[PATCH v3 3/4] drm/xe/sysctrl: Add loopback test debugfs interface

From: Anoop Vijay <[email protected]>

Add debugfs interface for testing sysctrl mailbox loopback
functionality. This allows userspace to send test payloads
and verify firmware communication.

Command details:
- Group ID: 0xFF (core group)
- Command ID: 0x03 (Inverted loopback)
- Usage: echo "0x11 0x22 0x33 0x44" > /sys/kernel/debug/dri/0/sc/loopback
         cat /sys/kernel/debug/dri/0/sc/loopback

Signed-off-by: Anoop Vijay <[email protected]>
---
 drivers/gpu/drm/xe/xe_sysctrl_debugfs.c       | 130 ++++++++++++++++++
 drivers/gpu/drm/xe/xe_sysctrl_mailbox_types.h |   2 +
 drivers/gpu/drm/xe/xe_sysctrl_types.h         |  28 ++++
 3 files changed, 160 insertions(+)

diff --git a/drivers/gpu/drm/xe/xe_sysctrl_debugfs.c b/drivers/gpu/drm/xe/xe_sysctrl_debugfs.c
index 0238ff093831..2144d9d43fba 100644
--- a/drivers/gpu/drm/xe/xe_sysctrl_debugfs.c
+++ b/drivers/gpu/drm/xe/xe_sysctrl_debugfs.c
@@ -3,13 +3,139 @@
  * Copyright © 2026 Intel Corporation
  */
 
+#include <linux/cleanup.h>
 #include <linux/debugfs.h>
+#include <linux/err.h>
+#include <linux/kstrtox.h>
+#include <linux/seq_file.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <linux/uaccess.h>
 
 #include "xe_device.h"
+#include "xe_pm.h"
+#include "xe_printk.h"
 #include "xe_sysctrl.h"
 #include "xe_sysctrl_debugfs.h"
+#include "xe_sysctrl_mailbox.h"
+#include "xe_sysctrl_mailbox_types.h"
 #include "xe_sysctrl_types.h"
 
+static ssize_t xe_sysctrl_loopback_write(struct file *file, const char __user *ubuf,
+					 size_t len, loff_t *offp)
+{
+	char *kbuf __free(kfree) = NULL;
+	u8 *input __free(kfree) = NULL;
+	struct seq_file *m = file->private_data;
+	struct xe_sysctrl_debugfs_entry *entry = m->private;
+	struct xe_device *xe = sc_to_xe(entry->sc);
+	struct xe_sysctrl_mailbox_command cmd = {};
+	char *token, *tmp;
+	unsigned long val;
+	size_t input_len = 0;
+	size_t max_input;
+	size_t out_len = 0;
+
+	if (len == 0 || len >= PAGE_SIZE)
+		return -EINVAL;
+
+	kbuf = kmalloc(len + 1, GFP_KERNEL);
+	if (!kbuf)
+		return -ENOMEM;
+
+	max_input = min_t(size_t, len, XE_SYSCTRL_MB_MAX_MESSAGE_SIZE);
+	input = kmalloc(max_input, GFP_KERNEL);
+	if (!input)
+		return -ENOMEM;
+
+	if (copy_from_user(kbuf, ubuf, len))
+		return -EFAULT;
+	kbuf[len] = '\0';
+
+	tmp = kbuf;
+	while ((token = strsep(&tmp, " \t\n")) != NULL) {
+		if (*token == '\0')
+			continue;
+
+		if (input_len >= max_input) {
+			xe_err(xe, "sysctrl: loopback payload too large (max %d bytes)\n",
+			       XE_SYSCTRL_MB_MAX_MESSAGE_SIZE);
+			return -EINVAL;
+		}
+
+		if (kstrtoul(token, 0, &val) || val > 0xFF) {
+			xe_err(xe, "sysctrl: invalid loopback token '%s'\n", token);
+			return -EINVAL;
+		}
+
+		input[input_len++] = (u8)val;
+	}
+
+	if (input_len == 0) {
+		xe_err(xe, "sysctrl: no loopback payload given\n");
+		return -EINVAL;
+	}
+
+	xe_sysctrl_create_command(&cmd, entry->group, entry->command,
+				  input, input_len, entry->response_buf, input_len);
+
+	guard(xe_pm_runtime)(xe);
+	entry->status = xe_sysctrl_send_command(entry->sc, &cmd, &out_len);
+	entry->response_len = entry->status ? 0 : out_len;
+
+	return entry->status ? entry->status : len;
+}
+
+static int xe_sysctrl_loopback_show(struct seq_file *m, void *data)
+{
+	struct xe_sysctrl_debugfs_entry *entry = m->private;
+	size_t i;
+
+	seq_printf(m, "Command: group=0x%02x cmd=0x%02x\n", entry->group, entry->command);
+	seq_printf(m, "Status: %d (%s)\n", entry->status, entry->status ? "FAILED" : "SUCCESS");
+	seq_printf(m, "Response: %zu bytes\n", entry->response_len);
+
+	if (entry->response_len) {
+		seq_puts(m, "Response data:\n");
+		for (i = 0; i < entry->response_len; i++) {
+			if (i && (i % 16) == 0)
+				seq_putc(m, '\n');
+			seq_printf(m, "%02x ", entry->response_buf[i]);
+		}
+		seq_putc(m, '\n');
+	}
+
+	return 0;
+}
+
+static int xe_sysctrl_loopback_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, xe_sysctrl_loopback_show, inode->i_private);
+}
+
+static const struct file_operations xe_sysctrl_loopback_fops = {
+	.owner = THIS_MODULE,
+	.open = xe_sysctrl_loopback_open,
+	.read = seq_read,
+	.write = xe_sysctrl_loopback_write,
+	.llseek = seq_lseek,
+	.release = single_release,
+};
+
+static void xe_sysctrl_register_entry(struct dentry *root, struct xe_sysctrl_debugfs_entry *entry,
+				      struct xe_sysctrl *sc, const char *name,
+				      u8 group, u8 command,
+				      const struct file_operations *fops)
+{
+	entry->sc = sc;
+	entry->group = group;
+	entry->command = command;
+	entry->response_len = 0;
+	entry->status = 0;
+
+	debugfs_create_file(name, 0644, root, entry, fops);
+}
+
 /**
  * xe_sysctrl_debugfs_register - Register debugfs entries for System Controller
  * @sc: xe_sysctrl instance
@@ -24,4 +150,8 @@ void xe_sysctrl_debugfs_register(struct xe_sysctrl *sc, struct dentry *parent)
 		return;
 
