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.
1. To get mmio registers value for iommu<x>
   # echo "0x18" > /sys/kernel/debug/iommu/amd/iommu00/mmio
   # cat /sys/kernel/debug/iommu/amd/iommu00/mmio_dump

Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
---
 drivers/iommu/amd/debugfs.c | 75 +++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)

diff --git a/drivers/iommu/amd/debugfs.c b/drivers/iommu/amd/debugfs.c
index ff9520e002be..30d6819e9a35 100644
--- a/drivers/iommu/amd/debugfs.c
+++ b/drivers/iommu/amd/debugfs.c
@@ -15,6 +15,76 @@
 static struct dentry *amd_iommu_debugfs;
 
 #define	MAX_NAME_LEN	20
+#define	OFS_IN_SZ	8
+
+static int mmio_offset = -1;
+
+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;
+	char *mmio_ptr_ofs;
+	int ret = cnt;
+
+	if (cnt > OFS_IN_SZ) {
+		ret = -EINVAL;
+		goto err;
+	}
+
+	mmio_ptr_ofs = memdup_user_nul(ubuf, cnt);
+	if (IS_ERR(mmio_ptr_ofs)) {
+		ret = PTR_ERR(mmio_ptr_ofs);
+		goto err;
+	}
+
+	if (kstrtou32(mmio_ptr_ofs, 0, &mmio_offset) < 0) {
+		ret = -EINVAL;
+		goto free;
+	}
+
+	if (mmio_offset > iommu->mmio_phys_end - 4) {
+		ret = -EINVAL;
+		goto free;
+	}
+
+	kfree(mmio_ptr_ofs);
+	*ppos += cnt;
+	return ret;
+
+free:
+	kfree(mmio_ptr_ofs);
+err:
+	mmio_offset = -1;
+	return ret;
+}
+
+static int iommu_mmio_show(struct seq_file *m, void *unused)
+{
+	if (mmio_offset >= 0)
+		seq_printf(m, "0x%x\n", mmio_offset);
+	else
+		seq_puts(m, "No or invalid input provided\n");
+	return 0;
+}
+DEFINE_SHOW_STORE_ATTRIBUTE(iommu_mmio);
+
+static int iommu_mmio_dump_show(struct seq_file *m, void *unused)
+{
+	struct amd_iommu *iommu = m->private;
+	u32 value;
+
+	if (mmio_offset < 0) {
+		seq_puts(m, "Please provide mmio register's offset\n");
+		return 0;
+	}
+
+	value = readl(iommu->mmio_base + mmio_offset);
+	seq_printf(m, "0x%08x\n", value);
+
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(iommu_mmio_dump);
 
 void amd_iommu_debugfs_setup(void)
 {
@@ -26,5 +96,10 @@ void amd_iommu_debugfs_setup(void)
 	for_each_iommu(iommu) {
 		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);
+		debugfs_create_file("mmio_dump", 0444, iommu->debugfs, iommu,
+				    &iommu_mmio_dump_fops);
 	}
 }
-- 
2.25.1