powerpc/pseries: Fix MSI-X allocation failure when quota is exceeded

powerpc/pseries: Fix MSI-X allocation failure when quota is exceeded

Nilay reported that since commit daaa574aba6f ("powerpc/pseries/msi: Switch
to msi_create_parent_irq_domain()"), the NVMe driver cannot enable MSI-X
when the device's MSI-X table size is larger than the firmware's MSI quota
for the device.

This is because the commit changes how rtas_prepare_msi_irqs() is called:

  - Before, it is called when interrupts are allocated at the global
    interrupt domain with nvec_in being the number of allocated interrupts.
    rtas_prepare_msi_irqs() can return a positive number and the allocation
    will be retried.

  - Now, it is called at the creation of per-device interrupt domain with
    nvec_in being the number of interrupts that the device supports. If
    rtas_prepare_msi_irqs() returns positive, domain creation just fails.

For Nilay's NVMe driver case, rtas_prepare_msi_irqs() returns a positive
number (the quota). This causes per-device interrupt domain creation to
fail and thus the NVMe driver cannot enable MSI-X.

Rework to make this scenario works again:

  - pseries_msi_ops_prepare() only prepares as many interrupts as the quota
    permit.

  - pseries_irq_domain_alloc() fails if the device's quota is exceeded.

Now, if the quota is exceeded, pseries_msi_ops_prepare() will only prepare
as allowed by the quota. If device drivers attempt to allocate more
interrupts than the quota permits, pseries_irq_domain_alloc() will return
an error code and msi_handle_pci_fail() will allow device drivers a retry.

Reported-by: Nilay Shroff <nilay@linux.ibm.com>
Closes: https://lore.kernel.org/linuxppc-dev/6af2c4c2-97f6-4758-be33-256638ef39e5@linux.ibm.com/
Fixes: daaa574aba6f ("powerpc/pseries/msi: Switch to msi_create_parent_irq_domain()")
Signed-off-by: Nam Cao <namcao@linutronix.de>
Cc: stable@vger.kernel.org
Tested-by: Nilay Shroff <nilay@linux.ibm.com>
Acked-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260107100230.1466093-1-namcao@linutronix.de