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Commit 19e8697b authored by Christopher James Halse Rogers's avatar Christopher James Halse Rogers Committed by Sumit Semwal
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dma-buf: Expose buffer size to userspace (v2) 



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: default avatarChristopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Reviewed-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: default avatarSumit Semwal <sumit.semwal@linaro.org>
parent 9022e24e

Documentation/dma-buf-sharing.txt

+12 −0
Original line number Diff line number Diff line
@@ -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

-------------------

drivers/base/dma-buf.c

+29 −0
Original line number Diff line number Diff line
@@ -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,

};



/*
@@ -137,6 +164,8 @@ struct dma_buf *dma_buf_export_named(void *priv, const struct dma_buf_ops *ops, 
		kfree(dmabuf);

		return ERR_CAST(file);

	}



	file->f_mode |= FMODE_LSEEK;

	dmabuf->file = file;



	mutex_init(&dmabuf->lock);