[PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v4)

[Christian König]

Am 11.03.20 um 04:43 schrieb Jason Ekstrand:
> Explicit synchronization is the future.  At least, that seems to be what
> most userspace APIs are agreeing on at this point.  However, most of our
> Linux APIs (both userspace and kernel UAPI) are currently built around
> implicit synchronization with dma-buf.  While work is ongoing to change
> many of the userspace APIs and protocols to an explicit synchronization
> model, switching over piecemeal is difficult due to the number of
> potential components involved.  On the kernel side, many drivers use
> dma-buf including GPU (3D/compute), display, v4l, and others.  In
> userspace, we have X11, several Wayland compositors, 3D drivers, compute
> drivers (OpenCL etc.), media encode/decode, and the list goes on.
>
> This patch provides a path forward by allowing userspace to manually
> manage the fences attached to a dma-buf.  Alternatively, one can think
> of this as making dma-buf's implicit synchronization simply a carrier
> for an explicit fence.  This is accomplished by adding two IOCTLs to
> dma-buf for importing and exporting a sync file to/from the dma-buf.
> This way a userspace component which is uses explicit synchronization,
> such as a Vulkan driver, can manually set the write fence on a buffer
> before handing it off to an implicitly synchronized component such as a
> Wayland compositor or video encoder.  In this way, each of the different
> components can be upgraded to an explicit synchronization model one at a
> time as long as the userspace pieces connecting them are aware of it and
> import/export fences at the right times.
>
> There is a potential race condition with this API if userspace is not
> careful.  A typical use case for implicit synchronization is to wait for
> the dma-buf to be ready, use it, and then signal it for some other
> component.  Because a sync_file cannot be created until it is guaranteed
> to complete in finite time, userspace can only signal the dma-buf after
> it has already submitted the work which uses it to the kernel and has
> received a sync_file back.  There is no way to atomically submit a
> wait-use-signal operation.  This is not, however, really a problem with
> this API so much as it is a problem with explicit synchronization
> itself.  The way this is typically handled is to have very explicit
> ownership transfer points in the API or protocol which ensure that only
> one component is using it at any given time.  Both X11 (via the PRESENT
> extension) and Wayland provide such ownership transfer points via
> explicit present and idle messages.
>
> The decision was intentionally made in this patch to make the import and
> export operations IOCTLs on the dma-buf itself rather than as a DRM
> IOCTL.  This makes it the import/export operation universal across all
> components which use dma-buf including GPU, display, v4l, and others.
> It also means that a userspace component can do the import/export
> without access to the DRM fd which may be tricky to get in cases where
> the client communicates with DRM via a userspace API such as OpenGL or
> Vulkan.  At a future date we may choose to add direct import/export APIs
> to components such as drm_syncobj to avoid allocating a file descriptor
> and going through two ioctls.  However, that seems to be something of a
> micro-optimization as import/export operations are likely to happen at a
> rate of a few per frame of rendered or decoded video.
>
> 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
>
> Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> ---
>   drivers/dma-buf/dma-buf.c    | 96 ++++++++++++++++++++++++++++++++++++
>   include/uapi/linux/dma-buf.h | 13 ++++-
>   2 files changed, 107 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index d4097856c86b..09973c689866 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -20,6 +20,7 @@
>   #include <linux/debugfs.h>
>   #include <linux/module.h>
>   #include <linux/seq_file.h>
> +#include <linux/sync_file.h>
>   #include <linux/poll.h>
>   #include <linux/dma-resv.h>
>   #include <linux/mm.h>
> @@ -348,6 +349,95 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
>   	return ret;
>   }
>   
> +static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,
> +				   const void __user *user_data)
> +{
> +	struct dma_buf_sync_file arg;
> +	struct dma_fence *fence;
> +	int ret = 0;
> +
> +	if (copy_from_user(&arg, user_data, sizeof(arg)))
> +		return -EFAULT;
> +
> +	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
> +		return -EINVAL;
> +
> +	fence = sync_file_get_fence(arg.fd);
> +	if (!fence)
> +		return -EINVAL;
> +
> +	dma_resv_lock(dmabuf->resv, NULL);
> +
> +	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
> +		struct dma_fence *singleton = NULL;
> +		ret = dma_resv_get_singleton(dmabuf->resv, fence, &singleton);
> +		if (!ret && singleton)
> +			dma_resv_add_excl_fence(dmabuf->resv, singleton);
> +	} else {
> +		dma_resv_add_shared_fence(dmabuf->resv, fence);
> +	}

