Add loopback driver for MHI endpoint devices. The driver receives
data on the uplink channel and echoes it back on the downlink
channel using a workqueue for asynchronous processing.

The driver is useful for testing MHI endpoint data path functionality
and debugging communication issues.

Co-developed-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
Signed-off-by: Sumit Kumar <sumit.kumar@oss.qualcomm.com>
---
 drivers/bus/mhi/ep/Kconfig           |   8 +++
 drivers/bus/mhi/ep/Makefile          |   1 +
 drivers/bus/mhi/ep/mhi_ep_loopback.c | 132 +++++++++++++++++++++++++++++++++++
 3 files changed, 141 insertions(+)

diff --git a/drivers/bus/mhi/ep/Kconfig b/drivers/bus/mhi/ep/Kconfig
index 90ab3b040672e0f04181d4802e3062afcc7cf782..ce7b63c2da82a6ca49528517687f4910552c35bb 100644
--- a/drivers/bus/mhi/ep/Kconfig
+++ b/drivers/bus/mhi/ep/Kconfig
@@ -8,3 +8,11 @@ config MHI_BUS_EP
 
 	  MHI_BUS_EP implements the MHI protocol for the endpoint devices,
 	  such as SDX55 modem connected to the host machine over PCIe.
+
+config MHI_BUS_EP_LOOPBACK
+	tristate "MHI Endpoint loopback driver"
+	depends on MHI_BUS_EP
+	help
+	  MHI endpoint loopback driver for data path testing.
+	  This driver receives data on the uplink channel and echoes
+	  it back on the downlink channel for testing purposes.
diff --git a/drivers/bus/mhi/ep/Makefile b/drivers/bus/mhi/ep/Makefile
index aad85f180b707fb997fcb541837eda9bbbb67437..02e4700e8dc3f860d40290476b0a852286683f8f 100644
--- a/drivers/bus/mhi/ep/Makefile
+++ b/drivers/bus/mhi/ep/Makefile
@@ -1,2 +1,3 @@
 obj-$(CONFIG_MHI_BUS_EP) += mhi_ep.o
 mhi_ep-y := main.o mmio.o ring.o sm.o
+obj-$(CONFIG_MHI_BUS_EP_LOOPBACK) += mhi_ep_loopback.o
diff --git a/drivers/bus/mhi/ep/mhi_ep_loopback.c b/drivers/bus/mhi/ep/mhi_ep_loopback.c
new file mode 100644
index 0000000000000000000000000000000000000000..89244aca684ffc959a38f8a0c3ad577b2d127c48
--- /dev/null
+++ b/drivers/bus/mhi/ep/mhi_ep_loopback.c
@@ -0,0 +1,132 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
+ */
+
+#include <linux/mhi_ep.h>
+#include <linux/mod_devicetable.h>
+#include <linux/module.h>
+#include <linux/skbuff.h>
+
+struct mhi_ep_loopback {
+	struct workqueue_struct *loopback_wq;
+	struct mhi_ep_device *mdev;
+};
+
+struct mhi_ep_loopback_work {
+	struct mhi_ep_device *mdev;
+	struct work_struct work;
+	struct sk_buff *skb;
+};
+
+static void mhi_ep_loopback_work_handler(struct work_struct *work)
+{
+	int ret;
+	struct mhi_ep_loopback_work *mhi_ep_lb_work = container_of(work,
+								struct mhi_ep_loopback_work, work);
+
+	ret = mhi_ep_queue_skb(mhi_ep_lb_work->mdev, mhi_ep_lb_work->skb);
+	if (ret) {
+		dev_err(&mhi_ep_lb_work->mdev->dev, "Failed to send the packet\n");
+		kfree_skb(mhi_ep_lb_work->skb);
+	}
+
+	kfree(mhi_ep_lb_work);
+}
+
+static void mhi_ep_loopback_ul_callback(struct mhi_ep_device *mhi_dev,
+					struct mhi_result *mhi_res)
+{
+	struct mhi_ep_loopback *mhi_ep_lb = dev_get_drvdata(&mhi_dev->dev);
+	struct mhi_ep_loopback_work *mhi_ep_lb_work;
+	struct sk_buff *skb;
+
+	if (!(mhi_res->transaction_status)) {
+		skb = alloc_skb(mhi_res->bytes_xferd, GFP_KERNEL);
+		if (!skb) {
+			dev_err(&mhi_dev->dev, "Failed to allocate skb\n");
+			return;
+		}
+
+		skb_put_data(skb, mhi_res->buf_addr, mhi_res->bytes_xferd);
+
+		mhi_ep_lb_work = kmalloc(sizeof(*mhi_ep_lb_work), GFP_KERNEL);
+		if (!mhi_ep_lb_work) {
+			dev_err(&mhi_dev->dev, "Unable to allocate the work structure\n");
+			kfree_skb(skb);
+			return;
+		}
+
+		INIT_WORK(&mhi_ep_lb_work->work, mhi_ep_loopback_work_handler);
+		mhi_ep_lb_work->mdev = mhi_dev;
+		mhi_ep_lb_work->skb = skb;
+
+		queue_work(mhi_ep_lb->loopback_wq, &mhi_ep_lb_work->work);
+	}
+}
+
+static void mhi_ep_loopback_dl_callback(struct mhi_ep_device *mhi_dev,
+					struct mhi_result *mhi_res)
+{
+	struct sk_buff *skb;
+
+	if (mhi_res->transaction_status)
+		return;
+
+	skb = mhi_res->buf_addr;
+	if (skb)
+		kfree_skb(skb);
+}
+
+static int mhi_ep_loopback_probe(struct mhi_ep_device *mhi_dev, const struct mhi_device_id *id)
+{
+	struct mhi_ep_loopback *mhi_ep_lb;
+
+	mhi_ep_lb = devm_kzalloc(&mhi_dev->dev, sizeof(struct mhi_ep_loopback), GFP_KERNEL);
+	if (!mhi_ep_lb)
+		return -ENOMEM;
+
+	mhi_ep_lb->loopback_wq = alloc_ordered_workqueue("mhi_loopback", WQ_MEM_RECLAIM);
+	if (!mhi_ep_lb->loopback_wq) {
+		dev_err(&mhi_dev->dev, "Failed to create workqueue.\n");
+		return -ENOMEM;
+	}
+
+	mhi_ep_lb->mdev = mhi_dev;
+	dev_set_drvdata(&mhi_dev->dev, mhi_ep_lb);
+
+	return 0;
+}
+
+static void mhi_ep_loopback_remove(struct mhi_ep_device *mhi_dev)
+{
+	struct mhi_ep_loopback *mhi_ep_lb = dev_get_drvdata(&mhi_dev->dev);
+
+	destroy_workqueue(mhi_ep_lb->loopback_wq);
+	dev_set_drvdata(&mhi_dev->dev, NULL);
+}
+
+static const struct mhi_device_id mhi_ep_loopback_id_table[] = {
+	{ .chan = "LOOPBACK"},
+	{}
+};
+MODULE_DEVICE_TABLE(mhi, mhi_ep_loopback_id_table);
+
+static struct mhi_ep_driver mhi_ep_loopback_driver = {
+	.probe = mhi_ep_loopback_probe,
+	.remove = mhi_ep_loopback_remove,
+	.dl_xfer_cb = mhi_ep_loopback_dl_callback,
+	.ul_xfer_cb = mhi_ep_loopback_ul_callback,
+	.id_table = mhi_ep_loopback_id_table,
+	.driver = {
+		.name = "mhi_ep_loopback",
+		.owner = THIS_MODULE,
+	},
+};
+
+module_mhi_ep_driver(mhi_ep_loopback_driver);
+
+MODULE_AUTHOR("Krishna chaitanya chundru <krishna.chundru@oss.qualcomm.com>");
+MODULE_AUTHOR("Sumit Kumar <sumit.kumar@oss.qualcomm.com>");
+MODULE_DESCRIPTION("MHI Endpoint Loopback driver");
+MODULE_LICENSE("GPL");