 	sc->debugfs.root = root;
+
+	xe_sysctrl_register_entry(root, &sc->debugfs.loopback, sc, "loopback",
+				  XE_SYSCTRL_GROUP_CORE, XE_SYSCTRL_CMD_LOOPBACK,
+				  &xe_sysctrl_loopback_fops);
 }
diff --git a/drivers/gpu/drm/xe/xe_sysctrl_mailbox_types.h b/drivers/gpu/drm/xe/xe_sysctrl_mailbox_types.h
index b507e1553cbb..ed80fe63e1c4 100644
--- a/drivers/gpu/drm/xe/xe_sysctrl_mailbox_types.h
+++ b/drivers/gpu/drm/xe/xe_sysctrl_mailbox_types.h
@@ -43,9 +43,11 @@ enum xe_sysctrl_gfsp_cmd {
 /**
  * enum xe_sysctrl_core_cmd - Commands supported by Core group
  *
+ * @XE_SYSCTRL_CMD_LOOPBACK: Loopback test command
  * @XE_SYSCTRL_CMD_GET_APP_STATUS_BY_ID: Retrieve application status by ID
  */
 enum xe_sysctrl_core_cmd {
+	XE_SYSCTRL_CMD_LOOPBACK				= 0x03,
 	XE_SYSCTRL_CMD_GET_APP_STATUS_BY_ID		= 0x05,
 };
 
diff --git a/drivers/gpu/drm/xe/xe_sysctrl_types.h b/drivers/gpu/drm/xe/xe_sysctrl_types.h
index d50a35fefb7a..53d82e61383a 100644
--- a/drivers/gpu/drm/xe/xe_sysctrl_types.h
+++ b/drivers/gpu/drm/xe/xe_sysctrl_types.h
@@ -10,9 +10,34 @@
 #include <linux/types.h>
 #include <linux/workqueue_types.h>
 
+#include "xe_sysctrl_mailbox_types.h"
+
 struct xe_mmio;
 struct dentry;
 
+/**
+ * struct xe_sysctrl_debugfs_entry - Debugfs entry for a raw mailbox test command
+ */
+struct xe_sysctrl_debugfs_entry {
+	/** @sc: Back pointer to parent sysctrl instance */
+	struct xe_sysctrl *sc;
+
+	/** @group: Command group ID */
+	u8 group;
+
+	/** @command: Command ID within group */
+	u8 command;
+
+	/** @response_buf: Response data buffer, sized to the maximum mailbox message */
+	u8 response_buf[XE_SYSCTRL_MB_MAX_MESSAGE_SIZE];
+
+	/** @response_len: Actual response length from firmware */
+	size_t response_len;
+
+	/** @status: Last command result */
+	int status;
+};
+
 /**
  * struct xe_sysctrl - System Controller driver context
  *
@@ -40,6 +65,9 @@ struct xe_sysctrl {
 	struct {
 		/** @debugfs.root: Root debugfs directory */
 		struct dentry *root;
+
+		/** @debugfs.loopback: Loopback test entry */
+		struct xe_sysctrl_debugfs_entry loopback;
 	} debugfs;
 };
 
-- 
2.43.0