You also need to create a singleton when adding a shared fences.

The problem is that shared fences must always signal after exclusive 
ones and you can't guarantee that for the fence you add here.

Regards,
Christian.

> +
> +	dma_resv_unlock(dmabuf->resv);
> +
> +	dma_fence_put(fence);
> +
> +	return ret;
> +}
> +
> +static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,
> +				     void __user *user_data)
> +{
> +	struct dma_buf_sync_file arg;
> +	struct dma_fence *fence = NULL;
> +	struct sync_file *sync_file;
> +	int fd, ret;
> +
> +	if (copy_from_user(&arg, user_data, sizeof(arg)))
> +		return -EFAULT;
> +
> +	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
> +		return -EINVAL;
> +
> +	fd = get_unused_fd_flags(O_CLOEXEC);
> +	if (fd < 0)
> +		return fd;
> +
> +	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
> +		/* We need to include both the exclusive fence and all of
> +		 * the shared fences in our fence.
> +		 */
> +		ret = dma_resv_get_singleton(dmabuf->resv, NULL, &fence);
> +		if (ret)
> +			goto err_put_fd;
> +	} else {
> +		fence = dma_resv_get_excl_rcu(dmabuf->resv);
> +	}
> +
> +	if (!fence)
> +		fence = dma_fence_get_stub();
> +
> +	sync_file = sync_file_create(fence);
> +
> +	dma_fence_put(fence);
> +
> +	if (!sync_file) {
> +		ret = -EINVAL;
> +		goto err_put_fd;
> +	}
> +
> +	fd_install(fd, sync_file->file);
> +
> +	arg.fd = fd;
> +	if (copy_to_user(user_data, &arg, sizeof(arg)))
> +		return -EFAULT;
> +
> +	return 0;
> +
> +err_put_fd:
> +	put_unused_fd(fd);
> +	return ret;
> +}
> +
>   static long dma_buf_ioctl(struct file *file,
>   			  unsigned int cmd, unsigned long arg)
>   {
> @@ -390,6 +480,12 @@ static long dma_buf_ioctl(struct file *file,
>   	case DMA_BUF_SET_NAME:
>   		return dma_buf_set_name(dmabuf, (const char __user *)arg);
>   
> +	case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
> +		return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
> +
> +	case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
> +		return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
> +
>   	default:
>   		return -ENOTTY;
>   	}
> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> index dbc7092e04b5..86e07acca90c 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -37,8 +37,17 @@ struct dma_buf_sync {
>   
>   #define DMA_BUF_NAME_LEN	32
>   
> +struct dma_buf_sync_file {
> +	__u32 flags;
> +	__s32 fd;
> +};
> +
> +#define DMA_BUF_SYNC_FILE_SYNC_WRITE	(1 << 0)
> +
>   #define DMA_BUF_BASE		'b'
> -#define DMA_BUF_IOCTL_SYNC	_IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
> -#define DMA_BUF_SET_NAME	_IOW(DMA_BUF_BASE, 1, const char *)
> +#define DMA_BUF_IOCTL_SYNC	    _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
> +#define DMA_BUF_SET_NAME	    _IOW(DMA_BUF_BASE, 1, const char *)
> +#define DMA_BUF_IOCTL_WAIT_SYNC_FILE	_IOW(DMA_BUF_BASE, 2, struct dma_buf_sync)
> +#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE	_IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync)
>   
>   #endif