Re: [PATCH 09/17] drm/radeon: use common fence implementation for fences

Christian König
2014-07-22T08:43:26Z

Am 22.07.2014 06:05, schrieb Dave Airlie:
> On 9 July 2014 22:29, Maarten Lankhorst <maarten.lankhorst@canonical.com> wrote:
>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
>> ---
>> drivers/gpu/drm/radeon/radeon.h | 15 +-
>> drivers/gpu/drm/radeon/radeon_device.c | 60 ++++++++-
>> drivers/gpu/drm/radeon/radeon_fence.c | 223 ++++++++++++++++++++++++++------
>> 3 files changed, 248 insertions(+), 50 deletions(-)
>>
> From what I can see this is still suffering from the problem that we
> need to find a proper solution to,
>
> My summary of the issues after talking to Jerome and Ben and
> re-reading things is:
>
> We really need to work out a better interface into the drivers to be
> able to avoid random atomic entrypoints,

Which is exactly what I criticized from the very first beginning. Good
to know that I'm not the only one thinking that this isn't such a good idea.

> I'm sure you have some ideas and I think you really need to
> investigate them to move this thing forward,
> even it if means some issues with android sync pts.

Actually I think that TTMs fence interface already gave quite a good
hint how it might look like. I can only guess that this won't fit with
the Android stuff, otherwise I can't see a good reason why we didn't
stick with that.

> but none of the two major drivers seem to want the interface as-is so
> something needs to give
>
> My major question is why we need an atomic callback here at all, what
> scenario does it cover?

Agree totally. As far as I can see all current uses of the interface are
of the kind of waiting for a fence to signal.

No need for any callback from one driver into another, especially not in
atomic context. If a driver needs such a functionality it should just
start up a kernel thread and do it's waiting there.

This obviously shouldn't be an obstacle for pure hardware
implementations where one driver signals a semaphore another driver is
waiting for, or a high signal on an interrupt line directly wired
between two chips. And I think this is a completely different topic and
not necessarily part of the common fence interface we should currently
focus on.

Christian.

> Surely we can use a workqueue based callback to ask a driver to check
> its signalling, is it really
> that urgent?
>
> Dave.