[PATCH] iio: industrialio-core: Fix debugfs read

[Alexandru Tachici]

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: e553f182d55b ("staging: iio: core: Introduce debugfs support, add support for direct register access")
Signed-off-by: Alexandru Tachici <alexandru.tachici@xxxxxxxxxx>
---
 drivers/iio/industrialio-core.c | 34 +++++++++++++++++++++++----------
 include/linux/iio/iio.h         |  2 ++
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index 65ff0d067018..637cea14afdb 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -297,26 +297,40 @@ static void __exit iio_exit(void)
 }
 
 #if defined(CONFIG_DEBUG_FS)
-static ssize_t iio_debugfs_read_reg(struct file *file, char __user *userbuf,
-			      size_t count, loff_t *ppos)
+static int iio_debugfs_get_reg_string(struct iio_dev *indio_dev)
 {
-	struct iio_dev *indio_dev = file->private_data;
-	char buf[20];
+	const struct iio_info *info = indio_dev->info;
 	unsigned val = 0;
-	ssize_t len;
 	int ret;
 
-	ret = indio_dev->info->debugfs_reg_access(indio_dev,
-						  indio_dev->cached_reg_addr,
-						  0, &val);
+	ret = info->debugfs_reg_access(indio_dev, indio_dev->cached_reg_addr,
+				       0, &val);
 	if (ret) {
 		dev_err(indio_dev->dev.parent, "%s: read failed\n", __func__);
 		return ret;
 	}
+	indio_dev->read_buf_len = snprintf(indio_dev->read_buf,
+					   sizeof(indio_dev->read_buf),
+					   "0x%X\n", val);
+	return 0;
+}
 
-	len = snprintf(buf, sizeof(buf), "0x%X\n", val);
+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;
+	loff_t pos = *ppos;
+	int ret;
+
+	if (pos == 0) {
+		ret = iio_debugfs_get_reg_string(indio_dev);
+		if (ret)
+			return ret;
+	}
 
-	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,
diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
index 862ce0019eba..eed58ed2f368 100644
--- a/include/linux/iio/iio.h
+++ b/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
 };
 
-- 
2.20.1