[PATCH v4 1/3] bus: mhi: host: clients: Add loopback driver with sysfs interface

The MHI specification defines a LOOPBACK channel. The endpoint firmware
echoes back whatever the host sends on this channel. Without a host-side
driver, there is no way to exercise this channel to validate MHI data path
integrity between host and endpoint.

Add a host-side loopback driver that binds to the LOOPBACK channel and
expose a sysfs interface for data path testing. The sysfs interface allows
users to configure TRE buffer size and count, trigger a loopback test, and
read the result.

Co-developed-by: Krishna Chaitanya Chundru <[email protected]>
Signed-off-by: Krishna Chaitanya Chundru <[email protected]>
Signed-off-by: Sumit Kumar <[email protected]>
---
 .../ABI/testing/sysfs-bus-mhi-devices-loopback     |  51 ++++
 MAINTAINERS                                        |   1 +
 drivers/bus/mhi/host/Kconfig                       |   1 +
 drivers/bus/mhi/host/Makefile                      |   1 +
 drivers/bus/mhi/host/clients/Kconfig               |  17 ++
 drivers/bus/mhi/host/clients/Makefile              |   2 +
 drivers/bus/mhi/host/clients/loopback.c            | 329 +++++++++++++++++++++
 7 files changed, 402 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-mhi-devices-loopback b/Documentation/ABI/testing/sysfs-bus-mhi-devices-loopback
new file mode 100644
index 0000000000000000000000000000000000000000..3bd770747799a3341a23903cc1a108e650e915b8
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-mhi-devices-loopback
@@ -0,0 +1,51 @@
+What:		/sys/bus/mhi/devices/mhi<N>_LOOPBACK/tre_size
+Date:		April 2026
+KernelVersion:	7.1
+Contact:	[email protected]
+Description:
+		(RW) Size of each Transfer Ring Element (TRE) buffer in bytes
+		used for the loopback test. Valid range is 1 to the value
+		reported by max_tre_size. Default value is 32 bytes.
+
+What:		/sys/bus/mhi/devices/mhi<N>_LOOPBACK/max_tre_size
+Date:		April 2026
+KernelVersion:	7.1
+Contact:	[email protected]
+Description:
+		(RO) Maximum allowed TRE size in bytes. Reading this file
+		returns the upper bound for the tre_size attribute.
+
+What:		/sys/bus/mhi/devices/mhi<N>_LOOPBACK/num_tre
+Date:		April 2026
+KernelVersion:	7.1
+Contact:	[email protected]
+Description:
+		(RW) Number of Transfer Ring Elements (TREs) to use per
+		loopback test. Must be greater than zero and must not exceed
+		the channel ring capacity. Default value is 1.
+
+What:		/sys/bus/mhi/devices/mhi<N>_LOOPBACK/start
+Date:		April 2026
+KernelVersion:	7.1
+Contact:	[email protected]
+Description:
+		(WO) Write any value to trigger a loopback test. The driver
+		sends random data to the endpoint using the configured tre_size
+		and num_tre parameters, waits for the endpoint to echo it back,
+		and verifies the received data matches what was sent.
+
+		This is a blocking write that returns when the test completes
+		or times out after 5 seconds.
+
+What:		/sys/bus/mhi/devices/mhi<N>_LOOPBACK/status
+Date:		April 2026
+KernelVersion:	7.1
+Contact:	[email protected]
+Description:
+		(RO) Result of the last loopback test. Returns one of:
+		  "pass"        - last test completed successfully
+		  "fail"        - last test failed
+		  "not started" - no test has been run yet
+
+		Reading this file while a test is in progress will block
+		until the test completes.
diff --git a/MAINTAINERS b/MAINTAINERS
index 6dcfbd11efef87927041f5cf58d70633dbb4b18d..ff12a6da48947ac853bc638359a7046fea85fc21 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -16441,6 +16441,7 @@ L:	[email protected]
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/mani/mhi.git
 F:	Documentation/ABI/stable/sysfs-bus-mhi
+F:	Documentation/ABI/testing/sysfs-bus-mhi-devices-loopback
 F:	Documentation/mhi/
 F:	drivers/bus/mhi/
 F:	drivers/pci/endpoint/functions/pci-epf-mhi.c
diff --git a/drivers/bus/mhi/host/Kconfig b/drivers/bus/mhi/host/Kconfig
index da5cd0c9fc620ab595e742c422f1a22a2a84c7b9..627c57948235aa52348179ae8b2d0826ebaed01e 100644
--- a/drivers/bus/mhi/host/Kconfig
+++ b/drivers/bus/mhi/host/Kconfig
@@ -29,3 +29,4 @@ config MHI_BUS_PCI_GENERIC
 	  This driver provides MHI PCI controller driver for devices such as
 	  Qualcomm SDX55 based PCIe modems.
