On Thu, May 5, 2022 at 3:23 AM Daniel Vetter <dan...@ffwll.ch> wrote:
> On Thu, May 05, 2022 at 03:05:44AM -0500, Jason Ekstrand wrote:
> > On Wed, May 4, 2022 at 5:49 PM Daniel Vetter <dan...@ffwll.ch> wrote:
> >
> > > On Wed, May 04, 2022 at 03:34:03PM -0500, Jason Ekstrand 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.
> > > >
> > > > By making this an ioctl on the dma-buf itself, it allows this new
> > > > functionality to be used in an entirely driver-agnostic way without
> > > > having access to a DRM fd. This makes it ideal for use in
> driver-generic
> > > > code in Mesa or in a client such as a compositor where the DRM fd
> may be
> > > > hard to reach.
> > > >
> > > > 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
> > > >
> > > > v7 (Jason Ekstrand):
> > > > - Require at least one sync flag
> > > > - Fix a refcounting bug: dma_resv_get_excl() doesn't take a
> reference
> > > > - Use _rcu helpers since we're accessing the dma_resv read-only
> > > >
> > > > v8 (Jason Ekstrand):
> > > > - Return -ENOMEM if the sync_file_create fails
> > > > - Predicate support on IS_ENABLED(CONFIG_SYNC_FILE)
> > > >
> > > > v9 (Jason Ekstrand):
> > > > - Add documentation for the new ioctl
> > > >
> > > > v10 (Jason Ekstrand):
> > > > - Go back to dma_buf_sync_file as the ioctl struct name
> > > >
> > > > v11 (Daniel Vetter):
> > > > - Go back to dma_buf_export_sync_file as the ioctl struct name
> > > > - Better kerneldoc describing what the read/write flags do
> > > >
> > > > v12 (Christian König):
> > > > - Document why we chose to make it an ioctl on dma-buf
> > > >
> > > > v12 (Jason Ekstrand):
> > > > - Rebase on Christian König's fence rework
> > > >
> > > > Signed-off-by: Jason Ekstrand <ja...@jlekstrand.net>
> > > > Acked-by: Simon Ser <cont...@emersion.fr>
> > > > Acked-by: Christian König <christian.koe...@amd.com>
> > > > Reviewed-by: Daniel Vetter <daniel.vet...@ffwll.ch>
> > >
> > > Not sure which version it was that I reviewed, but with dma_resv_usage
> > > this all looks neat and tidy. One nit below.
> > >
> > > > Cc: Sumit Semwal <sumit.sem...@linaro.org>
> > > > Cc: Maarten Lankhorst <maarten.lankho...@linux.intel.com>
> > > > ---
> > > > drivers/dma-buf/dma-buf.c | 64
> ++++++++++++++++++++++++++++++++++++
> > > > include/uapi/linux/dma-buf.h | 35 ++++++++++++++++++++
> > > > 2 files changed, 99 insertions(+)
> > > >
> > > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > > > index 79795857be3e..529e0611e53b 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>
> > > > @@ -192,6 +193,9 @@ static loff_t dma_buf_llseek(struct file *file,
> > > loff_t offset, int whence)
> > > > * Note that this only signals the completion of the respective
> fences,
> > > i.e. the
> > > > * DMA transfers are complete. Cache flushing and any other
> necessary
> > > > * preparations before CPU access can begin still need to happen.
> > > > + *
> > > > + * As an alternative to poll(), the set of fences on DMA buffer can
> be
> > > > + * exported as a &sync_file using &dma_buf_sync_file_export.
> > > > */
> > > >
> > > > static void dma_buf_poll_cb(struct dma_fence *fence, struct
> > > dma_fence_cb *cb)
> > > > @@ -326,6 +330,61 @@ static long dma_buf_set_name(struct dma_buf
> > > *dmabuf, const char __user *buf)
> > > > return 0;
> > > > }
> > > >
> > > > +#if IS_ENABLED(CONFIG_SYNC_FILE)
> > > > +static long dma_buf_export_sync_file(struct dma_buf *dmabuf,
> > > > + void __user *user_data)
> > > > +{
> > > > + struct dma_buf_export_sync_file arg;
> > > > + enum dma_resv_usage usage;
> > > > + 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;
> > > > +
> > > > + if ((arg.flags & DMA_BUF_SYNC_RW) == 0)
> > > > + return -EINVAL;
> > >
> > > We allow userspace to set both SYNC_READ and SYNC_WRITE here, I think
> > >
> > > if ((arg.flags & DMA_BUF_SYNC_RW) == DMA_BUF_SYNC_RW)
> > > return -EINVAL;
> > >
> > > is missing?
> > >
> >
> > We could, but I don't really get why we should disallow that. SYNC_READ
> |
> > SYNC_WRITE is the same as SYNC_WRITE and that seems like perfectly sane
> > behavior to me.
>
> Yeah, but it's resulting in some really confusing semantics:
>
> - SYNC_WRITE gives you the write fences
> - SYNC_READ gives you the read fences _and_ the write fences
> - SYNC_WRITE | SYNC_READ gives you only the write fences
>
> Someone will get this wrong. Also pondering some more we reuse the sync
> flags from the cpu flush helpers, and there you need to set them for the
> access you're about to do. And that's also how all the drivers use, which
> means maybe the more natural meaning if these flags would be:
>
> - SYNC_WRITE | SYNC_READ (or just SYNC_WRITE) gives you both read and
> write fences, since those are the fences you need to wait on before you
> start writing
> - SYNC_READ only gives you the read fence
>
> This is also what Christian implemented in the dma_resv_usage_rw() helper
> for implicit sync.
