iio: backend: add debugFs interface

This adds a basic debugfs interface for backends. Two new ops are being
added:

 * debugfs_reg_access: Analogous to the core IIO one but for backend
   devices.
 * debugfs_print_chan_status: One useful usecase for this one is for
   testing test tones in a digital interface and "ask" the backend to
   dump more details on why a test tone might have errors.
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240802-dev-iio-backend-add-debugfs-v2-2-4cb62852f0d0@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/debugfs-iio-backend

+What:		/sys/kernel/debug/iio/iio:deviceX/backendY/name
+KernelVersion:	6.11
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Name of Backend Y connected to device X.
+
+What:		/sys/kernel/debug/iio/iio:deviceX/backendY/direct_reg_access
+KernelVersion:	6.11
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Directly access the registers of backend Y. Typical usage is:
+
+		Reading address 0x50
+		echo 0x50 > direct_reg_access
+		cat direct_reg_access
+
+		Writing address 0x50
+		echo 0x50 0x3 > direct_reg_access
+		//readback address 0x50
+		cat direct_reg_access

MAINTAINERS

@@ -10894,6 +10894,7 @@ M:	Nuno Sa <nuno.sa@analog.com>
 R:	Olivier Moysan <olivier.moysan@foss.st.com>
 L:	linux-iio@vger.kernel.org
 S:	Maintained
+F:	Documentation/ABI/testing/debugfs-iio-backend
 F:	drivers/iio/industrialio-backend.c
 F:	include/linux/iio/backend.h
 

drivers/iio/industrialio-backend.c

@@ -32,6 +32,7 @@
 #define dev_fmt(fmt) "iio-backend: " fmt
 
 #include <linux/cleanup.h>
+#include <linux/debugfs.h>
 #include <linux/device.h>
 #include <linux/err.h>
 #include <linux/errno.h>
@@ -53,6 +54,14 @@ struct iio_backend {
 	struct device *dev;
 	struct module *owner;
 	void *priv;
+	const char *name;
+	unsigned int cached_reg_addr;
+	/*
+	 * This index is relative to the frontend. Meaning that for
+	 * frontends with multiple backends, this will be the index of this
+	 * backend. Used for the debugfs directory name.
+	 */
+	u8 idx;
 };
 
 /*
@@ -117,6 +126,138 @@ static DEFINE_MUTEX(iio_back_lock);
 			__stringify(op));			\
 }
 
