[PATCH] dma-buf: Expose buffer size to userspace (v2)

[Christopher James Halse Rogers]

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.

v2: Added Daniel's sugeested documentation, with minor fixups

Signed-off-by: Christopher James Halse Rogers <christopher....@canonical.com>
Reviewed-by: Daniel Vetter <daniel...@ffwll.ch>
Tested-by: Daniel Vetter <daniel...@ffwll.ch>
---
Documentation/dma-buf-sharing.txt | 12 ++++++++++++
drivers/base/dma-buf.c | 28 ++++++++++++++++++++++++++++
2 files changed, 40 insertions(+)

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index 0b23261..849e982 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 allow
+ 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 llseek.
+
Miscellaneous notes
-------------------

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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[Martin Peres]

Hi Christopher,
Le 04/09/2013 05:15, Christopher James Halse Rogers a �crit :

Shouldn't it be "supported" instead of "support"?

Anyway, I'm just curious, in which case is it important to know the size?
Do we already have a way to get the dimensions (x, y and stripe)?

[Daniel Vetter]

Size is an invariant part of a dma-buf. Everything else wrt metadata isn't
tracked in the kernel, so doesn't make much sense to expose it ;-)
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

[Martin Peres]

Le 04/09/2013 14:05, Daniel Vetter a �crit :

ACK, thanks Daniel.