[RFC PATCH v2 5/5] iommu/arm-smmu-v3: Add Context Descriptor display to debugfs

[Qinxin Xia]

Add Context Descriptor (CD) display functionality to debugfs.
This allow inspecting CD contents for all Substream IDs including:
- CD validity and translation parameters
- TTBR0 and TCR configurations
- Raw CD data

/sys/kernel/debug/iommu/arm_smmu_v3/smmu0/stream_table/
└── <sid>/
    ├─── ste
    ├─── cd
    └─── <dev_name>

Signed-off-by: Qinxin Xia <xiaqinxin at huawei.com>
---
 .../arm/arm-smmu-v3/arm-smmu-v3-debugfs.c     | 94 +++++++++++++++++++
 1 file changed, 94 insertions(+)

diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-debugfs.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-debugfs.c
index dbcc8fce6d8e..501437432809 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-debugfs.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-debugfs.c
@@ -10,6 +10,7 @@
  *     └── stream_table
  *	   └─── <sid>/                                # Stream ID
  *	       ├─── ste                               # Stream Table Entry
+ *	       ├── cd                                 # Context Descriptor
  *	       └── <dev_name>                         # Symlink to device sysfs directory
  *
  * The capabilities file provides detailed information about:
@@ -26,6 +27,12 @@
  * - Stage 1 and Stage 2 context pointers
  * - Raw STE data
  *
+ * CD Information Displayed:
+ * - T0SZ: Input address space size configuration
+ * - EPD0/EPD1: Stage 1 translation enable flags
+ * - TTBR0: Stage 1 translation table base address
+ * - Raw Data: Complete CD structure in hexadecimal format
+
  * Copyright (C) 2026 HiSilicon Limited.
  * Author: Qinxin Xia <xiaqinxin at huawei.com>
  */
@@ -284,6 +291,91 @@ static const struct file_operations smmu_debugfs_ste_fops = {
 	.release = smmu_debugfs_ste_release,
 };
 
+/**
+ * smmu_debug_dump_cd() - Dump a single Context Descriptor
+ * @seq: seq_file to write to
+ * @cd: pointer to the Context Descriptor to dump
+ */
+static void smmu_debug_dump_cd(struct seq_file *seq, struct arm_smmu_cd *cd)
+{
+	u64 data;
+	int i;
+
+	/* CD 0 */
+	data = le64_to_cpu(cd->data[0]);
+	seq_printf(seq, "  T0SZ: 0x%llx\n", data & CTXDESC_CD_0_TCR_T0SZ);
+	seq_printf(seq, "  EPD0: %s\n", data & CTXDESC_CD_0_TCR_EPD0 ? "Yes" : "No");
+	seq_printf(seq, "  EPD1: %s\n", data & CTXDESC_CD_0_TCR_EPD1 ? "Yes" : "No");
+
+	/* CD 1 */
+	data = le64_to_cpu(cd->data[1]);
+	seq_printf(seq, "  TTBR0: 0x%016llx\n", data & CTXDESC_CD_1_TTB0_MASK);
+
+	/* Display raw CD data */
+	seq_puts(seq, "  Raw Data:\n");
+	for (i = 0; i < CTXDESC_CD_DWORDS; i++)
+		seq_printf(seq, "    CD[%d]: 0x%016llx\n", i,
+			   le64_to_cpu(cd->data[i]));
+}
+
+static int smmu_debugfs_cd_show(struct seq_file *seq, void *unused)
+{
+	struct device *dev = seq->private;
+	struct arm_smmu_master *master = dev_iommu_priv_get(dev);
+	u32 max_ssids, ssid;
+
+	if (!master) {
+		seq_puts(seq, "No master data\n");
+		return 0;
+	}
+
+	mutex_lock(&arm_smmu_asid_lock);
+	max_ssids = 1 << master->ssid_bits;
+	seq_printf(seq, "Context Descriptors for device (max SSIDs: %u):\n",
+		   max_ssids);
+
+	for (ssid = 0; ssid < max_ssids; ssid++) {
+		struct arm_smmu_cd *cd = arm_smmu_get_cd_ptr(master, ssid);
+
+		if (cd && (le64_to_cpu(cd->data[0]) & CTXDESC_CD_0_V)) {
+			seq_printf(seq, "\n--- SSID %u ---\n", ssid);
+			smmu_debug_dump_cd(seq, cd);
+		}
+	}
+
+	mutex_unlock(&arm_smmu_asid_lock);
+	return 0;
+}
+
+static int smmu_debugfs_cd_open(struct inode *inode, struct file *file)
+{
+	struct device *dev = inode->i_private;
+
+	if (!dev || !get_device(dev))
+		return -ENODEV;
+
+	return single_open(file, smmu_debugfs_cd_show, dev);
+}
+
+static int smmu_debugfs_cd_release(struct inode *inode, struct file *file)
+{
+	struct seq_file *seq = file->private_data;
+	struct device *dev = seq->private;
+
+	single_release(inode, file);
+	if (dev)
+		put_device(dev);
+	return 0;
+}
+
+static const struct file_operations smmu_debugfs_cd_fops = {
+	.owner   = THIS_MODULE,
+	.open    = smmu_debugfs_cd_open,
+	.read    = seq_read,
+	.llseek  = seq_lseek,
+	.release = smmu_debugfs_cd_release,
+};
+
 /**
  * arm_smmu_debugfs_create_stream_table() - Create debugfs entries for stream table
  * @dev: device to create entries for
@@ -348,6 +440,8 @@ int arm_smmu_debugfs_create_stream_table(struct device *dev,
 		}
 
 		kfree(path);
+		/* Create CD file to dump all valid Context Descriptors */
+		debugfs_create_file("cd", 0444, dev_dir, dev, &smmu_debugfs_cd_fops);
 	}
 
 	return 0;
-- 
2.33.0