[RFC PATCH] media: vb2: use ssize_t for vb2_read/vb2_write

[Zile Xiong]

vb2_read() and vb2_write() return size_t, but propagate
negative errno values from __vb2_perform_fileio().

This relies on implicit signed/unsigned conversions in callers
(e.g. vb2_fop_read()) to recover error codes:

    __vb2_perform_fileio() -> -EINVAL
    vb2_read()             -> (size_t)-EINVAL
    vb2_fop_read()         -> -EINVAL

This relies on implicit conversions that are not obvious.

These helpers are exported (EXPORT_SYMBOL_GPL) and part of the
vb2 API, so changing their return type may affect existing users.

However, they conceptually follow read/write semantics, where
ssize_t is typically used to return either a byte count or a
negative error code.

Switch vb2_read() and vb2_write() to ssize_t, and update
__vb2_perform_fileio() accordingly.

This is an RFC to gather feedback.

Signed-off-by: Zile Xiong <xiongzile99@xxxxxxxxx>
---
 drivers/media/common/videobuf2/videobuf2-core.c | 6 +++---
 include/media/videobuf2-core.h                  | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index adf668b213c2..8a2b8156e9da 100644
--- a/drivers/media/common/videobuf2/videobuf2-core.c
+++ b/drivers/media/common/videobuf2/videobuf2-core.c
@@ -2990,7 +2990,7 @@ static int __vb2_cleanup_fileio(struct vb2_queue *q)
  * @nonblock:	mode selector (1 means blocking calls, 0 means nonblocking)
  * @read:	access mode selector (1 means read, 0 means write)
  */
-static size_t __vb2_perform_fileio(struct vb2_queue *q, char __user *data, size_t count,
+static ssize_t __vb2_perform_fileio(struct vb2_queue *q, char __user *data, size_t count,
 		loff_t *ppos, int nonblock, int read)
 {
 	struct vb2_fileio_data *fileio;
@@ -3154,14 +3154,14 @@ static size_t __vb2_perform_fileio(struct vb2_queue *q, char __user *data, size_
 	return ret;
 }
 
-size_t vb2_read(struct vb2_queue *q, char __user *data, size_t count,
+ssize_t vb2_read(struct vb2_queue *q, char __user *data, size_t count,
 		loff_t *ppos, int nonblocking)
 {
 	return __vb2_perform_fileio(q, data, count, ppos, nonblocking, 1);
 }
 EXPORT_SYMBOL_GPL(vb2_read);
 
-size_t vb2_write(struct vb2_queue *q, const char __user *data, size_t count,
+ssize_t vb2_write(struct vb2_queue *q, const char __user *data, size_t count,
 		loff_t *ppos, int nonblocking)
 {
 	return __vb2_perform_fileio(q, (char __user *) data, count,
diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
index 4424d481d7f7..bf20bcb1d366 100644
--- a/include/media/videobuf2-core.h
+++ b/include/media/videobuf2-core.h
@@ -1093,7 +1093,7 @@ __poll_t vb2_core_poll(struct vb2_queue *q, struct file *file,
  * @ppos:	file handle position tracking pointer
  * @nonblock:	mode selector (1 means blocking calls, 0 means nonblocking)
  */
-size_t vb2_read(struct vb2_queue *q, char __user *data, size_t count,
+ssize_t vb2_read(struct vb2_queue *q, char __user *data, size_t count,
 		loff_t *ppos, int nonblock);
 /**
  * vb2_write() - implements write() syscall logic.
@@ -1103,7 +1103,7 @@ size_t vb2_read(struct vb2_queue *q, char __user *data, size_t count,
  * @ppos:	file handle position tracking pointer
  * @nonblock:	mode selector (1 means blocking calls, 0 means nonblocking)
  */
-size_t vb2_write(struct vb2_queue *q, const char __user *data, size_t count,
+ssize_t vb2_write(struct vb2_queue *q, const char __user *data, size_t count,
 		loff_t *ppos, int nonblock);
 
 /**
-- 
2.39.5