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@xxxxxxx>
> ---
> 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
