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SubjectRe: [PATCH v3 1/4] dma-buf: add support for virtio exported objects
On Wed, Mar 11, 2020 at 12:20 PM David Stevens <stevensd@chromium.org> wrote:
>
> This change adds a new dma-buf operation that allows dma-bufs to be used
> by virtio drivers to share exported objects. The new operation allows
> the importing driver to query the exporting driver for the UUID which
> identifies the underlying exported object.
>
> Signed-off-by: David Stevens <stevensd@chromium.org>

Adding Tomasz Figa, I've discussed this with him at elce last year I
think. Just to make sure.

Bunch of things:
- obviously we need the users of this in a few drivers, can't really
review anything stand-alone
- adding very specific ops to the generic interface is rather awkward,
eventually everyone wants that and we end up in a mess. I think the
best solution here would be if we create a struct virtio_dma_buf which
subclasses dma-buf, add a (hopefully safe) runtime upcasting
functions, and then a virtio_dma_buf_get_uuid() function. Just storing
the uuid should be doable (assuming this doesn't change during the
lifetime of the buffer), so no need for a callback.
- for the runtime upcasting the usual approach is to check the ->ops
pointer. Which means that would need to be the same for all virtio
dma_bufs, which might get a bit awkward. But I'd really prefer we not
add allocator specific stuff like this to dma-buf.
-Daniel

> ---
> drivers/dma-buf/dma-buf.c | 12 ++++++++++++
> include/linux/dma-buf.h | 18 ++++++++++++++++++
> 2 files changed, 30 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index d4097856c86b..fa5210ba6aaa 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -1158,6 +1158,18 @@ void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
> }
> EXPORT_SYMBOL_GPL(dma_buf_vunmap);
>
> +int dma_buf_get_uuid(struct dma_buf *dmabuf, uuid_t *uuid)
> +{
> + if (WARN_ON(!dmabuf) || !uuid)
> + return -EINVAL;
> +
> + if (!dmabuf->ops->get_uuid)
> + return -ENODEV;
> +
> + return dmabuf->ops->get_uuid(dmabuf, uuid);
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_get_uuid);
> +
> #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 abf5459a5b9d..00758523597d 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -251,6 +251,21 @@ struct dma_buf_ops {
>
> void *(*vmap)(struct dma_buf *);
> void (*vunmap)(struct dma_buf *, void *vaddr);
> +
> + /**
> + * @get_uuid
> + *
> + * This is called by dma_buf_get_uuid to get the UUID which identifies
> + * the buffer to virtio devices.
> + *
> + * This callback is optional.
> + *
> + * Returns:
> + *
> + * 0 on success or a negative error code on failure. On success uuid
> + * will be populated with the buffer's UUID.
> + */
> + int (*get_uuid)(struct dma_buf *dmabuf, uuid_t *uuid);
> };
>
> /**
> @@ -444,4 +459,7 @@ int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
> unsigned long);
> void *dma_buf_vmap(struct dma_buf *);
> void dma_buf_vunmap(struct dma_buf *, void *vaddr);
> +
> +int dma_buf_get_uuid(struct dma_buf *dmabuf, uuid_t *uuid);
> +
> #endif /* __DMA_BUF_H__ */
> --
> 2.25.1.481.gfbce0eb801-goog
>


--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

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