Re: [RFC 0/6] drm/fences: add in-fences to DRM

[Daniel Stone]

Hi Inki,

On 28 March 2016 at 02:26, Inki Dae <inki.dae@xxxxxxxxxxx> wrote:
> 2016ë 03ì 25ì 21:10ì Daniel Stone ì(ê) ì ê:
>> Second, really. Vulkan avoids implicit sync entirely, and exposes
>> fence-like primitives throughout its whole API. These include being
>> able to pass prerequisite fences for display (what Gustavo is adding
>> here: something to block on before display), and also when the user
>> acquires a buffer as a render target, it is given another prerequisite
>> fence (the other side of what Gustavo is suggesting, i.e. the fence
>> triggers when the buffer is no longer displayed and becomes available
>> for rendering).
>>
>> In order to implement this correctly, and avoid performance bubbles,
>> we need a primitive like this exposed through the KMS API, from both
>> sides. This is especially important when you take the case of
>> userspace suballocation, where userspace allocates larger blocks and
>> divides the allocation internally for different uses. Implicit sync
>> does not work at all for that case.
>
> Can you give me more details why implicit sync cannot take care of the case of userspace suballocation?

Implicit sync does not know about suballocation, so implicit will
operate for every range in the buffer, not just the one you want.

Say you have one kernel buffer, which userspace subdivides into four
independent buffers. It can perform operations on these buffers which
are completely independent of each other, and an explicit sync model
allows this independence to be kept. Implicit sync ties them together,
so that you cannot do any operations on buffer 1 until all operations
on buffer 2 have completed.

> And is there any reason that fence fd shouldn't dependent of DMABUF - now fence fd is a new file, not DMABUF fd?

Because dmabuf is for buffer sharing, and fences aren't buffers (they
will never export page ranges). Is there any particular benefit you
think you would get from doing this?

>> good thing. This is also the model that ChromeOS is moving towards, so
>> it becomes more important from that point of view as well.
>
> I think Gustavo should had explaned this path series enough to other people when posting them - ie, what relationship explict and implicit fences have, and why implicit fence - which is independent of DMABUF - is required, and what use cases there are in real users, and etc.

Fair enough, the summary could perhaps contain something like this.

Cheers,
Daniel