iio: industrialio-core: Fix debugfs read

Currently iio_debugfs_read_reg calls debugfs_reg_access
every time it is ran. Reading the same hardware register
multiple times during the same reading of a debugfs file
can cause unintended effects.

For example for each: cat iio:device0/direct_reg_access
the file_operations.read function will be called at least
twice. First will return the full length of the string in
bytes  and the second will return 0.

This patch makes iio_debugfs_read_reg to call debugfs_reg_access
only when the user's buffer position (*ppos) is 0. (meaning
it is the beginning of a new reading of the debugfs file).

Fixes: e553f182 ("staging: iio: core: Introduce debugfs support, add support for direct register access")
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-core.c

@@ -301,11 +301,14 @@ static ssize_t iio_debugfs_read_reg(struct file *file, char __user *userbuf,
 			      size_t count, loff_t *ppos)
 {
 	struct iio_dev *indio_dev = file->private_data;
-	char buf[20];
 	unsigned val = 0;
-	ssize_t len;
 	int ret;
 
+	if (*ppos > 0)
+		return simple_read_from_buffer(userbuf, count, ppos,
+					       indio_dev->read_buf,
+					       indio_dev->read_buf_len);
+
 	ret = indio_dev->info->debugfs_reg_access(indio_dev,
 						  indio_dev->cached_reg_addr,
 						  0, &val);
@@ -314,9 +317,13 @@ static ssize_t iio_debugfs_read_reg(struct file *file, char __user *userbuf,
 		return ret;
 	}
 
-	len = snprintf(buf, sizeof(buf), "0x%X\n", val);
+	indio_dev->read_buf_len = snprintf(indio_dev->read_buf,
+					   sizeof(indio_dev->read_buf),
+					   "0x%X\n", val);
 
-	return simple_read_from_buffer(userbuf, count, ppos, buf, len);
+	return simple_read_from_buffer(userbuf, count, ppos,
+				       indio_dev->read_buf,
+				       indio_dev->read_buf_len);
 }
 
 static ssize_t iio_debugfs_write_reg(struct file *file,

include/linux/iio/iio.h

@@ -568,6 +568,8 @@ struct iio_dev {
 #if defined(CONFIG_DEBUG_FS)
 	struct dentry			*debugfs_dentry;
 	unsigned			cached_reg_addr;
+	char				read_buf[20];
+	unsigned int			read_buf_len;
 #endif
 };