Re: [PATCH 01/10] irq: move some interrupt arch_* functions into struct irq_chip.

[Ian Campbell <Ian.Campbell@citrix.com>]

On Wed, 2010-03-24 at 17:44 +0000, Thomas Gleixner wrote: 
> On Wed, 24 Mar 2010, Ian Campbell wrote:
> 
> > On Mon, 2010-03-22 at 10:19 +0000, Thomas Gleixner wrote:
> > > On Sun, 21 Mar 2010, Yinghai Lu wrote:
> > > 
> > > > From: Ian Campbell <ian.campbell@citrix.com>
> > > > 
> > > > 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.
> > > 
> > > Not sure about that. These functions are solely used by x86 and there
> > > is really no need to generalize them.
> > 
> > I thought the idea of struct irq_chip was to allow the potential for
> > multiple IRQ controllers in a system? Given that it seems that struct
> > irq_desc->chip_data ought to be available for use by whichever struct
> > irq_chip is managing a given interrupt. At the moment this is not
> > possible because we step around the abstraction using these arch_*
> > methods.
> 
> Right, but you have exactly _ONE_ irq_chip associated to an irq_desc,
> but that same irq_chip can be associated to several irq_descs. So
> irq_desc->data is there to provide data for the irq_chip functions
> depending on what irq they handle (e.g. base_address ...). 
> 
> irq_desc->chip_data is set when the irq_chip is assigned to the
> irq_desc.
> 
> So there is no point in having functions in irq_chip to set
> irq_desc->chip_data.

So how do you know which is the appropriate irq_chip specific function
to call given an irq_desc that you want to copy/free/migrate? The
contents of the chip_data pointer will take different forms for
different irq_chips. The way the generic code is currently structured it
appears you can't (or at least don't) just do a shallow copy by copying
the irq_desc->chip_data pointer itself -- you need to do a deep copy
using a function which understands that type of chip_data.

How is this operation different to having pointers in irq_chip for
enabling/disabling/masking interrupts for each irq_chip?

Ian.