genirq/msi: Expose MSI message data in debugfs

When debugging MSI-related hardware issues (e.g. interrupt delivery
failures), developers currently need to either:

  1. Recompile the kernel with dynamic debug for tracing msi_desc.
  2. Manually read device registers through low-level tools.

Both approaches become challenging in production environments where
dynamic debugging is often disabled.

The interrupt core provides a debugfs interface for inspection of interrupt
related data, which contains the per interrupt information in the view of
the hierarchical interrupt domains. Though this interface does not expose
the MSI address/data pair, which is important information to:

  - Verify whether the MSI configuration matches the hardware expectations
  - Diagnose interrupt routing errors (e.g., mismatched destination ID)
  - Validate remapping behavior in virtualized environments

Implement the debug_show() callback for the generic MSI interrupt domains,
and use it to expose the MSI address/data pair in the per interrupt
diagnostics.

Sample output:
  address_hi: 0x00000000
  address_lo: 0xfe670040
  msg_data:   0x00000001

[ tglx: Massaged change log. Use irq_data_get_msi_desc() to avoid pointless
  	lookup. ]

Signed-off-by: Hans Zhang <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

Author: Hans Zhang

kernel/irq/msi.c

@@ -15,6 +15,7 @@
 #include <linux/mutex.h>
 #include <linux/pci.h>
 #include <linux/slab.h>
+#include <linux/seq_file.h>
 #include <linux/sysfs.h>
 #include <linux/types.h>
 #include <linux/xarray.h>
@@ -756,12 +757,30 @@ static int msi_domain_translate(struct irq_domain *domain, struct irq_fwspec *fw
 	return info->ops->msi_translate(domain, fwspec, hwirq, type);
 }
 
+#ifdef CONFIG_GENERIC_IRQ_DEBUGFS
+static void msi_domain_debug_show(struct seq_file *m, struct irq_domain *d,
+				  struct irq_data *irqd, int ind)
+{
+	struct msi_desc *desc = irq_data_get_msi_desc(irqd);
+
+	if (!desc)
+		return;
+
+	seq_printf(m, "\n%*saddress_hi: 0x%08x", ind + 1, "", desc->msg.address_hi);
+	seq_printf(m, "\n%*saddress_lo: 0x%08x", ind + 1, "", desc->msg.address_lo);
+	seq_printf(m, "\n%*smsg_data:   0x%08x\n", ind + 1, "", desc->msg.data);
+}
+#endif
+
 static const struct irq_domain_ops msi_domain_ops = {
 	.alloc		= msi_domain_alloc,
 	.free		= msi_domain_free,
 	.activate	= msi_domain_activate,
 	.deactivate	= msi_domain_deactivate,
 	.translate	= msi_domain_translate,
+#ifdef CONFIG_GENERIC_IRQ_DEBUGFS
+	.debug_show     = msi_domain_debug_show,
+#endif
 };
 
 static irq_hw_number_t msi_domain_ops_get_hwirq(struct msi_domain_info *info,