 
+source "drivers/bus/mhi/host/clients/Kconfig"
diff --git a/drivers/bus/mhi/host/Makefile b/drivers/bus/mhi/host/Makefile
index 859c2f38451c669b3d3014c374b2b957c99a1cfe..2a16008aeb38127494782bbff4e1656428d2b776 100644
--- a/drivers/bus/mhi/host/Makefile
+++ b/drivers/bus/mhi/host/Makefile
@@ -4,3 +4,4 @@ mhi-$(CONFIG_MHI_BUS_DEBUG) += debugfs.o
 
 obj-$(CONFIG_MHI_BUS_PCI_GENERIC) += mhi_pci_generic.o
 mhi_pci_generic-y += pci_generic.o
+obj-y += clients/
diff --git a/drivers/bus/mhi/host/clients/Kconfig b/drivers/bus/mhi/host/clients/Kconfig
new file mode 100644
index 0000000000000000000000000000000000000000..d1463c3e0df0da461c815afaec623ba349b51dda
--- /dev/null
+++ b/drivers/bus/mhi/host/clients/Kconfig
@@ -0,0 +1,17 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config MHI_BUS_LOOPBACK
+	tristate "MHI LOOPBACK client driver"
+	depends on MHI_BUS
+	help
+	  MHI LOOPBACK client driver that binds to the MHI LOOPBACK channel
+	  as defined in the MHI specification. The LOOPBACK channel is
+	  implemented by MHI-based devices (modems, WLAN) in the field, where
+	  the endpoint firmware echoes back whatever the host sends.
+
+	  This driver exposes a sysfs interface for testing MHI data path
+	  integrity between host and endpoint. Users can configure the TRE
+	  size and count, trigger a loopback test, and read the result.
+
+	  To compile this driver as a module, choose M here. The module
+	  will be called mhi_loopback.
diff --git a/drivers/bus/mhi/host/clients/Makefile b/drivers/bus/mhi/host/clients/Makefile
new file mode 100644
index 0000000000000000000000000000000000000000..3811b6928f42b38f94b1167941cf3b0fe512d32b
--- /dev/null
+++ b/drivers/bus/mhi/host/clients/Makefile
@@ -0,0 +1,2 @@
+obj-$(CONFIG_MHI_BUS_LOOPBACK) += mhi_loopback.o
+mhi_loopback-y += loopback.o
diff --git a/drivers/bus/mhi/host/clients/loopback.c b/drivers/bus/mhi/host/clients/loopback.c
new file mode 100644
index 0000000000000000000000000000000000000000..693691fff26dc8fa0d58931b98ce5f287fbd5c3e
--- /dev/null
+++ b/drivers/bus/mhi/host/clients/loopback.c
@@ -0,0 +1,329 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
+ */
+
+/*
+ * The MHI LOOPBACK channel is defined in the MHI specification and is
+ * implemented by MHI-based devices (modems, WLAN) already deployed in the
+ * field. The endpoint firmware echoes back whatever the host sends on this
+ * channel. This driver binds to the LOOPBACK channel and exposes a sysfs
+ * interface for testing MHI data path integrity between host and endpoint.
+ * The sysfs interface is stable ABI because the wire protocol is fixed by
+ * the endpoint firmware and cannot be changed.
+ */
+
+#include <linux/atomic.h>
+#include <linux/cleanup.h>
+#include <linux/completion.h>
+#include <linux/errno.h>
+#include <linux/mhi.h>
+#include <linux/mod_devicetable.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/random.h>
+#include <linux/sizes.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <linux/sysfs.h>
+#include <linux/types.h>
+
+#define MHI_LOOPBACK_DEFAULT_TRE_SIZE	32
+#define MHI_LOOPBACK_DEFAULT_NUM_TRE	1
+#define MHI_LOOPBACK_TIMEOUT_MS		5000
+#define MHI_LOOPBACK_MAX_TRE_SIZE	(SZ_64K - 1)
+
+struct mhi_loopback {
+	struct mhi_device *mdev;
+	struct mutex lb_mutex;
+	struct completion comp;
+	atomic_t tres_pending;
+	const char *result;
+	u32 num_tre;
+	u32 tre_size;
+};
+
+static ssize_t tre_size_show(struct device *dev,
+			     struct device_attribute *attr, char *buf)
+{
