[PATCH] PCI/MSI: Clarify the irq sysfs ABI for PCI devices

[Barry Song]

From: Barry Song <song.bao.hua@xxxxxxxxxxxxx>

/sys/bus/pci/devices/.../irq has been there for many years but it has never
been documented. This patch is trying to document it. Plus, irq ABI is very
confusing at this moment especially for MSI and MSI-x cases. MSI sets irq
to the first number in the vector, but MSI-X does nothing for this though
it saves default_irq in msix_setup_entries(). Weird the saved default_irq
for MSI-X is never used in pci_msix_shutdown(), which is quite different
with pci_msi_shutdown(). Thus, this patch also moves to show the first IRQ
number which is from the first msi_entry for MSI-X. Hopefully, this can
make irq ABI more clear and more consistent.

Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
---
 Documentation/ABI/testing/sysfs-bus-pci | 8 ++++++++
 drivers/pci/msi.c                       | 6 ++++++
 2 files changed, 14 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
index 793cbb7..8d42385 100644
--- a/Documentation/ABI/testing/sysfs-bus-pci
+++ b/Documentation/ABI/testing/sysfs-bus-pci
@@ -96,6 +96,14 @@ Description:
 		This attribute indicates the mode that the irq vector named by
 		the file is in (msi vs. msix)
 
+What:		/sys/bus/pci/devices/.../irq
+Date:		August 2021
+Contact:	Barry Song <song.bao.hua@xxxxxxxxxxxxx>
+Description:
+		Historically this attribute represent the IRQ line which runs
+		from the PCI device to the Interrupt controller. With MSI and
+		MSI-X, this attribute is the first IRQ number of IRQ vectors.
+
 What:		/sys/bus/pci/devices/.../remove
 Date:		January 2009
 Contact:	Linux PCI developers <linux-pci@xxxxxxxxxxxxxxx>
diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
index 9232255..6bbf81b 100644
--- a/drivers/pci/msi.c
+++ b/drivers/pci/msi.c
@@ -771,6 +771,7 @@ static int msix_capability_init(struct pci_dev *dev, struct msix_entry *entries,
 	int ret;
 	u16 control;
 	void __iomem *base;
+	struct msi_desc *desc;
 
 	/* Ensure MSI-X is disabled while it is set up */
 	pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_ENABLE, 0);
@@ -814,6 +815,10 @@ static int msix_capability_init(struct pci_dev *dev, struct msix_entry *entries,
 	pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_MASKALL, 0);
 
 	pcibios_free_irq(dev);
+
+	desc = first_pci_msi_entry(dev);
+	dev->irq = desc->irq;
+
 	return 0;
 
 out_avail:
@@ -1024,6 +1029,7 @@ static void pci_msix_shutdown(struct pci_dev *dev)
 	pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_ENABLE, 0);
 	pci_intx_for_msi(dev, 1);
 	dev->msix_enabled = 0;
+	dev->irq = entry->msi_attrib.default_irq;
 	pcibios_alloc_irq(dev);
 }
 
-- 
1.8.3.1