iommu/amd: Add debugfs support to dump IOMMU MMIO registers

Analyzing IOMMU MMIO registers gives a view of what IOMMU is
configured with on the system and is helpful to debug issues
with IOMMU.

eg.
-> To get mmio registers value at offset 0x18 for iommu<x> (say, iommu00)
   # echo "0x18" > /sys/kernel/debug/iommu/amd/iommu00/mmio
   # cat /sys/kernel/debug/iommu/amd/iommu00/mmio

Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Link: https://lore.kernel.org/r/20250702093804.849-3-dheerajkumar.srivastava@amd.com
Signed-off-by: Will Deacon <will@kernel.org>

drivers/iommu/amd/amd_iommu_types.h

@@ -795,6 +795,7 @@ struct amd_iommu {
 #ifdef CONFIG_AMD_IOMMU_DEBUGFS
 	/* DebugFS Info */
 	struct dentry *debugfs;
+	int dbg_mmio_offset;
 #endif
 
 	/* IOPF support */

drivers/iommu/amd/debugfs.c

@@ -15,6 +15,48 @@
 static struct dentry *amd_iommu_debugfs;
 
 #define	MAX_NAME_LEN	20
+#define	OFS_IN_SZ	8
+
+static ssize_t iommu_mmio_write(struct file *filp, const char __user *ubuf,
+				size_t cnt, loff_t *ppos)
+{
+	struct seq_file *m = filp->private_data;
+	struct amd_iommu *iommu = m->private;
+	int ret;
+
+	iommu->dbg_mmio_offset = -1;
+
+	if (cnt > OFS_IN_SZ)
+		return -EINVAL;
+
+	ret = kstrtou32_from_user(ubuf, cnt, 0, &iommu->dbg_mmio_offset);
+	if (ret)
+		return ret;
+
+	if (iommu->dbg_mmio_offset > iommu->mmio_phys_end - 4) {
+		iommu->dbg_mmio_offset = -1;
+		return  -EINVAL;
+	}
+
+	return cnt;
+}
+
+static int iommu_mmio_show(struct seq_file *m, void *unused)
+{
+	struct amd_iommu *iommu = m->private;
+	u64 value;
+
+	if (iommu->dbg_mmio_offset < 0) {
+		seq_puts(m, "Please provide mmio register's offset\n");
+		return 0;
+	}
+
+	value = readq(iommu->mmio_base + iommu->dbg_mmio_offset);
+	seq_printf(m, "Offset:0x%x Value:0x%016llx\n", iommu->dbg_mmio_offset, value);
+
+	return 0;
+}
+DEFINE_SHOW_STORE_ATTRIBUTE(iommu_mmio);
 
 void amd_iommu_debugfs_setup(void)
 {
@@ -24,7 +66,12 @@ void amd_iommu_debugfs_setup(void)
 	amd_iommu_debugfs = debugfs_create_dir("amd", iommu_debugfs_dir);
 
 	for_each_iommu(iommu) {
+		iommu->dbg_mmio_offset = -1;
+
 		snprintf(name, MAX_NAME_LEN, "iommu%02d", iommu->index);
 		iommu->debugfs = debugfs_create_dir(name, amd_iommu_debugfs);
+
+		debugfs_create_file("mmio", 0644, iommu->debugfs, iommu,
+				    &iommu_mmio_fops);
 	}
 }