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    SubjectRe: [PATCH 01/10] irq: move some interrupt arch_* functions into struct irq_chip.
    On Wed, 24 Mar 2010, Ian Campbell wrote:
    > 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.

    The design of sparse_irq or to be honest the lack of design grew that
    crap and it's not only this detail which is a nightmare. That pointer
    should probably be simply copied. Either that or if the chip data
    require to be node bound we need something along the line:

    struct sparse_irq_chip_data {
    void *data;
    void (*copy)(...);
    void (*free)(...);
    };

    and a corresponding field in irq_desc.

    I'm looking into sparse_irq right now anyway because it has other way
    more serious short comings.

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

    Because that's the purpose of the irq_chip perhaps ?

    Thanks,

    tglx


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