >
>> > I've retained the chip_data behaviour for uv_irq although it isn't
>> > clear to me if these interrupt types support migration or how closely
>> > related to the APIC modes they really are. If it weren't for this the
>> > ioapic_{init,copy,free}_chip_data functions could be static to
>> > io_apic.c.
>> >
>> > I've tested by booting on a 64 bit system, but it's not clear to me
>> > what actions I need to take to actually exercise some of these code
>> > paths.
>> >
>> 
>> can you just add another pointer field in irq_desc?
>> 
>> some kind of *irq_info etc.
>
> I think I don't understand what you are suggesting.

YH another field doesn't make much sense.  Xen is a bizarre subarch
with an incompatible irq model.  Xen simply needs the ability to
handle the entire lifetime of an irq_chip.

All we need between the Xen and the rest of x86 is a convention
so that we never manage the same irqs.   At least for domU we are
in an either/or situation so I don't see even that being a problem.

> There is already a pointer for irq_chip specific use i.e.
> irq_desc->chip_data. This patchset is just about ensuring that the field
> really is available to any chip implementation rather than just assuming
> it is always used for the acpi chip types (on x86 at least).

Ian Xen in this sense is simply not x86.  irq_cfg is not acpi or ioapic
or anything but x86 specific.  It has everything to do with having a per
cpu vector table of 256 entries and architecturally receiving a vector
number when an interrupt is fired.

It totally makes sense for Xen to do something different because
architecturally it has a completely different irq subsystem.

At the same time let's not pretend that the reason for this is anything
except that Xen has a completely different notion of interrupt delivery
than the rest of x86 and so it is it's own bizarre subarch.

This is not a case where you simply need a driver because something is
a bit different but fits into the existing model.

So the best solution here seems to be a parameter that we pass into
irq_to_desc_alloc_node that does what is needed.  The second best
would be to have arch_init_chip_data to call something like
platfrom_init_chip_data().    But I think we can avoid that in
this case.

Eric