[PATCH v3 3/8] dma-buf: Add vmap_local and vnumap_local operations

[Daniel Vetter]

On Wed, Dec 09, 2020 at 03:25:22PM +0100, Thomas Zimmermann wrote:
> The existing dma-buf calls dma_buf_vmap() and dma_buf_vunmap() are
> allowed to pin the buffer or acquire the buffer's reservation object
> lock.
> 
> This is a problem for callers that only require a short-term mapping
> of the buffer without the pinning, or callers that have special locking
> requirements. These may suffer from unnecessary overhead or interfere
> with regular pin operations.
> 
> The new interfaces dma_buf_vmap_local(), dma_buf_vunmapo_local(), and
> their rsp callbacks in struct dma_buf_ops provide an alternative without
> pinning or reservation locking. Callers are responsible for these
> operations.
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
> ---
>  drivers/dma-buf/dma-buf.c | 80 +++++++++++++++++++++++++++++++++++++++
>  include/linux/dma-buf.h   | 34 +++++++++++++++++
>  2 files changed, 114 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index e63684d4cd90..be9f80190a66 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -1265,6 +1265,86 @@ void dma_buf_vunmap(struct dma_buf *dmabuf, struct dma_buf_map *map)
>  }
>  EXPORT_SYMBOL_GPL(dma_buf_vunmap);
>  
> +/**
> + * dma_buf_vmap_local - Create virtual mapping for the buffer object into kernel
> + * address space.
> + * @dmabuf:	[in]	buffer to vmap
> + * @map:	[out]	returns the vmap pointer
> + *
> + * 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.

We also need to specify whether callers need to call dma_buf_begin/end_cpu
access around these or not. For current implementations it doesn't matter,
but if you want to convert udl/gm12u320, it will.

I think requiring an explicit call would be good, for more consistency
with how normal vmap works.
-Daniel

> + *
> + * Returns:
> + * 0 on success, or a negative errno code otherwise.
> + */
> +int dma_buf_vmap_local(struct dma_buf *dmabuf, struct dma_buf_map *map)
> +{
> +	struct dma_buf_map ptr;
> +	int ret = 0;
> +
> +	dma_buf_map_clear(map);
> +
> +	if (WARN_ON(!dmabuf))
> +		return -EINVAL;
> +
> +	dma_resv_assert_held(dmabuf->resv);
> +
> +	if (!dmabuf->ops->vmap_local)
> +		return -EINVAL;
> +
> +	mutex_lock(&dmabuf->lock);
> +	if (dmabuf->vmapping_counter) {
> +		dmabuf->vmapping_counter++;
> +		BUG_ON(dma_buf_map_is_null(&dmabuf->vmap_ptr));
> +		*map = dmabuf->vmap_ptr;
> +		goto out_unlock;
> +	}
> +
> +	BUG_ON(dma_buf_map_is_set(&dmabuf->vmap_ptr));
> +
> +	ret = dmabuf->ops->vmap_local(dmabuf, &ptr);
> +	if (WARN_ON_ONCE(ret))
> +		goto out_unlock;
> +
> +	dmabuf->vmap_ptr = ptr;
> +	dmabuf->vmapping_counter = 1;
> +
> +	*map = dmabuf->vmap_ptr;
> +
> +out_unlock:
> +	mutex_unlock(&dmabuf->lock);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_vmap_local);
> +
> +/**
> + * dma_buf_vunmap_local - Unmap a vmap obtained by dma_buf_vmap_local.
> + * @dmabuf:	[in]	buffer to vunmap
> + * @map:	[in]	vmap pointer to vunmap
> + */
> +void dma_buf_vunmap_local(struct dma_buf *dmabuf, struct dma_buf_map *map)
> +{
> +	if (WARN_ON(!dmabuf))
> +		return;
> +
> +	dma_resv_assert_held(dmabuf->resv);
> +
> +	BUG_ON(dma_buf_map_is_null(&dmabuf->vmap_ptr));
> +	BUG_ON(dmabuf->vmapping_counter == 0);
> +	BUG_ON(!dma_buf_map_is_equal(&dmabuf->vmap_ptr, map));
> +
> +	mutex_lock(&dmabuf->lock);
> +	if (--dmabuf->vmapping_counter == 0) {
> +		if (dmabuf->ops->vunmap_local)
> +			dmabuf->ops->vunmap_local(dmabuf, map);
> +		dma_buf_map_clear(&dmabuf->vmap_ptr);
> +	}
> +	mutex_unlock(&dmabuf->lock);
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_vunmap_local);
> +
>  #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 cf72699cb2bc..f66580d23a9b 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -269,6 +269,38 @@ struct dma_buf_ops {
>  
>  	int (*vmap)(struct dma_buf *dmabuf, struct dma_buf_map *map);
>  	void (*vunmap)(struct dma_buf *dmabuf, struct dma_buf_map *map);
> +
> +	/**
> +	 * @vmap_local:
> +	 *
> +	 * Creates a virtual mapping for the buffer into kernel address space.
> +	 *
> +	 * This callback establishes short-term mappings for situations where
> +	 * callers only use the buffer for a bounded amount of time; such as
> +	 * updates to the framebuffer or reading back contained information.
> +	 * In contrast to the regular @vmap callback, vmap_local does never pin
> +	 * the buffer to a specific domain or acquire the buffer's reservation
> +	 * lock.
> +	 *
> +	 * This is called with the dmabuf->resv object locked. Callers must hold
> +	 * the lock until after removing the mapping with @vunmap_local.
> +	 *
> +	 * This callback is optional.
> +	 *
> +	 * Returns:
> +	 *
> +	 * 0 on success or a negative error code on failure.
> +	 */
> +	int (*vmap_local)(struct dma_buf *dmabuf, struct dma_buf_map *map);
> +
> +	/**
> +	 * @vunmap_local:
> +	 *
> +	 * Removes a virtual mapping that wa sestablished by @vmap_local.
> +	 *
> +	 * This callback is optional.
> +	 */
> +	void (*vunmap_local)(struct dma_buf *dmabuf, struct dma_buf_map *map);
>  };
>  
>  /**
> @@ -506,4 +538,6 @@ int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
>  		 unsigned long);
>  int dma_buf_vmap(struct dma_buf *dmabuf, struct dma_buf_map *map);
>  void dma_buf_vunmap(struct dma_buf *dmabuf, struct dma_buf_map *map);
> +int dma_buf_vmap_local(struct dma_buf *dmabuf, struct dma_buf_map *map);
> +void dma_buf_vunmap_local(struct dma_buf *dmabuf, struct dma_buf_map *map);
>  #endif /* __DMA_BUF_H__ */
> -- 
> 2.29.2
> 

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