DMA_BUF_IOCTL_SYNC doesn't work

[jbeich]

EDIT: Description has been rewritten as the initial test case was incorrect.

Waypipe (package, workaround) relies on DMA_BUF_IOCTL_SYNC to check if a given file descripter is backed by DMABUF. On FreeBSD the ioctl always fails with EBADF which is unexpected according to the implementation:

kms-drm/linuxkpi/gplv2/src/linux_dmabuf.c

Lines 215 to 260 in e31e5fc

	static int
	dma_buf_ioctl(struct file *fp, u_long com, void *data,
	struct ucred *active_cred, struct thread *td)
	{
	struct dma_buf *db;
	struct dma_buf_sync *sync;
	enum dma_data_direction dir;
	int rc;
	if (!fp_is_db(fp))
	return (EINVAL);
	db = fp->f_data;
	sync = data;
	rc = 0;
	switch (com) {
	case DMA_BUF_IOCTL_SYNC:
	if (sync->flags & ~DMA_BUF_SYNC_VALID_FLAGS_MASK)
	return (EINVAL);
	switch (sync->flags & DMA_BUF_SYNC_RW) {
	case DMA_BUF_SYNC_READ:
	dir = DMA_FROM_DEVICE;
	break;
	case DMA_BUF_SYNC_WRITE:
	dir = DMA_TO_DEVICE;
	break;
	case DMA_BUF_SYNC_RW:
	dir = DMA_BIDIRECTIONAL;
	break;
	default:
	return (EINVAL);
	}
	if (sync->flags & DMA_BUF_SYNC_END)
	rc = dma_buf_end_cpu_access(db, dir);
	else
	rc = dma_buf_begin_cpu_access(db, dir);
	return (-rc);
	default:
	return (ENOTTY);
	}
	/* UNREACHED */
	return (0);
	}

Test case: https://gitlab.freedesktop.org/snippets/621

[mstoeckl]

The invalid flags used by waypipe are something of a hack, for lack of an easier way to generically detect DMABUFs on Linux without trying to import with GBM or using driver-specific code.

Also, FYI, the Linux version is https://github.com/torvalds/linux/blob/master/drivers/dma-buf/dma-buf.c#L279.

Edit: The sample code tries the ioctl on an fd for a render node, not for a DMABUF.

[jbeich]

Can someone try reproducing using DMA-BUF reimplementation ? drm-subtree doesn't seem to have i915 or amdgpu, so testing on amd64 is not currently possible.

[mstoeckl]

For future reference: Waypipe no longer relies on this behavior on FreeBSD, since v0.8.0, which was released more than a year ago. As of recent git commits, it has also stopped using the EBADF hack on Linux.