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

   interesting ways depending upong the exporter (if userspace starts depending

   upon this implicit synchronization).

Other Interfaces Exposed to Userspace on the dma-buf FD

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- 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

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

	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,

};

/*

		kfree(dmabuf);

		return ERR_CAST(file);

	}

	file->f_mode |= FMODE_LSEEK;

	dmabuf->file = file;

	mutex_init(&dmabuf->lock);