-- 
2.34.1

Hi Sumit,

kernel test robot noticed the following build errors:

[auto build test ERROR on e6b9dce0aeeb91dfc0974ab87f02454e24566182]

url:    https://github.com/intel-lab-lkp/linux/commits/Sumit-Kumar/bus-mhi-host-Add-loopback-driver-with-sysfs-interface/20250923-144109
base:   e6b9dce0aeeb91dfc0974ab87f02454e24566182
patch link:    https://lore.kernel.org/r/20250923-loopback_mhi-v1-2-8618f31f44aa%40oss.qualcomm.com
patch subject: [PATCH 2/2] bus: mhi: ep: Add loopback driver for data path testing
config: loongarch-randconfig-001-20250929 (https://download.01.org/0day-ci/archive/20250929/202509290238.4kkJL0x5-lkp@intel.com/config)
compiler: loongarch64-linux-gcc (GCC) 12.5.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250929/202509290238.4kkJL0x5-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202509290238.4kkJL0x5-lkp@intel.com/

All errors (new ones prefixed by >>):

   loongarch64-linux-ld: drivers/bus/mhi/ep/mhi_ep_loopback.o: in function `mhi_ep_loopback_dl_callback':
>> mhi_ep_loopback.c:(.text+0x20): undefined reference to `sk_skb_reason_drop'
   loongarch64-linux-ld: drivers/bus/mhi/ep/mhi_ep_loopback.o: in function `mhi_ep_loopback_work_handler':
   mhi_ep_loopback.c:(.text+0x74): undefined reference to `sk_skb_reason_drop'
   loongarch64-linux-ld: drivers/bus/mhi/ep/mhi_ep_loopback.o: in function `mhi_ep_loopback_ul_callback':
>> mhi_ep_loopback.c:(.text+0x19c): undefined reference to `__alloc_skb'
>> loongarch64-linux-ld: mhi_ep_loopback.c:(.text+0x1e4): undefined reference to `skb_put'
>> loongarch64-linux-ld: mhi_ep_loopback.c:(.text+0x2b8): undefined reference to `sk_skb_reason_drop'

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki