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

[Oleksandr Andrushchenko]

On 01/26/2017 12:45 PM, Chris Wilson wrote:
> 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().
Will use this in v1
> Just the emphasize the symmetry (because of the internal magics)?
> -Chris
>