+static ssize_t iio_backend_debugfs_read_reg(struct file *file,
+					    char __user *userbuf,
+					    size_t count, loff_t *ppos)
+{
+	struct iio_backend *back = file->private_data;
+	char read_buf[20];
+	unsigned int val;
+	int ret, len;
+
+	ret = iio_backend_op_call(back, debugfs_reg_access,
+				  back->cached_reg_addr, 0, &val);
+	if (ret)
+		return ret;
+
+	len = scnprintf(read_buf, sizeof(read_buf), "0x%X\n", val);
+
+	return simple_read_from_buffer(userbuf, count, ppos, read_buf, len);
+}
+
+static ssize_t iio_backend_debugfs_write_reg(struct file *file,
+					     const char __user *userbuf,
+					     size_t count, loff_t *ppos)
+{
+	struct iio_backend *back = file->private_data;
+	unsigned int val;
+	char buf[80];
+	ssize_t rc;
+	int ret;
+
+	rc = simple_write_to_buffer(buf, sizeof(buf), ppos, userbuf, count);
+	if (rc < 0)
+		return rc;
+
+	ret = sscanf(buf, "%i %i", &back->cached_reg_addr, &val);
+
+	switch (ret) {
+	case 1:
+		return count;
+	case 2:
+		ret = iio_backend_op_call(back, debugfs_reg_access,
+					  back->cached_reg_addr, val, NULL);
+		if (ret)
+			return ret;
+		return count;
+	default:
+		return -EINVAL;
+	}
+}
+
+static const struct file_operations iio_backend_debugfs_reg_fops = {
+	.open = simple_open,
+	.read = iio_backend_debugfs_read_reg,
+	.write = iio_backend_debugfs_write_reg,
+};
+
+static ssize_t iio_backend_debugfs_read_name(struct file *file,
+					     char __user *userbuf,
+					     size_t count, loff_t *ppos)
+{
+	struct iio_backend *back = file->private_data;
+	char name[128];
+	int len;
+
+	len = scnprintf(name, sizeof(name), "%s\n", back->name);
+
+	return simple_read_from_buffer(userbuf, count, ppos, name, len);
+}
+
+static const struct file_operations iio_backend_debugfs_name_fops = {
+	.open = simple_open,
+	.read = iio_backend_debugfs_read_name,
+};
+
+/**
+ * iio_backend_debugfs_add - Add debugfs interfaces for Backends
+ * @back: Backend device
+ * @indio_dev: IIO device
+ */
+void iio_backend_debugfs_add(struct iio_backend *back,
+			     struct iio_dev *indio_dev)
+{
+	struct dentry *d = iio_get_debugfs_dentry(indio_dev);
+	struct dentry *back_d;
+	char name[128];
+
+	if (!IS_ENABLED(CONFIG_DEBUG_FS) || !d)
+		return;
+	if (!back->ops->debugfs_reg_access && !back->name)
+		return;
+
+	snprintf(name, sizeof(name), "backend%d", back->idx);
+
+	back_d = debugfs_create_dir(name, d);
+	if (!back_d)
+		return;
+
+	if (back->ops->debugfs_reg_access)
+		debugfs_create_file("direct_reg_access", 0600, back_d, back,
+				    &iio_backend_debugfs_reg_fops);
+
+	if (back->name)
+		debugfs_create_file("name", 0400, back_d, back,
+				    &iio_backend_debugfs_name_fops);
+}
+EXPORT_SYMBOL_NS_GPL(iio_backend_debugfs_add, IIO_BACKEND);
+
+/**
+ * iio_backend_debugfs_print_chan_status - Print channel status
+ * @back: Backend device
+ * @chan: Channel number
+ * @buf: Buffer where to print the status
+ * @len: Available space
+ *
+ * One usecase where this is useful is for testing test tones in a digital
+ * interface and "ask" the backend to dump more details on why a test tone might
+ * have errors.
+ *
+ * RETURNS:
+ * Number of copied bytes on success, negative error code on failure.
+ */
+ssize_t iio_backend_debugfs_print_chan_status(struct iio_backend *back,
+					      unsigned int chan, char *buf,
+					      size_t len)
+{
+	if (!IS_ENABLED(CONFIG_DEBUG_FS))
+		return -ENODEV;
+
+	return iio_backend_op_call(back, debugfs_print_chan_status, chan, buf,
+				   len);
+}
+EXPORT_SYMBOL_NS_GPL(iio_backend_debugfs_print_chan_status, IIO_BACKEND);
+
 /**
  * iio_backend_chan_enable - Enable a backend channel
  * @back: Backend device
@@ -577,6 +718,9 @@ struct iio_backend *devm_iio_backend_get(struct device *dev, const char *name)
 		if (ret)
 			return ERR_PTR(ret);
 
+		if (name)
+			back->idx = index;
+
 		return back;
 	}
 
@@ -668,6 +812,7 @@ int devm_iio_backend_register(struct device *dev,
 		return -ENOMEM;
 
 	back->ops = info->ops;
+	back->name = info->name;
 	back->owner = dev->driver->owner;
 	back->dev = dev;
 	back->priv = priv;

include/linux/iio/backend.h

@@ -22,6 +22,8 @@ enum iio_backend_data_source {
 	IIO_BACKEND_DATA_SOURCE_MAX
 };
 
+#define iio_backend_debugfs_ptr(ptr)	PTR_IF(IS_ENABLED(CONFIG_DEBUG_FS), ptr)
+
 /**
  * IIO_BACKEND_EX_INFO - Helper for an IIO extended channel attribute
  * @_name: Attribute name
@@ -81,6 +83,8 @@ enum iio_backend_sample_trigger {
  * @extend_chan_spec: Extend an IIO channel.
  * @ext_info_set: Extended info setter.
  * @ext_info_get: Extended info getter.
+ * @debugfs_print_chan_status: Print channel status into a buffer.
+ * @debugfs_reg_access: Read or write register value of backend.
  **/
 struct iio_backend_ops {
 	int (*enable)(struct iio_backend *back);
@@ -113,6 +117,11 @@ struct iio_backend_ops {
 			    const char *buf, size_t len);
 	int (*ext_info_get)(struct iio_backend *back, uintptr_t private,
 			    const struct iio_chan_spec *chan, char *buf);
+	int (*debugfs_print_chan_status)(struct iio_backend *back,
+					 unsigned int chan, char *buf,
+					 size_t len);
+	int (*debugfs_reg_access)(struct iio_backend *back, unsigned int reg,
+				  unsigned int writeval, unsigned int *readval);
 };
 
 /**
@@ -163,4 +172,9 @@ __devm_iio_backend_get_from_fwnode_lookup(struct device *dev,
 int devm_iio_backend_register(struct device *dev,
 			      const struct iio_backend_info *info, void *priv);
 
+ssize_t iio_backend_debugfs_print_chan_status(struct iio_backend *back,
+					      unsigned int chan, char *buf,
+					      size_t len);
+void iio_backend_debugfs_add(struct iio_backend *back,
+			     struct iio_dev *indio_dev);
 #endif