>
Yup. I've reworked to use dma_rev_usage_rw() to fix the bug.
--Jason
> -Daniel
>
> >
> > --Jason
> >
> >
> > > Also maybe a case to add to your igt.
> > >
> > > > +
> > > > + fd = get_unused_fd_flags(O_CLOEXEC);
> > > > + if (fd < 0)
> > > > + return fd;
> > > > +
> > > > + usage = (arg.flags & DMA_BUF_SYNC_WRITE) ?
> DMA_RESV_USAGE_WRITE :
> > > > + DMA_RESV_USAGE_READ;
> > > > + ret = dma_resv_get_singleton(dmabuf->resv, usage, &fence);
> > > > + if (ret)
> > > > + goto err_put_fd;
> > > > +
> > > > + if (!fence)
> > > > + fence = dma_fence_get_stub();
> > > > +
> > > > + sync_file = sync_file_create(fence);
> > > > +
> > > > + dma_fence_put(fence);
> > > > +
> > > > + if (!sync_file) {
> > > > + ret = -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;
> > > > +}
> > > > +#endif
> > > > +
> > > > static long dma_buf_ioctl(struct file *file,
> > > > unsigned int cmd, unsigned long arg)
> > > > {
> > > > @@ -369,6 +428,11 @@ static long dma_buf_ioctl(struct file *file,
> > > > case DMA_BUF_SET_NAME_B:
> > > > return dma_buf_set_name(dmabuf, (const char __user
> *)arg);
> > > >
> > > > +#if IS_ENABLED(CONFIG_SYNC_FILE)
> > > > + case DMA_BUF_IOCTL_EXPORT_SYNC_FILE:
> > > > + return dma_buf_export_sync_file(dmabuf, (void __user
> > > *)arg);
> > > > +#endif
> > > > +
> > > > default:
> > > > return -ENOTTY;
> > > > }
> > > > diff --git a/include/uapi/linux/dma-buf.h
> b/include/uapi/linux/dma-buf.h
> > > > index 8e4a2ca0bcbf..46f1e3e98b02 100644
> > > > --- a/include/uapi/linux/dma-buf.h
> > > > +++ b/include/uapi/linux/dma-buf.h
> > > > @@ -85,6 +85,40 @@ struct dma_buf_sync {
> > > >
> > > > #define DMA_BUF_NAME_LEN 32
> > > >
> > > > +/**
> > > > + * struct dma_buf_export_sync_file - Get a sync_file from a dma-buf
> > > > + *
> > > > + * Userspace can perform a DMA_BUF_IOCTL_EXPORT_SYNC_FILE to
> retrieve
> > > the
> > > > + * current set of fences on a dma-buf file descriptor as a
> sync_file.
> > > CPU
> > > > + * waits via poll() or other driver-specific mechanisms typically
> wait
> > > on
> > > > + * whatever fences are on the dma-buf at the time the wait begins.
> This
> > > > + * is similar except that it takes a snapshot of the current fences
> on
> > > the
> > > > + * dma-buf for waiting later instead of waiting immediately. This
> is
> > > > + * useful for modern graphics APIs such as Vulkan which assume an
> > > explicit
> > > > + * synchronization model but still need to inter-operate with
> dma-buf.
> > > > + */
> > > > +struct dma_buf_export_sync_file {
> > > > + /**
> > > > + * @flags: Read/write flags
> > > > + *
> > > > + * Must be DMA_BUF_SYNC_READ, DMA_BUF_SYNC_WRITE, or both.
> > > > + *
> > > > + * If DMA_BUF_SYNC_READ is set and DMA_BUF_SYNC_WRITE is not
> set,
> > > > + * the returned sync file waits on any writers of the dma-buf
> to
> > > > + * complete. Waiting on the returned sync file is equivalent
> to
> > > > + * poll() with POLLIN.
> > > > + *
> > > > + * If DMA_BUF_SYNC_WRITE is set, the returned sync file waits
> on
> > > > + * any users of the dma-buf (read or write) to complete.
> Waiting
> > > > + * on the returned sync file is equivalent to poll() with
> POLLOUT.
> > > > + * If both DMA_BUF_SYNC_WRITE and DMA_BUF_SYNC_READ are set,
> this
> > > > + * is equivalent to just DMA_BUF_SYNC_WRITE.
> > > > + */
> > > > + __u32 flags;
> > > > + /** @fd: Returned sync file descriptor */
> > > > + __s32 fd;
> > > > +};
> > > > +
> > > > #define DMA_BUF_BASE 'b'
> > > > #define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct
> dma_buf_sync)
> > > >
> > > > @@ -94,5 +128,6 @@ 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_export_sync_file)
> > >
> > > With the one nit fixed for this version:
> > >
> > > Reviewed-by: Daniel Vetter <daniel.vet...@ffwll.ch>
> > >
> > > >
> > > > #endif
> > > > --
> > > > 2.36.0
> > > >
> > >
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
> > >
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
>
