bus: mhi: ep: clients: Add loopback driver for data path testing

When an MHI endpoint device runs Linux, there is no firmware to implement
the LOOPBACK channel echo that real modem firmware provides. Without an
endpoint-side driver, the host loopback test has no software echo partner
and cannot exercise the full end-to-end MHI data path.

Add an endpoint-side loopback driver that binds to the LOOPBACK channel and
echoes received data back to the host. An ordered workqueue is used for
asynchronous processing to preserve packet ordering. Together with the
host-side loopback driver, this enables complete MHI data path validation
for Linux-based endpoint devices.

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>
Link: https://patch.msgid.link/20261005-lb-v7-3-36a268b6386f@oss.qualcomm.com
[mani: fixed checkpatch warnings]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>

drivers/bus/mhi/ep/Kconfig

@@ -8,3 +8,5 @@ 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.
+
+source "drivers/bus/mhi/ep/clients/Kconfig"

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-y += clients/

drivers/bus/mhi/ep/clients/Kconfig

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config MHI_BUS_EP_LOOPBACK
+	tristate "MHI Endpoint LOOPBACK client driver"
+	depends on MHI_BUS_EP
+	help
+	  MHI Endpoint LOOPBACK client driver that binds to the MHI LOOPBACK
+	  channel as defined in the MHI specification. The LOOPBACK channel is
+	  implemented by MHI-based endpoint devices (modems, WLAN) in the field,
+	  where the endpoint firmware echoes back whatever the host sends.
+
+	  This driver receives data on the uplink channel and echoes it back on
+	  the downlink channel for testing the MHI endpoint data path.
+
+	  To compile this driver as a module, choose M here. The module
+	  will be called mhi_ep_loopback.

drivers/bus/mhi/ep/clients/Makefile

@@ -0,0 +1,2 @@
+obj-$(CONFIG_MHI_BUS_EP_LOOPBACK) += mhi_ep_loopback.o
+mhi_ep_loopback-y += loopback.o

drivers/bus/mhi/ep/clients/loopback.c

@@ -0,0 +1,126 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
+ */
+
+#include <linux/mhi_ep.h>
+#include <linux/module.h>
+#include <linux/string.h>
+
+struct mhi_ep_loopback {
+	struct workqueue_struct *wq;
+	struct mhi_ep_device *mdev;
+};
+
+struct mhi_ep_loopback_work {
+	struct mhi_ep_device *mdev;
+	struct work_struct work;
+	void *buf;
+	size_t len;
+};
+
+static void mhi_ep_loopback_work_handler(struct work_struct *work)
+{
+	struct mhi_ep_loopback_work *mhi_ep_lb_work = container_of(work,
+								struct mhi_ep_loopback_work, work);
+	int ret;
+
+	ret = mhi_ep_queue_buf(mhi_ep_lb_work->mdev, mhi_ep_lb_work->buf,
+			       mhi_ep_lb_work->len);
+	if (ret) {
+		dev_err(&mhi_ep_lb_work->mdev->dev, "Failed to queue buffer: %d\n", ret);
+		kfree(mhi_ep_lb_work->buf);
+	}
+
+	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;
+	void *buf;
+
+	if (!mhi_ep_lb)
+		return;
+
+	if (!mhi_res->transaction_status) {
+		if (!mhi_res->bytes_xferd)
+			return;
+
+		buf = kmemdup(mhi_res->buf_addr, mhi_res->bytes_xferd, GFP_KERNEL);
+		if (!buf)
+			return;
+
+		mhi_ep_lb_work = kmalloc_obj(*mhi_ep_lb_work, GFP_KERNEL);
+		if (!mhi_ep_lb_work) {
+			kfree(buf);
+			return;
+		}
+
+		INIT_WORK(&mhi_ep_lb_work->work, mhi_ep_loopback_work_handler);
+		mhi_ep_lb_work->mdev = mhi_dev;
+		mhi_ep_lb_work->buf = buf;
+		mhi_ep_lb_work->len = mhi_res->bytes_xferd;
+
+		queue_work(mhi_ep_lb->wq, &mhi_ep_lb_work->work);
+	}
+}
+
+static void mhi_ep_loopback_dl_callback(struct mhi_ep_device *mhi_dev,
+					struct mhi_result *mhi_res)
+{
+	kfree(mhi_res->buf_addr);
+}
+
+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(*mhi_ep_lb), GFP_KERNEL);
+	if (!mhi_ep_lb)
+		return -ENOMEM;
+
+	mhi_ep_lb->wq = alloc_ordered_workqueue("mhi_ep_loopback", WQ_MEM_RECLAIM);
+	if (!mhi_ep_lb->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->wq);
+}
+
+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",
+	},
+};
+
+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");