+	struct mhi_loopback *loopback = dev_get_drvdata(dev);
+
+	if (!loopback)
+		return -ENODEV;
+
+	return sysfs_emit(buf, "%u\n", loopback->tre_size);
+}
+
+static ssize_t tre_size_store(struct device *dev,
+			      struct device_attribute *attr,
+			      const char *buf, size_t count)
+{
+	struct mhi_loopback *loopback = dev_get_drvdata(dev);
+	u32 val;
+
+	if (!loopback)
+		return -ENODEV;
+
+	if (kstrtou32(buf, 0, &val))
+		return -EINVAL;
+
+	if (val == 0 || val > MHI_LOOPBACK_MAX_TRE_SIZE)
+		return -EINVAL;
+
+	guard(mutex)(&loopback->lb_mutex);
+	loopback->tre_size = val;
+
+	return count;
+}
+static DEVICE_ATTR_RW(tre_size);
+
+static ssize_t max_tre_size_show(struct device *dev,
+				 struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%u\n", MHI_LOOPBACK_MAX_TRE_SIZE);
+}
+static DEVICE_ATTR_RO(max_tre_size);
+
+static ssize_t num_tre_show(struct device *dev,
+			    struct device_attribute *attr, char *buf)
+{
+	struct mhi_loopback *loopback = dev_get_drvdata(dev);
+
+	if (!loopback)
+		return -ENODEV;
+
+	return sysfs_emit(buf, "%u\n", loopback->num_tre);
+}
+
+static ssize_t num_tre_store(struct device *dev,
+			     struct device_attribute *attr,
+			     const char *buf, size_t count)
+{
+	struct mhi_loopback *loopback = dev_get_drvdata(dev);
+	u32 val;
+	int el_num;
+
+	if (!loopback)
+		return -ENODEV;
+
+	if (kstrtou32(buf, 0, &val))
+		return -EINVAL;
+
+	if (val == 0)
+		return -EINVAL;
+
+	guard(mutex)(&loopback->lb_mutex);
+
+	el_num = mhi_get_free_desc_count(loopback->mdev, DMA_TO_DEVICE);
+	if (val > el_num) {
+		dev_err(dev, "num_tre (%u) exceeds ring capacity (%d)\n", val, el_num);
+		return -EINVAL;
+	}
+
+	loopback->num_tre = val;
+
+	return count;
+}
+static DEVICE_ATTR_RW(num_tre);
+
+static ssize_t start_store(struct device *dev,
+			   struct device_attribute *attr,
+			   const char *buf, size_t count)
+{
+	struct mhi_loopback *loopback = dev_get_drvdata(dev);
+	u32 total_size, tre_count, tre_size;
+	int i;
+
+	if (!loopback)
+		return -ENODEV;
+
+	guard(mutex)(&loopback->lb_mutex);
+
+	tre_size = loopback->tre_size;
+	tre_count = loopback->num_tre;
+	total_size = size_mul(tre_count, tre_size);
+
+	if (total_size > KMALLOC_MAX_SIZE)
+		return -EINVAL;
+
+	void *recv_buf __free(kfree) = kzalloc(total_size, GFP_KERNEL);
+	if (!recv_buf)
+		return -ENOMEM;
+
+	void *send_buf __free(kfree) = kzalloc(total_size, GFP_KERNEL);
+	if (!send_buf)
+		return -ENOMEM;
+
+	get_random_bytes(send_buf, total_size);
+
+	atomic_set(&loopback->tres_pending, tre_count);
+	reinit_completion(&loopback->comp);
+
+	for (i = 0; i < tre_count; i++) {
+		int ret = mhi_queue_buf(loopback->mdev, DMA_FROM_DEVICE,
+					recv_buf + (i * tre_size), tre_size, MHI_EOT);
+		if (ret) {
+			dev_err(dev, "Unable to queue read TRE %d: %d\n", i, ret);
+			loopback->result = "fail";
+			if (atomic_sub_and_test(tre_count - i, &loopback->tres_pending))
+				complete(&loopback->comp);
+			return ret;
+		}
+	}
+
+	for (i = 0; i < tre_count - 1; i++) {
+		int ret = mhi_queue_buf(loopback->mdev, DMA_TO_DEVICE,
+					send_buf + (i * tre_size), tre_size, MHI_CHAIN);
+		if (ret) {
+			dev_err(dev, "Unable to queue send TRE %d: %d\n", i, ret);
+			loopback->result = "fail";
+			return ret;
+		}
+	}
+
+	int ret = mhi_queue_buf(loopback->mdev, DMA_TO_DEVICE,
+				send_buf + (i * tre_size), tre_size, MHI_EOT);
+	if (ret) {
+		dev_err(dev, "Unable to queue final TRE: %d\n", ret);
+		loopback->result = "fail";
+		return ret;
+	}
+
+	if (!wait_for_completion_timeout(&loopback->comp,
