platform-msi: Add ABI to show msi_irqs of platform devices

PCI devices expose the associated MSI interrupts via sysfs, but platform
devices which utilize MSI interrupts do not. This information is important
for user space tools to optimize affinity settings.

Utilize the generic MSI sysfs facility to expose this information for
platform MSI.

Signed-off-by: Barry Song <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Acked-by: Bjorn Helgaas <[e-mail]>
Acked-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/ABI/testing/sysfs-bus-platform

···
 		value comes from an ACPI _PXM method or a similar firmware
 		source. Initial users for this file would be devices like
 		arm smmu which are populated by arm64 acpi_iort.
+
+What:		/sys/bus/platform/devices/.../msi_irqs/
+Date:		August 2021
+Contact:	Barry Song <[e-mail]>
+Description:
+		The /sys/devices/.../msi_irqs directory contains a variable set
+		of files, with each file being named after a corresponding msi
+		irq vector allocated to that device.
+
+What:		/sys/bus/platform/devices/.../msi_irqs/<N>
+Date:		August 2021
+Contact:	Barry Song <[e-mail]>
+Description:
+		This attribute will show "msi" if <N> is a valid msi irq

drivers/base/platform-msi.c

···
  * and the callback to write the MSI message.
  */
 struct platform_msi_priv_data {
-	struct device		*dev;
-	void 			*host_data;
-	msi_alloc_info_t	arg;
-	irq_write_msi_msg_t	write_msg;
-	int			devid;
+	struct device			*dev;
+	void				*host_data;
+	const struct attribute_group    **msi_irq_groups;
+	msi_alloc_info_t		arg;
+	irq_write_msi_msg_t		write_msg;
+	int				devid;
 };
 
 /* The devid allocator */
···
 	if (err)
 		goto out_free_desc;
 
+	priv_data->msi_irq_groups = msi_populate_sysfs(dev);
+	if (IS_ERR(priv_data->msi_irq_groups)) {
+		err = PTR_ERR(priv_data->msi_irq_groups);
+		goto out_free_irqs;
+	}
+
 	return 0;
 
+out_free_irqs:
+	msi_domain_free_irqs(dev->msi_domain, dev);
 out_free_desc:
 	platform_msi_free_descs(dev, 0, nvec);
 out_free_priv_data:
···
 		struct msi_desc *desc;
 
 		desc = first_msi_entry(dev);
+		msi_destroy_sysfs(dev, desc->platform.msi_priv_data->msi_irq_groups);
 		platform_msi_free_priv_data(desc->platform.msi_priv_data);
 	}