[PATCH 3/6] dma-buf: Document DMA_BUF_IOCTL_SYNC (v2)

[Pekka Paalanen]

On Thu, 10 Jun 2021 16:14:42 -0500
Jason Ekstrand <jason at jlekstrand.net> wrote:

> This adds a new "DMA Buffer ioctls" section to the dma-buf docs and adds
> documentation for DMA_BUF_IOCTL_SYNC.
> 
> v2 (Daniel Vetter):
>  - Fix a couple typos
>  - Add commentary about synchronization with other devices
>  - Use item list format for describing flags
> 
> Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Christian König <christian.koenig at amd.com>
> Cc: Sumit Semwal <sumit.semwal at linaro.org>
> ---
>  Documentation/driver-api/dma-buf.rst |  8 +++++
>  include/uapi/linux/dma-buf.h         | 46 +++++++++++++++++++++++++++-
>  2 files changed, 53 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
> index 7f21425d9435a..0d4c13ec1a800 100644
> --- a/Documentation/driver-api/dma-buf.rst
> +++ b/Documentation/driver-api/dma-buf.rst
> @@ -88,6 +88,9 @@ consider though:
>  - The DMA buffer FD is also pollable, see `Implicit Fence Poll Support`_ below for
>    details.
>  
> +- The DMA buffer FD also supports a few dma-buf-specific ioctls, see
> +  `DMA Buffer ioctls`_ below for details.
> +
>  Basic Operation and Device DMA Access
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>  
> @@ -106,6 +109,11 @@ Implicit Fence Poll Support
>  .. kernel-doc:: drivers/dma-buf/dma-buf.c
>     :doc: implicit fence polling
>  
> +DMA Buffer ioctls
> +~~~~~~~~~~~~~~~~~
> +
> +.. kernel-doc:: include/uapi/linux/dma-buf.h
> +
>  Kernel Functions and Structures Reference
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>  
> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> index 7f30393b92c3b..1c131002fe1ee 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -22,8 +22,52 @@
>  
>  #include <linux/types.h>
>  
> -/* begin/end dma-buf functions used for userspace mmap. */
> +/**
> + * struct dma_buf_sync - Synchronize with CPU access.
> + *
> + * When a DMA buffer is accessed from the CPU via mmap, it is not always
> + * possible to guarantee coherency between the CPU-visible map and underlying
> + * memory.  To manage coherency, DMA_BUF_IOCTL_SYNC must be used to bracket
> + * any CPU access to give the kernel the chance to shuffle memory around if
> + * needed.
> + *
> + * Prior to accessing the map, the client must call DMA_BUF_IOCTL_SYNC
> + * with DMA_BUF_SYNC_START and the appropriate read/write flags.  Once the
> + * access is complete, the client should call DMA_BUF_IOCTL_SYNC with
> + * DMA_BUF_SYNC_END and the same read/write flags.
> + *
> + * The synchronization provided via DMA_BUF_IOCTL_SYNC only provides cache
> + * coherency.  It does not prevent other processes or devices from
> + * accessing the memory at the same time.  If synchronization with a GPU or
> + * other device driver is required, it is the client's responsibility to
> + * wait for buffer to be ready for reading or writing.

... before calling this ioctl.

Maybe that would be worthwhile to add?

Likewise, submit follow-up work to GPU et al. only after calling this
ioctl with SYNC_END?

Anyway, looks nice to me.

Acked-by: Pekka Paalanen <pekka.paalanen at collabora.com>


Thanks,
pq

>  If the driver or
> + * API with which the client is interacting uses implicit synchronization,
> + * this 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.
> + */
>  struct dma_buf_sync {
> +	/**
> +	 * @flags: Set of access flags
> +	 *
> +	 * DMA_BUF_SYNC_START:
> +	 *     Indicates the start of a map access session.
> +	 *
> +	 * DMA_BUF_SYNC_END:
> +	 *     Indicates the end of a map access session.
> +	 *
> +	 * DMA_BUF_SYNC_READ:
> +	 *     Indicates that the mapped DMA buffer will be read by the
> +	 *     client via the CPU map.
> +	 *
> +	 * DMA_BUF_SYNC_WRITE:
> +	 *     Indicates that the mapped DMA buffer will be written by the
> +	 *     client via the CPU map.
> +	 *
> +	 * DMA_BUF_SYNC_RW:
> +	 *     An alias for DMA_BUF_SYNC_READ | DMA_BUF_SYNC_WRITE.
> +	 */
>  	__u64 flags;
>  };
>  

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