Re: [PATCH 1/4] dma-buf: Remove kmap kerneldoc vestiges

["Christian König" <christian.koenig@amd.com>]

Am 11.12.20 um 16:58 schrieb Daniel Vetter:
> Also try to clarify a bit when dma_buf_begin/end_cpu_access should
> be called.
>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: "Christian König" <christian.koenig@amd.com>
> Cc: linux-media@vger.kernel.org
> Cc: linaro-mm-sig@lists.linaro.org
> ---
>   drivers/dma-buf/dma-buf.c | 20 ++++++++++++++------
>   include/linux/dma-buf.h   | 25 +++++++++----------------
>   2 files changed, 23 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index e63684d4cd90..a12fdffa130f 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -1001,15 +1001,15 @@ EXPORT_SYMBOL_GPL(dma_buf_move_notify);
>    *   vmalloc space might be limited and result in vmap calls failing.
>    *
>    *   Interfaces::
> + *
>    *      void \*dma_buf_vmap(struct dma_buf \*dmabuf)
>    *      void dma_buf_vunmap(struct dma_buf \*dmabuf, void \*vaddr)
>    *
>    *   The vmap call can fail if there is no vmap support in the exporter, or if
> - *   it runs out of vmalloc space. Fallback to kmap should be implemented. Note
> - *   that the dma-buf layer keeps a reference count for all vmap access and
> - *   calls down into the exporter's vmap function only when no vmapping exists,
> - *   and only unmaps it once. Protection against concurrent vmap/vunmap calls is
> - *   provided by taking the dma_buf->lock mutex.
> + *   it runs out of vmalloc space. Note that the dma-buf layer keeps a reference
> + *   count for all vmap access and calls down into the exporter's vmap function
> + *   only when no vmapping exists, and only unmaps it once. Protection against
> + *   concurrent vmap/vunmap calls is provided by taking the &dma_buf.lock mutex.

Who is talking the lock? The caller of the dma_buf_vmap/vunmap() 
functions, the functions itself or the callback inside the exporter?

Christian.

>    *
>    * - For full compatibility on the importer side with existing userspace
>    *   interfaces, which might already support mmap'ing buffers. This is needed in
> @@ -1098,6 +1098,11 @@ static int __dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
>    * dma_buf_end_cpu_access(). Only when cpu access is braketed by both calls is
>    * it guaranteed to be coherent with other DMA access.
>    *
> + * This function will also wait for any DMA transactions tracked through
> + * implicit synchronization in &dma_buf.resv. For DMA transactions with explicit
> + * synchronization this function will only ensure cache coherency, callers must
> + * ensure synchronization with such DMA transactions on their own.
> + *
>    * Can return negative error values, returns 0 on success.
>    */
>   int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
> @@ -1199,7 +1204,10 @@ EXPORT_SYMBOL_GPL(dma_buf_mmap);
>    * This call may fail due to lack of virtual mapping address space.
>    * These calls are optional in drivers. The intended use for them
>    * is for mapping objects linear in kernel space for high use objects.
> - * Please attempt to use kmap/kunmap before thinking about these interfaces.
> + *
> + * To ensure coherency users must call dma_buf_begin_cpu_access() and
> + * dma_buf_end_cpu_access() around any cpu access performed through this
> + * mapping.
>    *
>    * Returns 0 on success, or a negative errno code otherwise.
>    */
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index cf72699cb2bc..7eca37c8b10c 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -183,24 +183,19 @@ struct dma_buf_ops {
>   	 * @begin_cpu_access:
>   	 *
>   	 * This is called from dma_buf_begin_cpu_access() and allows the
> -	 * exporter to ensure that the memory is actually available for cpu
> -	 * access - the exporter might need to allocate or swap-in and pin the
> -	 * backing storage. The exporter also needs to ensure that cpu access is
> -	 * coherent for the access direction. The direction can be used by the
> -	 * exporter to optimize the cache flushing, i.e. access with a different
> +	 * exporter to ensure that the memory is actually coherent for cpu
> +	 * access. The exporter also needs to ensure that cpu access is coherent
> +	 * for the access direction. The direction can be used by the exporter
> +	 * to optimize the cache flushing, i.e. access with a different
>   	 * direction (read instead of write) might return stale or even bogus
>   	 * data (e.g. when the exporter needs to copy the data to temporary
>   	 * storage).
>   	 *
> -	 * This callback is optional.
> +	 * Note that this is both called through the DMA_BUF_IOCTL_SYNC IOCTL
> +	 * command for userspace mappings established through @mmap, and also
> +	 * for kernel mappings established with @vmap.
>   	 *
> -	 * FIXME: This is both called through the DMA_BUF_IOCTL_SYNC command
> -	 * from userspace (where storage shouldn't be pinned to avoid handing
> -	 * de-factor mlock rights to userspace) and for the kernel-internal
> -	 * users of the various kmap interfaces, where the backing storage must
> -	 * be pinned to guarantee that the atomic kmap calls can succeed. Since
> -	 * there's no in-kernel users of the kmap interfaces yet this isn't a
> -	 * real problem.
> +	 * This callback is optional.
>   	 *
>   	 * Returns:
>   	 *
> @@ -216,9 +211,7 @@ struct dma_buf_ops {
>   	 *
>   	 * This is called from dma_buf_end_cpu_access() when the importer is
>   	 * done accessing the CPU. The exporter can use this to flush caches and
> -	 * unpin any resources pinned in @begin_cpu_access.
> -	 * The result of any dma_buf kmap calls after end_cpu_access is
> -	 * undefined.
> +	 * undo anything else done in @begin_cpu_access.
>   	 *
>   	 * This callback is optional.
>   	 *