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 at gmail.com> > >> 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. > > /Thomas > >
