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@xxxxxxxxxxxxx>
---
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
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