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: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

2 files changed, 41 insertions, 0 deletions

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index e31a2a9d2b07..505e71172ae7 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 2d5ac1a1842e..1e16cbd61da2 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,
 };
 
 /*
@@ -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);