On Fri, Feb 24, 2023 at 2:24 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote: > > On Fri, 24 Feb 2023 09:41:46 +0000 > Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote: > > > On 24/02/2023 09:26, Pekka Paalanen wrote: > > > On Thu, 23 Feb 2023 10:51:48 -0800 > > > Rob Clark <robdclark@xxxxxxxxx> wrote: > > > > > >> On Thu, Feb 23, 2023 at 1:38 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote: > > >>> > > >>> On Wed, 22 Feb 2023 07:37:26 -0800 > > >>> Rob Clark <robdclark@xxxxxxxxx> wrote: > > >>> > > >>>> On Wed, Feb 22, 2023 at 1:49 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote: > > > > > > ... > > > > > >>>>> On another matter, if the application uses SET_DEADLINE with one > > >>>>> timestamp, and the compositor uses SET_DEADLINE on the same thing with > > >>>>> another timestamp, what should happen? > > >>>> > > >>>> The expectation is that many deadline hints can be set on a fence. > > >>>> The fence signaller should track the soonest deadline. > > >>> > > >>> You need to document that as UAPI, since it is observable to userspace. > > >>> It would be bad if drivers or subsystems would differ in behaviour. > > >>> > > >> > > >> It is in the end a hint. It is about giving the driver more > > >> information so that it can make better choices. But the driver is > > >> even free to ignore it. So maybe "expectation" is too strong of a > > >> word. Rather, any other behavior doesn't really make sense. But it > > >> could end up being dictated by how the hw and/or fw works. > > > > > > It will stop being a hint once it has been implemented and used in the > > > wild long enough. The kernel userspace regression rules make sure of > > > that. > > > > Yeah, tricky and maybe a gray area in this case. I think we eluded > > elsewhere in the thread that renaming the thing might be an option. > > > > So maybe instead of deadline, which is a very strong word, use something > > along the lines of "present time hint", or "signalled time hint"? Maybe > > reads clumsy. Just throwing some ideas for a start. > > You can try, but I fear that if it ever changes behaviour and > someone notices that, it's labelled as a kernel regression. I don't > think documentation has ever been the authoritative definition of UABI > in Linux, it just guides drivers and userspace towards a common > understanding and common usage patterns. > > So even if the UABI contract is not documented (ugh), you need to be > prepared to set the UABI contract through kernel implementation. > > If you do not document the UABI contract, then different drivers are > likely to implement it differently, leading to differing behaviour. > Also userspace will invent wild ways to abuse the UABI if there is no > documentation guiding it on proper use. If userspace or end users > observe different behaviour, that's bad even if it's not a regression. > > I don't like the situation either, but it is what it is. UABI stability > trumps everything regardless of whether it was documented or not. > > I bet userspace is going to use this as a "make it faster, make it > hotter" button. I would not be surprised if someone wrote a LD_PRELOAD > library that stamps any and all fences with an expired deadline to > just squeeze out a little more through some weird side-effect. Userspace already has various (driver specific) debugfs/sysfs that it can use if it wants to make it hotter and drain batteries faster, so I'm not seeing a strong need to cater to the "turn it up to eleven" crowd here. And really your point feels like a good reason to _not_ document this as anything more than a hint. Back in the real world, mobile games are already well aware of the fps vs battery-life (and therefore gameplay) tradeoff. But what is missing is a way to inform the kernel of userspace's intentions, so that gpu dvfs can make intelligent decisions. This series is meant to bridge that gap. BR, -R > Well, that's hopefully overboard in scaring, but in the end, I would > like to see UABI documented so I can have a feeling of what it is for > and how it was intended to be used. That's all. > > > Thanks, > pq
