[PATCH] dma-buf: Document DMA_BUF_IOCTL_SYNC (v3)

[Daniel Vetter]

On Thu, Jun 17, 2021 at 02:42:58PM -0500, Jason Ekstrand 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
> 
> v3 (Pekka Paalanen):
>  - Clarify stalling requirements.
>  - Be more clear that that DMA_BUF_IOCTL_SYNC with SINC_END has to be
>    called before more GPU work happens.
> 
> Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> Acked-by: Christian König <christian.koenig at amd.com>
> Acked-by: Pekka Paalanen <pekka.paalanen at collabora.com>
> Cc: Sumit Semwal <sumit.semwal at linaro.org>

Merged to drm-misc-next, thanks.
-Daniel

> ---
>  Documentation/driver-api/dma-buf.rst |  8 +++++
>  include/uapi/linux/dma-buf.h         | 50 +++++++++++++++++++++++++++-
>  2 files changed, 57 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..8e4a2ca0bcbf7 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -22,8 +22,56 @@
>  
>  #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 with DMA_BUF_SYNC_START.  Likewise, the client must ensure that
> + * follow-up work is not submitted to GPU or other device driver until
> + * after this ioctl has been called with DMA_BUF_SYNC_END?
> + *
> + * 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.
> + */
>  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;
>  };
>  
> -- 
> 2.31.1
> 

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