[PATCH v11 07/16] genirq/msi: Cache MSI message in irq_chip_write_msi_msg()

[Pranjal Shrivastava <praan@google.com>]

Currently, PCI MSI caches the composed MSI message into the MSI
descriptor inside __pci_write_msi_msg(), whereas platform MSI and other
device MSI domains do not cache the message in the core, forcing
individual drivers to manually copy the message into desc->msg if they
need to restore it across power transitions.

Cache desc->msg in irq_chip_write_msi_msg() before invoking the chip
callback so all hierarchical MSI domains automatically record the
latest message. Document why the redundant assignment in
__pci_write_msi_msg() must remain for direct callers outside the
irq_chip hierarchy.

Suggested-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Pranjal Shrivastava <praan@google.com>
---
 drivers/pci/msi/msi.c | 6 ++++++
 kernel/irq/msi.c      | 9 +++++++++
 2 files changed, 15 insertions(+)

diff --git a/drivers/pci/msi/msi.c b/drivers/pci/msi/msi.c
index 80a9db417dc8..f7bedc31d486 100644
--- a/drivers/pci/msi/msi.c
+++ b/drivers/pci/msi/msi.c
@@ -257,6 +257,12 @@ void __pci_write_msi_msg(struct msi_desc *entry, struct msi_msg *msg)
 		pci_write_msg_msi(dev, entry, msg);
 	}
 
+	/*
+	 * Although irq_chip_write_msi_msg() already caches the message,
+	 * keep the assignment here for callers that invoke
+	 * __pci_write_msi_msg() or pci_write_msi_msg() directly outside
+	 * the irq_chip hierarchy.
+	 */
 	entry->msg = *msg;
 
 	if (entry->write_msi_msg)
diff --git a/kernel/irq/msi.c b/kernel/irq/msi.c
index fb5f372215bf..df460a574d80 100644
--- a/kernel/irq/msi.c
+++ b/kernel/irq/msi.c
@@ -634,6 +634,15 @@ static unsigned int msi_domain_get_hwsize(struct device *dev, unsigned int domid
 static inline void irq_chip_write_msi_msg(struct irq_data *data,
 					  struct msi_msg *msg)
 {
+	struct msi_desc *desc = irq_data_get_msi_desc(data);
+
+	/*
+	 * PCI Multi-MSI shares a single descriptor across all vectors and
+	 * the device stores only the base message. Cache the message of the
+	 * first interrupt only, so that sub-vectors cannot overwrite it.
+	 */
+	if (desc->irq == data->irq)
+		desc->msg = *msg;
 	data->chip->irq_write_msi_msg(data, msg);
 }
 
-- 
2.56.0.rc1.315.gc6ed9934b7-goog