RFC for ideas to clean up PCI MSI code

[Jiang Liu <jiang.liu@linux.intel.com>]

Hi Bjorn, Yijing and Thomas,
	Recently Yijing is trying to kill some weak functions in
drivers/pci/msi.c by using msi_chip mechanism. And it inspired me
that we could go one step further to kill those weak functions
at all and move common PCI MSI code into PCI core by using the
new hierarch irqdomain framework.
	The way to achieve the goal is:
1) arch code creates irqdomain for PCI MSI controllers. By default,
there's only one global PCI MSI irqdomain. But there's may be multiple
for some architecture like x86. So arch also need to implement a weak
function (I know you don't like weak functions):
	struct irq_domain *arch_get_pci_msi_irq_domain(struct pci_dev *);

2) replace
arch_setup_msi_irqs()/arch_setup_msi_irqs()/arch_teardown_msi_irqs()/arch_teardown_msi_irq()
with
irq_domain interfaces.

3) kill arch_msi_mask_irq()/arch_msi_mask_irq() by directly setting
irq_chip.irq_mask()/irq_unmask().

4) we could also find a way to kill arch_restore_msi_irqs() by using
new irqdomain interfaces.

For arch code to support the new MSI, it needs to implement callbacks in
msi_domain_ops and irq_chip. All other PCI MSI code becomes common
and will be moved into pci/msi.c. Proposed interfaces as below:

struct irq_chip {
        void            (*irq_compose_msi_msg)(struct irq_data *data,
struct msi_msg *msg);
};

struct msi_domain_ops {
        void (*calc_hwirq)(struct msi_domain_info *info, void *arg,
                           struct msi_desc *desc);
        irq_hw_number_t (*get_hwirq)(struct msi_domain_info *info, void
*arg);
        int (*msi_init)(struct irq_domain *domain, struct
msi_domain_info *info,
                        unsigned int virq, irq_hw_number_t hwirq, void
*arg);
        void (*msi_free)(struct irq_domain *domain,
                         struct msi_domain_info *info, unsigned int virq);
};

struct msi_domain_info {
        struct msi_domain_ops *ops;
        struct irq_chip *chip;
        void *data;
};

struct irq_domain *msi_create_irq_domain(struct device_node *of_node,
                                         struct msi_domain_info *info,
                                         struct irq_domain *parent);

So, what's your thoughts about the proposal? If now objection, I may
send out a draft version for x86 within about one week:)
Thanks!
Gerry