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: e553f182d55b ("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>

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index 0b14666..eac63c1 100644 (file)
--- a/drivers/iio/industrialio-core.c
+++ b/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,

diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
index 862ce00..eed58ed 100644 (file)
--- 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
 };