[PATCH] dma-buf: document how to find size, mention kernel versions

[PATCH] dma-buf: document how to find size, mention kernel versions

[Simon Ser]

Document how to obtain the size of a DMA-BUF. This is what
Wayland compositors are doing.

Mention the kernel version numbers from which DMA-BUF features are
available.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jason Ekstrand <jason@jlekstrand.net>
---
 include/uapi/linux/dma-buf.h | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
index 8e4a2ca0bcbf..c95f6d3457d2 100644
--- a/include/uapi/linux/dma-buf.h
+++ b/include/uapi/linux/dma-buf.h
@@ -47,9 +47,13 @@
  *
  * If the driver or API with which the client is interacting uses implicit
  * synchronization, waiting for prior work to complete can be done via
- * poll() on the DMA buffer file descriptor.  If the driver or API requires
- * explicit synchronization, the client may have to wait on a sync_file or
- * other synchronization primitive outside the scope of the DMA buffer API.
+ * poll() on the DMA buffer file descriptor from kernel version 3.17.  If the
+ * driver or API requires explicit synchronization, the client may have to wait
+ * on a sync_file or other synchronization primitive outside the scope of the
+ * DMA buffer API.
+ *
+ * From kernel version 3.12, user-space can use llseek(2) with the ``SEEK_END``
+ * whence to obtain the size of a DMA-BUF.
  */
 struct dma_buf_sync {
 	/**
-- 
2.36.1

Re: [PATCH] dma-buf: document how to find size, mention kernel versions

[Daniel Vetter]

On Thu, 26 May 2022 at 18:30, Simon Ser <contact@emersion.fr> wrote:
>
> Document how to obtain the size of a DMA-BUF. This is what
> Wayland compositors are doing.
>
> Mention the kernel version numbers from which DMA-BUF features are
> available.
>
> Signed-off-by: Simon Ser <contact@emersion.fr>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Jason Ekstrand <jason@jlekstrand.net>
> ---
>  include/uapi/linux/dma-buf.h | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> index 8e4a2ca0bcbf..c95f6d3457d2 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -47,9 +47,13 @@
>   *
>   * If the driver or API with which the client is interacting uses implicit
>   * synchronization, waiting for prior work to complete can be done via
> - * poll() on the DMA buffer file descriptor.  If the driver or API requires
> - * explicit synchronization, the client may have to wait on a sync_file or
> - * other synchronization primitive outside the scope of the DMA buffer API.
> + * poll() on the DMA buffer file descriptor from kernel version 3.17.  If the
> + * driver or API requires explicit synchronization, the client may have to wait
> + * on a sync_file or other synchronization primitive outside the scope of the
> + * DMA buffer API.

This looks good, but you missed the DOC: implicit fence polling in
dma-buf.c. Probably good to update that too.

> + *
> + * From kernel version 3.12, user-space can use llseek(2) with the ``SEEK_END``
> + * whence to obtain the size of a DMA-BUF.

This feels misplaced, especially since this ends up under the "DMA
Buffer ioctls" heading.

Maybe simplest to just put a "DMA Buffer misc uAPI" section directly
into dma-buf.rst and put that section there? Also probably good to add
that the size is invariant.

Or if you don't want a new heading, put it somewhere sensible in the
mmap section? Also just noticed that the DOC: cpu access section could
perhaps link to the relevant ioctl docs here to connect things better.
-Daniel


>   */
>  struct dma_buf_sync {
>         /**
> --
> 2.36.1
>
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch