Re: [PATCH] dma-buf: Update docs for SYNC ioctl

From: Hans Verkuil <hverkuil@xs4all.nl>
To: Daniel Vetter <daniel.vetter@ffwll.ch>,
	DRI Development <dri-devel@lists.freedesktop.org>
Cc: "Chris Wilson" <chris@chris-wilson.co.uk>,
	"Tiago Vignatti" <tiago.vignatti@intel.com>,
	"Stéphane Marchesin" <marcheu@chromium.org>,
	"David Herrmann" <dh.herrmann@gmail.com>,
	"Sumit Semwal" <sumit.semwal@linaro.org>,
	"Daniel Vetter" <daniel.vetter@intel.com>,
	linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org,
	intel-gfx@lists.freedesktop.org, devel@driverdev.osuosl.org
Subject: Re: [PATCH] dma-buf: Update docs for SYNC ioctl
Date: Mon, 21 Mar 2016 08:53:18 +0100	[thread overview]
Message-ID: <56EFA86E.5030602@xs4all.nl> (raw)
In-Reply-To: <1458546705-3564-1-git-send-email-daniel.vetter@ffwll.ch>

On 03/21/2016 08:51 AM, Daniel Vetter wrote:
> Just a bit of wording polish plus mentioning that it can fail and must
> be restarted.
> 
> Requested by Sumit.
> 
> v2: Fix them typos (Hans).
> 
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tiago Vignatti <tiago.vignatti@intel.com>
> Cc: Stéphane Marchesin <marcheu@chromium.org>
> Cc: David Herrmann <dh.herrmann@gmail.com>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: Daniel Vetter <daniel.vetter@intel.com>
> CC: linux-media@vger.kernel.org
> Cc: dri-devel@lists.freedesktop.org
> Cc: linaro-mm-sig@lists.linaro.org
> Cc: intel-gfx@lists.freedesktop.org
> Cc: devel@driverdev.osuosl.org
> Cc: Hans Verkuil <hverkuil@xs4all.nl>
> Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  Documentation/dma-buf-sharing.txt | 11 ++++++-----
>  drivers/dma-buf/dma-buf.c         |  2 +-
>  2 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
> index 32ac32e773e1..ca44c5820585 100644
> --- a/Documentation/dma-buf-sharing.txt
> +++ b/Documentation/dma-buf-sharing.txt
> @@ -352,7 +352,8 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
>  
>     No special interfaces, userspace simply calls mmap on the dma-buf fd, making
>     sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
> -   used when the access happens. This is discussed next paragraphs.
> +   used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with
> +   -EAGAIN or -EINTR, in which case it must be restarted.
>  
>     Some systems might need some sort of cache coherency management e.g. when
>     CPU and GPU domains are being accessed through dma-buf at the same time. To
> @@ -366,10 +367,10 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
>         want (with the new data being consumed by the GPU or say scanout device)
>       - munmap once you don't need the buffer any more
>  
> -    Therefore, for correctness and optimal performance, systems with the memory
> -    cache shared by the GPU and CPU i.e. the "coherent" and also the
> -    "incoherent" are always required to use SYNC_START and SYNC_END before and
> -    after, respectively, when accessing the mapped address.
> +    For correctness and optimal performance, it is always required to use
> +    SYNC_START and SYNC_END before and after, respectively, when accessing the
> +    mapped address. Userspace cannot rely on coherent access, even when there
> +    are systems where it just works without calling these ioctls.
>  
>  2. Supporting existing mmap interfaces in importers
>  
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 774a60f4309a..4a2c07ee6677 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -612,7 +612,7 @@ EXPORT_SYMBOL_GPL(dma_buf_begin_cpu_access);
>   * @dmabuf:	[in]	buffer to complete cpu access for.
>   * @direction:	[in]	length of range for cpu access.
>   *
> - * This call must always succeed.
> + * Can return negative error values, returns 0 on success.
>   */
>  int dma_buf_end_cpu_access(struct dma_buf *dmabuf,
>  			   enum dma_data_direction direction)
> 

Much better :-)

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>

Regards,

	Hans