irqchip/msi-lib: Honor the MSI_FLAG_PCI_MSI_MASK_PARENT flag

For systems that implement interrupt masking at the interrupt controller
level, the MSI library offers MSI_FLAG_PCI_MSI_MASK_PARENT. It indicates
that it isn't enough to only unmask the interrupt at the PCI device level,
but that the interrupt controller must also be involved.

However, the way this is currently done is less than optimal, as the
masking/unmasking is done on both sides, always. It would be far cheaper to
unmask both at the start of times, and then only deal with the interrupt
controller mask, which is cheaper than a round-trip to the PCI endpoint.

Now that the PCI/MSI layer implements irq_startup() and irq_shutdown()
callbacks, which [un]mask at the PCI level and honor the request to
[un]mask the parent, this can be trivially done.

Overwrite the irq_mask/unmask() callbacks of the device domain interrupt
chip with irq_[un]mask_parent() when the parent domain asks for it.

[ tglx: Adopted to the PCI/MSI changes ]

Signed-off-by: Marc Zyngier <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Tested-by: Marc Zyngier <[e-mail]>
Acked-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]

drivers/irqchip/irq-msi-lib.c

···
 	 */
 	if (!chip->irq_set_affinity && !(info->flags & MSI_FLAG_NO_AFFINITY))
 		chip->irq_set_affinity = msi_domain_set_affinity;
+
+	/*
+	 * If the parent domain insists on being in charge of masking, obey
+	 * blindly. The interrupt is un-masked at the PCI level on startup
+	 * and masked on shutdown to prevent rogue interrupts after the
+	 * driver freed the interrupt. Not masking it at the PCI level
+	 * speeds up operation for disable/enable_irq() as it avoids
+	 * getting all the way out to the PCI device.
+	 */
+	if (info->flags & MSI_FLAG_PCI_MSI_MASK_PARENT) {
+		chip->irq_mask		= irq_chip_mask_parent;
+		chip->irq_unmask	= irq_chip_unmask_parent;
+	}
+
 	return true;
 }
 EXPORT_SYMBOL_GPL(msi_lib_init_dev_msi_info);