On Mon, Mar 15, 2021 at 4:05 PM Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote:
>
> Modern userspace APIs like Vulkan are built on an explicit
> synchronization model. This doesn't always play nicely with the
> implicit synchronization used in the kernel and assumed by X11 and
> Wayland. The client -> compositor half of the synchronization isn't too
> bad, at least on intel, because we can control whether or not i915
> synchronizes on the buffer and whether or not it's considered written.
>
> The harder part is the compositor -> client synchronization when we get
> the buffer back from the compositor. We're required to be able to
> provide the client with a VkSemaphore and VkFence representing the point
> in time where the window system (compositor and/or display) finished
> using the buffer. With current APIs, it's very hard to do this in such
> a way that we don't get confused by the Vulkan driver's access of the
> buffer. In particular, once we tell the kernel that we're rendering to
> the buffer again, any CPU waits on the buffer or GPU dependencies will
> wait on some of the client rendering and not just the compositor.
>
> This new IOCTL solves this problem by allowing us to get a snapshot of
> the implicit synchronization state of a given dma-buf in the form of a
> sync file. It's effectively the same as a poll() or I915_GEM_WAIT only,
> instead of CPU waiting directly, it encapsulates the wait operation, at
> the current moment in time, in a sync_file so we can check/wait on it
> later. As long as the Vulkan driver does the sync_file export from the
> dma-buf before we re-introduce it for rendering, it will only contain
> fences from the compositor or display. This allows to accurately turn
> it into a VkFence or VkSemaphore without any over- synchronization.
>
> 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
>
> v6 (Jason Ekstrand):
> - Drop the sync_file import as it was all-around sketchy and not nearly
> as useful as import.
> - Re-introduce READ/WRITE flag support for export
> - Rework the commit message
>
> Signed-off-by: Jason Ekstrand <jason@xxxxxxxxxxxxxx>
> ---
> drivers/dma-buf/dma-buf.c | 55 ++++++++++++++++++++++++++++++++++++
> include/uapi/linux/dma-buf.h | 6 ++++
> 2 files changed, 61 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index f264b70c383eb..e7f9dd62c19a9 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>
> @@ -362,6 +363,57 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
> return ret;
> }
>
> +static long dma_buf_export_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 & ~DMA_BUF_SYNC_RW)
> + return -EINVAL;
> +
> + fd = get_unused_fd_flags(O_CLOEXEC);
> + if (fd < 0)
> + return fd;
> +
> + if (arg.flags & DMA_BUF_SYNC_WRITE) {
> + ret = dma_resv_get_singleton(dmabuf->resv, NULL, &fence);
> + if (ret)
> + goto err_put_fd;
> + } else if (arg.flags & DMA_BUF_SYNC_READ) {
> + fence = dma_resv_get_excl(dmabuf->resv);
> + }
> +
> + if (!fence)
> + fence = dma_fence_get_stub();
> +
> + sync_file = sync_file_create(fence);
> +
> + dma_fence_put(fence);
> +
> + if (!sync_file) {
> + ret = -EINVAL;
Should this be -EINVAL or -ENOMEM?
> + 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)
> {
> @@ -405,6 +457,9 @@ static long dma_buf_ioctl(struct file *file,
> case DMA_BUF_SET_NAME_B:
> return dma_buf_set_name(dmabuf, (const char __user *)arg);
>
> + case DMA_BUF_IOCTL_EXPORT_SYNC_FILE:
> + return dma_buf_export_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 7f30393b92c3b..9bce1e8bd31d3 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -37,6 +37,11 @@ struct dma_buf_sync {
>
> #define DMA_BUF_NAME_LEN 32
>
> +struct dma_buf_sync_file {
> + __u32 flags;
> + __s32 fd;
> +};
> +
> #define DMA_BUF_BASE 'b'
> #define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
>
> @@ -46,5 +51,6 @@ struct dma_buf_sync {
> #define DMA_BUF_SET_NAME _IOW(DMA_BUF_BASE, 1, const char *)
> #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)
>
> #endif
> --
> 2.29.2
>
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