[PATCH v5 7/7] bus: mhi: Expose DDR training data via controller sysfs

[Kishore Batta]

DDR training data captured during Sahara command mode needs to be
accessible to userspace so it can be persisted and reused on subsequent
boots. Currently, the training data is stored internally in the driver
but has no external visibility once the Sahara channel is torn down.

Expose the captured DDR training data via a read-only binary sysfs
attribute on the MHI controller device:

/sys/bus/mhi/devices/<mhi_cntrl>/ddr_training_data

The sysfs read callback serves data directly from controller scoped storage
and protects access with the controller training data lock. The attribute
lifetime is tied to the controller device via devres, allowing the data to
remain readable after Sahara channel teardown and ensuring automatic
cleanup when controller device is removed.

Userspace flow:
1. For each controller device, userspace reads the ddr_training_data sysfs
   attribute.
2. If the read returns non-zero data, userspace persists it using a
   serial specific filename (for example, mdmddr_0x<serial_no>.mbn).
3. On subsequent boots, the Sahara driver attempts to load this serial
   specific DDR training image before falling back to the default
   training image, restoring DDR calibration data and avoiding retraining.

Add ABI documentation for the DDR training data sysfs attribute exposed by
Sahara MHI driver.

Signed-off-by: Kishore Batta <kishore.batta@xxxxxxxxxxxxxxxx>
---
 .../ABI/testing/sysfs-bus-mhi-ddr_training_data    | 19 ++++++
 drivers/bus/mhi/host/clients/sahara/sahara.c       | 69 ++++++++++++++++++++++
 2 files changed, 88 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-mhi-ddr_training_data b/Documentation/ABI/testing/sysfs-bus-mhi-ddr_training_data
new file mode 100644
index 0000000000000000000000000000000000000000..810b487b5a5fdba133d81255f9879844e3938a10
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-mhi-ddr_training_data
@@ -0,0 +1,19 @@
+What:                   /sys/bus/mhi/devices/<mhi-cntrl>/ddr_training_data
+
+Date:                   March 2026
+
+Contact:                Kishore Batta <kishore.batta@xxxxxxxxxxxxxxxx>
+
+Description:            Contains the DDR training data for the Qualcomm device
+                        connected. MHI driver populates different controller
+                        nodes for each device. The DDR training data is exposed
+                        to userspace to read and save the training data file to
+                        the filesystem. In the subsequent boot up of the device,
+                        the training data is restored from host to device
+                        optimizing the boot up time of the device.
+
+Usage:                  Example for reading DDR training data:
+                        cat /sys/bus/mhi/devices/mhi0/ddr_training_data
+
+Permissions:            The file permissions are set to 0444 allowing read
+                        access.
diff --git a/drivers/bus/mhi/host/clients/sahara/sahara.c b/drivers/bus/mhi/host/clients/sahara/sahara.c
index 07bc743aa061dd2fa85638067d494562152474e3..fef5dc1d8884133397d204f23361584fd1d9b075 100644
--- a/drivers/bus/mhi/host/clients/sahara/sahara.c
+++ b/drivers/bus/mhi/host/clients/sahara/sahara.c
@@ -273,6 +273,73 @@ static struct sahara_cntrl_training_data *sahara_cntrl_training_get(struct devic
 	return ct;
 }
 
+static ssize_t ddr_training_data_read(struct file *filp, struct kobject *kobj,
+				      const struct bin_attribute *attr, char *buf,
+				      loff_t offset, size_t count)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct sahara_cntrl_training_data *ct;
+	size_t available;
+
+	ct = sahara_cntrl_training_get(dev);
+	if (!ct)
+		return -ENODEV;
+
+	mutex_lock(&ct->lock);
+
+	/* No data yet or offset past end */
+	if (!ct->data || offset >= ct->size) {
+		mutex_unlock(&ct->lock);
+		return 0;
+	}
+
+	available = ct->size - offset;
+	count = min(count, available);
+	memcpy(buf, (u8 *)ct->data + offset, count);
+
+	mutex_unlock(&ct->lock);
+
+	return count;
+}
+
+static const struct bin_attribute ddr_training_data_attr = {
+	.attr = {
+		.name = "ddr_training_data",
+		.mode = 0444,
+	},
+	.read = ddr_training_data_read,
+};
+
+static void sahara_sysfs_devres_release(struct device *dev, void *res)
+{
+	device_remove_bin_file(dev, &ddr_training_data_attr);
+}
+
+static void sahara_sysfs_create(struct mhi_device *mhi_dev)
+{
+	struct device *dev = &mhi_dev->mhi_cntrl->mhi_dev->dev;
+	void *cookie;
+	int ret;
+
+	if (devres_find(dev, sahara_sysfs_devres_release, NULL, NULL))
+		return;
+
+	ret = device_create_bin_file(dev, &ddr_training_data_attr);
+	if (ret) {
+		dev_warn(&mhi_dev->dev,
+			 "Failed to create DDR training sysfs node (%d)\n", ret);
+		return;
+	}
+
+	cookie = devres_alloc(sahara_sysfs_devres_release, 1, GFP_KERNEL);
+	if (!cookie) {
+		device_remove_bin_file(dev, &ddr_training_data_attr);
+		return;
+	}
+
+	devres_add(dev, cookie);
+}
+
 static int sahara_find_image(struct sahara_context *context, u32 image_id)
 {
 	char *fw_path;
@@ -1131,6 +1198,8 @@ static int sahara_mhi_probe(struct mhi_device *mhi_dev, const struct mhi_device_
 		return ret;
 	}
 
+	sahara_sysfs_create(mhi_dev);
+
 	return 0;
 }
 

-- 
2.34.1