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

[Marek Szyprowski]

On 20.03.2026 07:54, Zile Xiong wrote:
> 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>

Acked-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>

vb2_fop_read() and vb2_fop_write() return ssize_t type, so the same 
should be used for vb2_read() and vb2_write(), as well as 
__vb2_perform_fileio(). This issue was there from the beginning of this 
code, so it would make sense to add:

Fixes: b25748fe6126 ("[media] v4l: videobuf2: add read() and write() 
emulator")

> ---
>   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);
>   
>   /**

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland