Re: [Linaro-mm-sig] [PATCH v5 1/6] dma-buf: Add dma_buf_{begin,end}_access()

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 19.01.24 um 15:13 schrieb Paul Cercueil:
> These functions should be used by device drivers when they start and
> stop accessing the data of DMABUF. It allows DMABUF importers to cache
> the dma_buf_attachment while ensuring that the data they want to access
> is available for their device when the DMA transfers take place.

As Daniel already noted as well this is a complete no-go from the 
DMA-buf design point of view.

Regards,
Christian.

>
> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
>
> ---
> v5: New patch
> ---
>   drivers/dma-buf/dma-buf.c | 66 +++++++++++++++++++++++++++++++++++++++
>   include/linux/dma-buf.h   | 37 ++++++++++++++++++++++
>   2 files changed, 103 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 8fe5aa67b167..a8bab6c18fcd 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -830,6 +830,8 @@ static struct sg_table * __map_dma_buf(struct dma_buf_attachment *attach,
>    *     - dma_buf_mmap()
>    *     - dma_buf_begin_cpu_access()
>    *     - dma_buf_end_cpu_access()
> + *     - dma_buf_begin_access()
> + *     - dma_buf_end_access()
>    *     - dma_buf_map_attachment_unlocked()
>    *     - dma_buf_unmap_attachment_unlocked()
>    *     - dma_buf_vmap_unlocked()
> @@ -1602,6 +1604,70 @@ void dma_buf_vunmap_unlocked(struct dma_buf *dmabuf, struct iosys_map *map)
>   }
>   EXPORT_SYMBOL_NS_GPL(dma_buf_vunmap_unlocked, DMA_BUF);
>   
> +/**
> + * @dma_buf_begin_access - Call before any hardware access from/to the DMABUF
> + * @attach:	[in]	attachment used for hardware access
> + * @sg_table:	[in]	scatterlist used for the DMA transfer
> + * @direction:  [in]    direction of DMA transfer
> + */
> +int dma_buf_begin_access(struct dma_buf_attachment *attach,
> +			 struct sg_table *sgt, enum dma_data_direction dir)
> +{
> +	struct dma_buf *dmabuf;
> +	bool cookie;
> +	int ret;
> +
> +	if (WARN_ON(!attach))
> +		return -EINVAL;
> +
> +	dmabuf = attach->dmabuf;
> +
> +	if (!dmabuf->ops->begin_access)
> +		return 0;
> +
> +	cookie = dma_fence_begin_signalling();
> +	ret = dmabuf->ops->begin_access(attach, sgt, dir);
> +	dma_fence_end_signalling(cookie);
> +
> +	if (WARN_ON_ONCE(ret))
> +		return ret;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_NS_GPL(dma_buf_begin_access, DMA_BUF);
> +
> +/**
> + * @dma_buf_end_access - Call after any hardware access from/to the DMABUF
> + * @attach:	[in]	attachment used for hardware access
> + * @sg_table:	[in]	scatterlist used for the DMA transfer
> + * @direction:  [in]    direction of DMA transfer
> + */
> +int dma_buf_end_access(struct dma_buf_attachment *attach,
> +		       struct sg_table *sgt, enum dma_data_direction dir)
> +{
> +	struct dma_buf *dmabuf;
> +	bool cookie;
> +	int ret;
> +
> +	if (WARN_ON(!attach))
> +		return -EINVAL;
> +
> +	dmabuf = attach->dmabuf;
> +
> +	if (!dmabuf->ops->end_access)
> +		return 0;
> +
> +	cookie = dma_fence_begin_signalling();
> +	ret = dmabuf->ops->end_access(attach, sgt, dir);
> +	dma_fence_end_signalling(cookie);
> +
> +	if (WARN_ON_ONCE(ret))
> +		return ret;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_NS_GPL(dma_buf_end_access, DMA_BUF);
> +
>   #ifdef CONFIG_DEBUG_FS
>   static int dma_buf_debug_show(struct seq_file *s, void *unused)
>   {
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 8ff4add71f88..8ba612c7cc16 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -246,6 +246,38 @@ struct dma_buf_ops {
>   	 */
>   	int (*end_cpu_access)(struct dma_buf *, enum dma_data_direction);
>   
> +	/**
> +	 * @begin_access:
> +	 *
> +	 * This is called from dma_buf_begin_access() when a device driver
> +	 * wants to access the data of the DMABUF. The exporter can use this
> +	 * to flush/sync the caches if needed.
> +	 *
> +	 * This callback is optional.
> +	 *
> +	 * Returns:
> +	 *
> +	 * 0 on success or a negative error code on failure.
> +	 */
> +	int (*begin_access)(struct dma_buf_attachment *, struct sg_table *,
> +			    enum dma_data_direction);
> +
> +	/**
> +	 * @end_access:
> +	 *
> +	 * This is called from dma_buf_end_access() when a device driver is
> +	 * done accessing the data of the DMABUF. The exporter can use this
> +	 * to flush/sync the caches if needed.
> +	 *
> +	 * This callback is optional.
> +	 *
> +	 * Returns:
> +	 *
> +	 * 0 on success or a negative error code on failure.
> +	 */
> +	int (*end_access)(struct dma_buf_attachment *, struct sg_table *,
> +			  enum dma_data_direction);
> +
>   	/**
>   	 * @mmap:
>   	 *
> @@ -606,6 +638,11 @@ void dma_buf_detach(struct dma_buf *dmabuf,
>   int dma_buf_pin(struct dma_buf_attachment *attach);
>   void dma_buf_unpin(struct dma_buf_attachment *attach);
>   
> +int dma_buf_begin_access(struct dma_buf_attachment *attach,
> +			 struct sg_table *sgt, enum dma_data_direction dir);
> +int dma_buf_end_access(struct dma_buf_attachment *attach,
> +		       struct sg_table *sgt, enum dma_data_direction dir);
> +
>   struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info);
>   
>   int dma_buf_fd(struct dma_buf *dmabuf, int flags);