Each dma-buf has an associated size and it's reasonable for userspace
to want to know what it is.
Since userspace already has an fd, expose the size using the
size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0);
idiom.
Signed-off-by: Christopher James Halse Rogers <[email protected]>
---
I've run into a point in the radeon DRM userspace where I need the
size of a dma-buf. I could add a radeon-specific mechanism to get that,
but this seems like something that would be more generally useful.
I'm not entirely sure about supporting both SEEK_END and SEEK_CUR; this
is somewhat of an abuse of lseek, as seeking obviously doesn't make sense.
It's the obivous idiom for getting the size of what's on the other end of a
file descriptor, though.
I didn't notice anywhere to document this; Documentation/dma-buf-api didn't
seem like the right place. Is there somewhere I've overlooked?
drivers/base/dma-buf.c | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)
diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
index 6687ba7..c33a857 100644
--- a/drivers/base/dma-buf.c
+++ b/drivers/base/dma-buf.c
@@ -77,9 +77,36 @@ static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
return dmabuf->ops->mmap(dmabuf, vma);
}
+static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
+{
+ struct dma_buf *dmabuf;
+ loff_t base;
+
+ if (!is_dma_buf_file(file))
+ return -EBADF;
+
+ dmabuf = file->private_data;
+
+ /* only support discovering the end of the buffer,
+ but also allow SEEK_SET to maintain the idiomatic
+ SEEK_END(0), SEEK_CUR(0) pattern */
+ if (whence == SEEK_END)
+ base = dmabuf->size;
+ else if (whence == SEEK_SET)
+ base = 0;
+ else
+ return -EINVAL;
+
+ if (offset != 0)
+ return -EINVAL;
+
+ return base + offset;
+}
+
static const struct file_operations dma_buf_fops = {
.release = dma_buf_release,
.mmap = dma_buf_mmap_internal,
+ .llseek = dma_buf_llseek,
};
/*
@@ -133,6 +160,7 @@ struct dma_buf *dma_buf_export_named(void *priv, const struct dma_buf_ops *ops,
dmabuf->exp_name = exp_name;
file = anon_inode_getfile("dmabuf", &dma_buf_fops, dmabuf, flags);
+ file->f_mode |= FMODE_LSEEK;
dmabuf->file = file;
--
1.8.3.2
On Mon, Aug 05, 2013 at 04:22:00PM +1000, Christopher James Halse Rogers wrote:
> Each dma-buf has an associated size and it's reasonable for userspace
> to want to know what it is.
>
> Since userspace already has an fd, expose the size using the
> size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0);
> idiom.
>
> Signed-off-by: Christopher James Halse Rogers <[email protected]>
Yeah, loosk good to me and rather useful, so (with the dma-buf docs
improved as suggested below):
Reviewed-by: Daniel Vetter <[email protected]>
I've also written some small prime tests in igt, so also:
Tested-by: Daniel Vetter <[email protected]>
> ---
>
> I've run into a point in the radeon DRM userspace where I need the
> size of a dma-buf. I could add a radeon-specific mechanism to get that,
> but this seems like something that would be more generally useful.
>
> I'm not entirely sure about supporting both SEEK_END and SEEK_CUR; this
> is somewhat of an abuse of lseek, as seeking obviously doesn't make sense.
> It's the obivous idiom for getting the size of what's on the other end of a
> file descriptor, though.
>
> I didn't notice anywhere to document this; Documentation/dma-buf-api didn't
> seem like the right place. Is there somewhere I've overlooked?
I think adding a section about various other userspace interfaces exposed
below the mmap support section would be good. Feel free to squash in the
belo diff for v2.
Cheers, Daniel
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index 0b23261..b3a8aa2 100644
--- a/Documentation/dma-buf-sharing.txt
+++ b/Documentation/dma-buf-sharing.txt
@@ -407,6 +407,18 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
interesting ways depending upong the exporter (if userspace starts depending
upon this implicit synchronization).
+Other Interfaces Exposed to Userspace on the dma-buf FD
+------------------------------------------------------
+
+- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
+ with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allowe
+ the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
+ llseek operation will report -EINVAL.
+
+ If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
+ cases. Userspace can use this to detect support for discovering the dma-buf
+ size using llsee.
+
Miscellaneous notes
-------------------
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch