> As Daniel said using fd is most likely the way we want to do it but this > remains vague. Separating the discussion if it should be an fd or not. Using an fd sounds fine to me in general, but I have some concerns as well.
For example what was the maximum number of opened FDs per process again? Could that become a problem? etc... Please comment, Christian. Am 12.09.2014 um 18:03 schrieb Jerome Glisse: > On Fri, Sep 12, 2014 at 05:58:09PM +0200, Christian K?nig wrote: >> Am 12.09.2014 um 17:48 schrieb Jerome Glisse: >>> On Fri, Sep 12, 2014 at 05:42:57PM +0200, Christian K?nig wrote: >>>> Am 12.09.2014 um 17:33 schrieb Jerome Glisse: >>>>> On Fri, Sep 12, 2014 at 11:25:12AM -0400, Alex Deucher wrote: >>>>>> On Fri, Sep 12, 2014 at 10:50 AM, Jerome Glisse <j.glisse at gmail.com> >>>>>> wrote: >>>>>>> On Fri, Sep 12, 2014 at 04:43:44PM +0200, Daniel Vetter wrote: >>>>>>>> On Fri, Sep 12, 2014 at 4:09 PM, Daniel Vetter <daniel at ffwll.ch> >>>>>>>> wrote: >>>>>>>>> On Fri, Sep 12, 2014 at 03:23:22PM +0200, Christian K?nig wrote: >>>>>>>>>> Hello everyone, >>>>>>>>>> >>>>>>>>>> to allow concurrent buffer access by different engines beyond the >>>>>>>>>> multiple >>>>>>>>>> readers/single writer model that we currently use in radeon and other >>>>>>>>>> drivers we need some kind of synchonization object exposed to >>>>>>>>>> userspace. >>>>>>>>>> >>>>>>>>>> My initial patch set for this used (or rather abused) zero sized GEM >>>>>>>>>> buffers >>>>>>>>>> as fence handles. This is obviously isn't the best way of doing this >>>>>>>>>> (to >>>>>>>>>> much overhead, rather ugly etc...), Jerome commented on this >>>>>>>>>> accordingly. >>>>>>>>>> >>>>>>>>>> So what should a driver expose instead? Android sync points? >>>>>>>>>> Something else? >>>>>>>>> I think actually exposing the struct fence objects as a fd, using >>>>>>>>> android >>>>>>>>> syncpts (or at least something compatible to it) is the way to go. >>>>>>>>> Problem >>>>>>>>> is that it's super-hard to get the android guys out of hiding for >>>>>>>>> this :( >>>>>>>>> >>>>>>>>> Adding a bunch of people in the hopes that something sticks. >>>>>>>> More people. >>>>>>> Just to re-iterate, exposing such thing while still using command stream >>>>>>> ioctl that use implicit synchronization is a waste and you can only get >>>>>>> the lowest common denominator which is implicit synchronization. So i do >>>>>>> not see the point of such api if you are not also adding a new cs ioctl >>>>>>> with explicit contract that it does not do any kind of synchronization >>>>>>> (it could be almost the exact same code modulo the do not wait for >>>>>>> previous cmd to complete). >>>>>> Our thinking was to allow explicit sync from a single process, but >>>>>> implicitly sync between processes. >>>>> This is a BIG NAK if you are using the same ioctl as it would mean you are >>>>> changing userspace API, well at least userspace expectation. Adding a new >>>>> cs flag might do the trick but it should not be about inter-process, or >>>>> any >>>>> thing special, it's just implicit sync or no synchronization. Converting >>>>> userspace is not that much of a big deal either, it can be broken into >>>>> several step. Like mesa use explicit synchronization all time but ddx use >>>>> implicit. >>>> The thinking here is that we need to be backward compatible for DRI2/3 and >>>> support all kind of different use cases like old DDX and new Mesa, or old >>>> Mesa and new DDX etc... >>>> >>>> So for my prototype if the kernel sees any access of a BO from two >>>> different >>>> clients it falls back to the old behavior of implicit synchronization of >>>> access to the same buffer object. That might not be the fastest approach, >>>> but is as far as I can see conservative and so should work under all >>>> conditions. >>>> >>>> Apart from that the planning so far was that we just hide this feature >>>> behind a couple of command submission flags and new chunks. >>> Just to reproduce IRC discussion, i think it's a lot simpler and not that >>> complex. For explicit cs ioctl you do not wait for any previous fence of >>> any of the buffer referenced in the cs ioctl, but you still associate a >>> new fence with all the buffer object referenced in the cs ioctl. So if the >>> next ioctl is an implicit sync ioctl it will wait properly and synchronize >>> properly with previous explicit cs ioctl. Hence you can easily have a mix >>> in userspace thing is you only get benefit once enough of your userspace >>> is using explicit. >> Yes, that's exactly what my patches currently implement. >> >> The only difference is that by current planning I implemented it as a per BO >> flag for the command submission, but that was just for testing. Having a >> single flag to switch between implicit and explicit synchronization for >> whole CS IOCTL would do equally well. > Doing it per BO sounds bogus to me. But otherwise yes we are in agreement. > As Daniel said using fd is most likely the way we want to do it but this > remains vague. > >>> Note that you still need a way to have explicit cs ioctl to wait on a >>> previos "explicit" fence so you need some api to expose fence per cs >>> submission. >> Exactly, that's what this mail thread is all about. >> >> As Daniel correctly noted you need something like a functionality to get a >> fence as the result of a command submission as well as pass in a list of >> fences to wait for before beginning a command submission. >> >> At least it looks like we are all on the same general line here, its just >> nobody has a good idea how the details should look like. >> >> Regards, >> Christian. >> >>> Cheers, >>> J?r?me >>> >>>> Regards, >>>> Christian. >>>> >>>>> Cheers, >>>>> J?r?me >>>>> >>>>>> Alex >>>>>> >>>>>>> Also one thing that the Android sync point does not have, AFAICT, is a >>>>>>> way to schedule synchronization as part of a cs ioctl so cpu never have >>>>>>> to be involve for cmd stream that deal only one gpu (assuming the driver >>>>>>> and hw can do such trick). >>>>>>> >>>>>>> Cheers, >>>>>>> J?r?me >>>>>>> >>>>>>>> -Daniel >>>>>>>> -- >>>>>>>> Daniel Vetter >>>>>>>> Software Engineer, Intel Corporation >>>>>>>> +41 (0) 79 365 57 48 - http://blog.ffwll.ch >>>>>>> _______________________________________________ >>>>>>> dri-devel mailing list >>>>>>> dri-devel at lists.freedesktop.org >>>>>>> http://lists.freedesktop.org/mailman/listinfo/dri-devel
