Jerome Glisse wrote: > On 5/4/07, Thomas Hellström <[EMAIL PROTECTED]> wrote: >> Keith Packard wrote: >> > On Thu, 2007-05-03 at 01:01 +0200, Thomas Hellström wrote: >> > >> > >> >> It might be possible to find schemes that work around this. One way >> >> could possibly be to have a buffer mapping -and validate order for >> >> shared buffers. >> >> >> > >> > If mapping never blocks on anything other than the fence, then there >> > isn't any dead lock possibility. What this says is that ordering of >> > rendering between clients is *not DRMs problem*. I think that's a good >> > solution though; I want to let multiple apps work on DRM-able memory >> > with their own CPU without contention. >> > >> > I don't recall if Eric layed out the proposed rules, but: >> > >> > 1) Map never blocks on map. Clients interested in dealing with this >> > are on their own. >> > >> > 2) Submit blocks on map. You must unmap all buffers before submitting >> > them. Doing the relocations in the kernel makes this all possible. >> > >> > 3) Map blocks on the fence from submit. We can play with pending the >> > flush until the app asks for the buffer back, or we can play with >> > figuring out when flushes are useful automatically. Doesn't matter >> > if the policy is in the kernel. >> > >> > I'm interested in making deadlock avoidence trivial and eliminating >> any >> > map-map contention. >> > >> > >> It's rare to have two clients access the same buffer at the same time. >> In what situation will this occur? >> >> If we think of map / unmap and validation / fence as taking a buffer >> mutex either for the CPU or for the GPU, that's the way implementation >> is done today. The CPU side of the mutex should IIRC be per-client >> recursive. OTOH, the TTM implementation won't stop the CPU from >> accessing the buffer when it is unmapped, but then you're on your own. >> "Mutexes" need to be taken in the correct order, otherwise a deadlock >> will occur, and GL will, as outlined in Eric's illustration, more or >> less encourage us to take buffers in the "incorrect" order. >> >> In essence what you propose is to eliminate the deadlock problem by just >> avoiding taking the buffer mutex unless we know the GPU has it. I see >> two problems with this: >> >> * It will encourage different DRI clients to simultaneously access >> the same buffer. >> * Inter-client and GPU data coherence can be guaranteed if we issue >> a mb() / write-combining flush with the unmap operation (which, >> BTW, I'm not sure is done today). Otherwise it is up to the >> clients, and very easy to forget. >> >> I'm a bit afraid we might in the future regret taking the easy way out? >> >> OTOH, letting DRM resolve the deadlock by unmapping and remapping shared >> buffers in the correct order might not be the best one either. It will >> certainly mean some CPU overhead and what if we have to do the same with >> buffer validation? (Yes for some operations with thousands and thousands >> of relocations, the user space validation might need to stay). >> >> Personally, I'm slightly biased towards having DRM resolve the deadlock, >> but I think any solution will do as long as the implications and why we >> choose a certain solution are totally clear. >> >> For item 3) above the kernel must have a way to issue a flush when >> needed for buffer eviction. >> The current implementation also requires the buffer to be completely >> flushed before mapping. >> Other than that the flushing policy is currently completely up to the >> DRM drivers. >> >> /Thomas > > I might say stupid things as i don't think i fully understand all > the input to this problem. Anyway here is my thought on all this: > > 1) First client map never block (as in Keith layout) except on > fence from drm side (point 3 in Keith layout) > But is there really a need for this except to avoid the above-mentioned deadlock? As I'm not too up to date with all the possibilities the servers and GL clients may be using shared buffers, I need some enlightenment :). Could we have an example, please?
> 4) We got 2 gpu queue: > - one with pending apps ask in which we do all stuff > necessary > to be done before submitting (locking buffer, > validation, ...) > for instance we might wait here for each buffer that are > still > mapped by some other apps in user space > - one run queue in which we add each apps ask that are now > ready to be submited to the gpu This is getting closer and closer to a GPU scheduler, an interesting topic indeed. Perhaps we should have a separate discussion on the needs and requirements for such a thing? Regards, /Thomas ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ -- _______________________________________________ Dri-devel mailing list Dri-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/dri-devel