+					 msecs_to_jiffies(MHI_LOOPBACK_TIMEOUT_MS))) {
+		dev_err(dev, "Loopback test timed out\n");
+		loopback->result = "fail";
+		return -ETIMEDOUT;
+	}
+
+	if (memcmp(send_buf, recv_buf, total_size)) {
+		dev_err(dev, "Loopback data mismatch\n");
+		loopback->result = "fail";
+		return -EIO;
+	}
+
+	loopback->result = "pass";
+
+	return count;
+}
+static DEVICE_ATTR_WO(start);
+
+static ssize_t status_show(struct device *dev,
+			   struct device_attribute *attr, char *buf)
+{
+	struct mhi_loopback *loopback = dev_get_drvdata(dev);
+
+	if (!loopback)
+		return -ENODEV;
+
+	guard(mutex)(&loopback->lb_mutex);
+
+	return sysfs_emit(buf, "%s\n", loopback->result);
+}
+static DEVICE_ATTR_RO(status);
+
+static void mhi_loopback_dl_callback(struct mhi_device *mhi_dev,
+				     struct mhi_result *mhi_res)
+{
+	struct mhi_loopback *loopback = dev_get_drvdata(&mhi_dev->dev);
+
+	if (!loopback)
+		return;
+
+	if (mhi_res->transaction_status && mhi_res->transaction_status != -ENOTCONN)
+		dev_err(&mhi_dev->dev, "DL callback error: status %d\n",
+			mhi_res->transaction_status);
+
+	if (atomic_dec_and_test(&loopback->tres_pending))
+		complete(&loopback->comp);
+}
+
+static void mhi_loopback_ul_callback(struct mhi_device *mhi_dev,
+				     struct mhi_result *mhi_res)
+{
+}
+
+static struct attribute *mhi_loopback_attrs[] = {
+	&dev_attr_tre_size.attr,
+	&dev_attr_max_tre_size.attr,
+	&dev_attr_num_tre.attr,
+	&dev_attr_start.attr,
+	&dev_attr_status.attr,
+	NULL,
+};
+
+static const struct attribute_group mhi_loopback_group = {
+	.attrs = mhi_loopback_attrs,
+};
+
+static int mhi_loopback_probe(struct mhi_device *mhi_dev,
+			      const struct mhi_device_id *id)
+{
+	struct mhi_loopback *loopback;
+	int rc;
+
+	loopback = devm_kzalloc(&mhi_dev->dev, sizeof(*loopback), GFP_KERNEL);
+	if (!loopback)
+		return -ENOMEM;
+
+	loopback->mdev = mhi_dev;
+	loopback->tre_size = MHI_LOOPBACK_DEFAULT_TRE_SIZE;
+	loopback->num_tre = MHI_LOOPBACK_DEFAULT_NUM_TRE;
+	loopback->result = "not started";
+
+	mutex_init(&loopback->lb_mutex);
+	init_completion(&loopback->comp);
+
+	dev_set_drvdata(&mhi_dev->dev, loopback);
+
+	rc = mhi_prepare_for_transfer(mhi_dev);
+	if (rc) {
+		dev_err(&mhi_dev->dev, "failed to prepare for transfers\n");
+		return rc;
+	}
+
+	rc = devm_device_add_group(&mhi_dev->dev, &mhi_loopback_group);
+	if (rc) {
+		dev_err(&mhi_dev->dev, "failed to create sysfs attributes\n");
+		mhi_unprepare_from_transfer(mhi_dev);
+	}
+
+	return rc;
+}
+
+static void mhi_loopback_remove(struct mhi_device *mhi_dev)
+{
+	struct mhi_loopback *loopback = dev_get_drvdata(&mhi_dev->dev);
+
+	complete(&loopback->comp);
+
+	mutex_lock(&loopback->lb_mutex);
+	mutex_unlock(&loopback->lb_mutex);
+
+	mhi_unprepare_from_transfer(mhi_dev);
+	dev_set_drvdata(&mhi_dev->dev, NULL);
+}
+
+static const struct mhi_device_id mhi_loopback_id_table[] = {
+	{ .chan = "LOOPBACK"},
+	{}
+};
+MODULE_DEVICE_TABLE(mhi, mhi_loopback_id_table);
+
+static struct mhi_driver mhi_loopback_driver = {
+	.probe = mhi_loopback_probe,
+	.remove = mhi_loopback_remove,
+	.dl_xfer_cb = mhi_loopback_dl_callback,
+	.ul_xfer_cb = mhi_loopback_ul_callback,
+	.id_table = mhi_loopback_id_table,
+	.driver = {
+		.name = "mhi_loopback",
+	},
+};
+
+module_mhi_driver(mhi_loopback_driver);
+
+MODULE_AUTHOR("Krishna Chaitanya Chundru <[email protected]>");
+MODULE_AUTHOR("Sumit Kumar <[email protected]>");
+MODULE_DESCRIPTION("MHI Host Loopback Driver");
+MODULE_LICENSE("GPL");

-- 
2.34.1