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 christopher.halse.rogers@canonical.com ---
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;