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

[Oleksandr Andrushchenko]

Thank you for comments,
I will update the patch and send v1.

On 01/26/2017 12:36 PM, 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().
ah, I am still new to DRM, so it is good to know
> 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()."
Will put that into the v1
>>    *
>>    * On the import the importing GEM object holds a reference to the
>>    * dma_buf (which in turn holds a ref to the exporting GEM object).
>> @@ -51,6 +51,13 @@
>>    * when the imported object is destroyed, we remove the attachment
>>    * and drop the reference to the dma_buf.
>>    *
>> + * When all the references to the dma_buf are dropped, e.g. when
>> + * userspace closes both handles to the imported (fd_to_handle_ioctl)
>> + * and exported (handle_to_fd_ioctl) dma_buf and closes the corresponding
>> + * file descriptor (handle_to_fd), then dma_buf gets destroyed.
>> + * This can also happen as a part of the clean up procedure in the
>> + * driver .release function if userspace fails to properly clean up.
> This isn't accurate, since the kernel can also internally hold references
> to the dma_buf, not just userspace by keeping the fd open. I'd drop this,
> it doesn't really explain things.
>
> Maybe add a note to the previous hunk like "Note that both the kernel and
> userspace (by keeeping the PRIME file descriptors open) can hold
> references onto a &dma_buf."
Will put that into the v1
> Thanks, Daniel
Thank you,
Oleksandr
>> + *
>>    * Thus the chain of references always flows in one direction
>>    * (avoiding loops): importing_gem -> dmabuf -> exporting_gem
>>    *
>> -- 
>> 2.7.4
>>