[PATCH] drm/prime: Clarify DMA-BUF/GEM Object lifetime

[Chris Wilson]

On Thu, Jan 26, 2017 at 11:36:05AM +0100, Daniel Vetter wrote:
> On Thu, Jan 26, 2017 at 09:22:46AM +0200, Oleksandr Andrushchenko wrote:
> > From: Oleksandr Andrushchenko <oleksandr_andrushchenko at epam.com>
> > 
> > From the description of the "DMA-BUF/GEM Object references
> > and lifetime overview" it is not clear when exactly
> > dma_buf gets destroyed and memory freed: only driver
> > .release function mentioned which makes confusion on the
> > real buffer's lifetime.
> > 
> > Add more description so all the paths are covered.
> > 
> > Cc: Rob Clark <robdclark at gmail.com>
> > Cc: Dave Airlie <airlied at linux.ie>
> > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> > Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko at epam.com>
> > ---
> >  drivers/gpu/drm/drm_prime.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> > index 8d77b2462594..c061a0b29819 100644
> > --- a/drivers/gpu/drm/drm_prime.c
> > +++ b/drivers/gpu/drm/drm_prime.c
> > @@ -41,7 +41,7 @@
> >   * object. It takes this reference in handle_to_fd_ioctl, when it
> >   * first calls .prime_export and stores the exporting GEM object in
> >   * the dma_buf priv. This reference is released when the dma_buf
> > - * object goes away in the driver .release function.
> > + * object goes away.
> 
> This was meant to talke about the release function of the dma-buf file,
> not the drm_driver. Note that not all drivers use the same _release
> function, e.g. vmwgfx is ttm-based, not gem and hence can't use
> drm_gem_dmabuf_release().
> 
> Maybe let's rewrite the entire sentence:
> 
> "This refernce needs to be released when the final reference to the
> &dma_buf itself is dropped and its &dma_buf_ops.release function is
> called. For GEM-based drivers this is done by using
> drm_gem_dmabuf_release()."

For GEM-based drivers, the dmabuf should be exported using
drm_gem_dmabuf_export() and then released by drm_gem_dmabuf_release().

Just the emphasize the symmetry (because of the internal magics)?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre