[PATCH 6/6] RFC: dma-buf: Add an API for importing sync files (v6)

[Daniel Vetter]

On Mon, May 24, 2021 at 03:59:54PM -0500, Jason Ekstrand wrote:
> This patch is analogous to the previous sync file export patch in that
> it allows you to import a sync_file into a dma-buf.  Unlike the previous
> patch, however, this does add genuinely new functionality to dma-buf.
> Without this, the only way to attach a sync_file to a dma-buf is to
> submit a batch to your driver of choice which waits on the sync_file and
> claims to write to the dma-buf.  Even if said batch is a no-op, a submit
> is typically way more overhead than just attaching a fence.  A submit
> may also imply extra synchronization with other work because it happens
> on a hardware queue.
> 
> In the Vulkan world, this is useful for dealing with the out-fence from
> vkQueuePresent.  Current Linux window-systems (X11, Wayland, etc.) all
> rely on dma-buf implicit sync.  Since Vulkan is an explicit sync API, we
> get a set of fences (VkSemaphores) in vkQueuePresent and have to stash
> those as an exclusive (write) fence on the dma-buf.  We handle it in
> Mesa today with the above mentioned dummy submit trick.  This ioctl
> would allow us to set it directly without the dummy submit.
> 
> This may also open up possibilities for GPU drivers to move away from
> implicit sync for their kernel driver uAPI and instead provide sync
> files and rely on dma-buf import/export for communicating with other
> implicit sync clients.
> 
> We make the explicit choice here to only allow setting RW fences which
> translates to an exclusive fence on the dma_resv.  There's no use for
> read-only fences for communicating with other implicit sync userspace
> and any such attempts are likely to be racy at best.  When we got to
> insert the RW fence, the actual fence we set as the new exclusive fence
> is a combination of the sync_file provided by the user and all the other
> fences on the dma_resv.  This ensures that the newly added exclusive
> fence will never signal before the old one would have and ensures that
> we don't break any dma_resv contracts.  We require userspace to specify
> RW in the flags for symmetry with the export ioctl and in case we ever
> want to support read fences in the future.
> 
> There is one downside here that's worth documenting:  If two clients
> writing to the same dma-buf using this API race with each other, their
> actions on the dma-buf may happen in parallel or in an undefined order.
> Both with and without this API, the pattern is the same:  Collect all
> the fences on dma-buf, submit work which depends on said fences, and
> then set a new exclusive (write) fence on the dma-buf which depends on
> said work.  The difference is that, when it's all handled by the GPU
> driver's submit ioctl, the three operations happen atomically under the
> dma_resv lock.  If two userspace submits race, one will happen before
> the other.  You aren't guaranteed which but you are guaranteed that
> they're strictly ordered.  If userspace manages the fences itself, then
> these three operations happen separately and the two render operations
> may happen genuinely in parallel or get interleaved.  However, this is a
> case of userspace racing with itself.  As long as we ensure userspace
> can't back the kernel into a corner, it should be fine.
> 
> v2 (Jason Ekstrand):
>  - Use a wrapper dma_fence_array of all fences including the new one
>    when importing an exclusive fence.
> 
> v3 (Jason Ekstrand):
>  - Lock around setting shared fences as well as exclusive
>  - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
>  - Initialize ret to 0 in dma_buf_wait_sync_file
> 
> v4 (Jason Ekstrand):
>  - Use the new dma_resv_get_singleton helper
> 
> v5 (Jason Ekstrand):
>  - Rename the IOCTLs to import/export rather than wait/signal
>  - Drop the WRITE flag and always get/set the exclusive fence
> 
> v5 (Jason Ekstrand):
>  - Split import and export into separate patches
>  - New commit message
> 
> Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> ---
>  drivers/dma-buf/dma-buf.c    | 34 ++++++++++++++++++++++++++++++++++
>  include/uapi/linux/dma-buf.h |  1 +
>  2 files changed, 35 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index f23d939e0e833..0a50c19dcf015 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -419,6 +419,38 @@ static long dma_buf_export_sync_file(struct dma_buf *dmabuf,
>  	put_unused_fd(fd);
>  	return ret;
>  }
> +
> +static long dma_buf_import_sync_file(struct dma_buf *dmabuf,
> +				     const void __user *user_data)
> +{
> +	struct dma_buf_sync_file arg;
> +	struct dma_fence *fence, *singleton = NULL;
> +	int ret = 0;
> +
> +	if (copy_from_user(&arg, user_data, sizeof(arg)))
> +		return -EFAULT;
> +
> +	if (arg.flags != DMA_BUF_SYNC_RW)
> +		return -EINVAL;
> +
> +	fence = sync_file_get_fence(arg.fd);
> +	if (!fence)
> +		return -EINVAL;
> +
> +	dma_resv_lock(dmabuf->resv, NULL);
> +
> +	singleton = dma_resv_get_singleton_unlocked(dmabuf->resv, fence);
> +	if (IS_ERR(singleton))
> +		ret = PTR_ERR(singleton);
> +	else if (singleton)
> +		dma_resv_add_excl_fence(dmabuf->resv, singleton);

We also need to add the new fence to the shared slots, to make sure that
the collective sum of shared fences still retires after the exclusive one.
Not holding this up will pretty surely allow userspace to pull a bunch of
ttm based drivers over the table I think.

Note that with dma-buf shared buffers there shouldn't be a problem here,
since as long as the buffer is in use by the other driver (which might
break the contract here) it's pinned. So nothing bad can happen.


Aside: The read-only version of this just adds the new fence, and the
exclusive fence to the shared array. I think that would be useful to have,
if just for completeness. I need to pester you how external images work
here with vulkan ...

> +
> +	dma_resv_unlock(dmabuf->resv);
> +
> +	dma_fence_put(fence);
> +
> +	return ret;
> +}
>  #endif
>  
>  static long dma_buf_ioctl(struct file *file,
> @@ -467,6 +499,8 @@ static long dma_buf_ioctl(struct file *file,
>  #if IS_ENABLED(CONFIG_SYNC_FILE)
>  	case DMA_BUF_IOCTL_EXPORT_SYNC_FILE:
>  		return dma_buf_export_sync_file(dmabuf, (void __user *)arg);
> +	case DMA_BUF_IOCTL_IMPORT_SYNC_FILE:
> +		return dma_buf_import_sync_file(dmabuf, (const void __user *)arg);
>  #endif
>  
>  	default:
> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> index f902cadcbdb56..75fdde4800267 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -70,5 +70,6 @@ struct dma_buf_sync_file {
>  #define DMA_BUF_SET_NAME_A	_IOW(DMA_BUF_BASE, 1, u32)
>  #define DMA_BUF_SET_NAME_B	_IOW(DMA_BUF_BASE, 1, u64)
>  #define DMA_BUF_IOCTL_EXPORT_SYNC_FILE	_IOWR(DMA_BUF_BASE, 2, struct dma_buf_sync_file)
> +#define DMA_BUF_IOCTL_IMPORT_SYNC_FILE	_IOW(DMA_BUF_BASE, 3, struct dma_buf_sync)

Uh wrong struct here. Not good :-)

Also more kerneldoc would be really nice, plus on 2nd thought I'm not
really sure saving the few bytes in storage is such a bright idea, and
maybe we should have distinct dma_buf_export/import_sync_file structures,
each with their appropriate kerneldoc and no confusion.

Aside from these I think this looks good. And as long as we keep up the
"shared fences in their entirety complete after the exclusive fence if
both are present", then I think we'll be fine.
-Daniel



>  
>  #endif
> -- 
> 2.31.1
> 

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