On 08/15/2014 04:52 PM, Jerome Glisse wrote: > On Fri, Aug 15, 2014 at 08:54:38AM +0200, Thomas Hellstrom wrote: >> On 08/14/2014 09:15 PM, Jerome Glisse wrote: >>> On Thu, Aug 14, 2014 at 08:47:16PM +0200, Daniel Vetter wrote: >>>> On Thu, Aug 14, 2014 at 8:18 PM, Jerome Glisse <j.glisse@xxxxxxxxx> wrote: >>>>> Sucks because you can not do weird synchronization like one i depicted in another >>>>> mail in this thread and for as long as cmdbuf_ioctl do not give you fence|syncpt >>>>> you can not such thing cleanly in non hackish way. >>>> Actually i915 can soon will do that that. >>> So you will return fence|syncpoint with each cmdbuf_ioctl ? >>> >>>>> Sucks because you have a fence object per buffer object and thus overhead grow >>>>> with the number of objects. Not even mentioning fence lifetime issue. >>>>> >>>>> Sucks because sub-buffer allocation is just one of many tricks that can not be >>>>> achieved properly and cleanly with implicit sync. >>>>> >>>>> ... >>>> Well I heard all those reasons and I'm well of aware of them. The >>>> problem is that with current hardware the kernel needs to know for >>>> each buffer how long it needs to be kept around since hw just can't do >>>> page faulting. Yeah you can pin them but for an uma design that >>>> doesn't go down well with folks. >>> I am not thinking with fancy hw in mind, on contrary i thought about all >>> this with the crappiest hw i could think of, in mind. >>> >>> Yes you can get rid of fence and not have to pin memory with current hw. >>> What matter for unpinning is to know that all hw block are done using the >>> memory. This is easily achievable with your beloved seqno. Have one seqno >>> per driver (one driver can have different block 3d, video decoding, crtc, >>> ...) each time a buffer is use as part of a command on one block inc the >>> common seqno and tag the buffer with that number. Have each hw block write >>> the lastest seqno that is done to a per block location. Now to determine >>> is buffer is done compare the buffer seqno with the max of all the signaled >>> seqno of all blocks. >>> >>> Cost 1 uint32 per buffer and simple if without locking to check status of >>> a buffer. >> Hmm? >> The trivial and first use of fence objects in the linux DRM was >> triggered by the fact that a >> 32-bit seqno wraps pretty quickly and a 32-bit solution just can't be >> made robust. >> Now a 64-bit seqno will probably be robust for forseeable future, but >> when it comes to implement that on 32-bit hardware and compare it to a >> simple fence object approach, > Using same kind of arithemic as use for jiffies would do it provided that > there is a checking that we never let someobject pass above a certain age. But wouldn't the search-for-max scheme break if blocks complete out of order? /Thomas _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
