On Tue, Dec 22, 2015 at 9:47 PM, Connor Abbott <cwabbo...@gmail.com> wrote: > On Tue, Dec 22, 2015 at 9:02 PM, Rob Clark <robdcl...@gmail.com> wrote: >> On Mon, Dec 21, 2015 at 1:48 PM, Jason Ekstrand <ja...@jlekstrand.net> wrote: >>> >>> I think two different concepts of ownership are getting conflated here: >>> Right/responsibility to delete and right to modify. >>> >>> The way I understand it, gallium, as it stands, gives neither to the driver. >>> A back-end using NIR requires the right to modify but who deletes it doesn't >>> ultimately matter. I think it's dangerous to pass one of these rights to >>> the driver and not the other but we need to think about both. >> >> yeah, uneasy about driver modifying the IR if the state tracker is >> still going to potentially spin off variants of the IR.. that sounds >> like madness. >> >> The refcnt'ing I proposed does deal w/ right to modify vs delete via >> nir_shader(_is)_mutable() which returns something that is guaranteed >> to be safe to modify (ie. has only a single reference) >> >>> What I'm trying to say is that we have two options here: >>> >>> 1) gallium passes IR to the back-end that it is free to modify and is >>> required to delete when it's done. >> >> with refcnt'ing, s/delete/unref/ >> >> The idea is, the st transfers ownership of the reference it passes to >> the driver. If the st wants to hang on to a reference itself, it must >> increment the refcnt before passing to the driver (backend). >> >> Without refcnt'ing, I suppose we could (since we don't have to follow >> TGSI semantics), just decree that the driver always takes ownership of >> the copy passed in, and if the st wants to hang on to a copy too, then >> it must clone. I suppose this would work well enough for >> freedreno/vc4, which both end up generating variants later. It does >> force an extra clone for drivers that immediately translate into their >> own backend IR and don't need to keep the NIR around, for example. >> Maybe that is not worth caring about (since at this point it is >> hypothetical). > > While always cloning does have this disadvantage, I don't think it's > really relevant here. Even if the driver throws away the NIR > immediately after consuming it, almost invariably it's going to want > to modify it. The generic NIR passed in by the state tracker (other > IR -> NIR + some optimizations) is almost never going to be the same > as the NIR after going through driver-specific lowering passes, which > means that drivers are never going to want a read-only version of the > IR. In light of that, I think making the driver own the IR passed in > seems like the most sensible thing.
well, unless the driver is already doing it's own lowering in it's own native IR.. Maybe it is too much of a hypothetical.. I still think refcnt'ing gives some nice flexibility to deal with various scenarios, and having to call nir_shader_unref() isn't so much of a burden. BR, -R >> >> (I guess nouveau is the one driver, that if it ever consumed NIR, >> would translate immediately into it's own backend IR?) >> >>> 2) gallium passes read-only IR to the back-end and it always makes a copy. >> >> This is how it is w/ TGSI, but I think with NIR we are free to make a >> clean break. And we *definitely* want to avoid >> the-driver-always-copies semantics.. >> >>> It sounds like, from what Marek is saying, that gallium is currently doing >>> (2) and changing it to (1) would be painful. I think reference counting is >>> more like an awkward option 1.5 than option 3. Reference counting would >>> mean that gallium passes a reference to the driver which it is expected to >>> unref but may keep a second reference if it wants to keep the driver from >>> modifying it. Then the driver may or may not make a copy based on the >>> number of references. Why don't we just make it explicit and add a >>> read-only bit and call it a day. >>> >>> One of the reasons I don't like passing a reference is that it effectively >>> puts allocation and freeing in different components of the driver. >> >> With refcnt'ing you should talk in terms of ref/unref rather than >> allocate/free.. imho. Although maybe that is what you meant. (In >> which case, yes, that was my idea, that passing in to driver transfers >> ownership of the passed reference.) >> >>> This >>> means that if and driver doesn't care at all about the shader that gets >>> passed in, it still has to under it to avoid a memory leak. You can't have >>> the driver take the reference because then, either it comes in with a >>> recount of 0 and should have been deleted, or the "can I modify this" check >>> becomes "recount <= 2" which makes no sense. >> >> hmm, no, if ownership of the reference is transferred to the driver, >> then it becomes "refcount == 1" (and refcount == 0 should be an >> assert, because something has gone horribly wrong) >> >> BR, >> -R _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
