> > > In Panfrost and Lima, we don't have this concept of "incremental > > > rendering", so when we fail the allocation, we just fail the GPU job > > > with an unhandled GPU fault. > > > > To be honest I think that this is enough to mark those two drivers as > > broken. It's documented that this approach is a no-go for upstream > > drivers. > > > > How widely is that used? > > It exists in lima and panfrost, and I wouldn't be surprised if a similar > mechanism was used in other drivers for tiler-based GPUs (etnaviv, > freedreno, powervr, ...), because ultimately that's how tilers work: > the amount of memory needed to store per-tile primitives (and metadata) > depends on what the geometry pipeline feeds the tiler with, and that > can't be predicted. If you over-provision, that's memory the system won't > be able to use while rendering takes place, even though only a small > portion might actually be used by the GPU. If your allocation is too > small, it will either trigger a GPU fault (for HW not supporting an > "incremental rendering" mode) or under-perform (because flushing > primitives has a huge cost on tilers).
Yes and no. Although we can't allocate more memory for /this/ frame, we know the required size is probably constant across its lifetime. That gives a simple heuristic to manage the tiler heap efficiently without allocations - even fallible ones - in the fence signal path: * Start with a small fixed size tiler heap * Try to render, let incremental rendering kick in when it's too small. * When cleaning up the job, check if we used incremental rendering. * If we did - double the size of the heap the next time we submit work. The tiler heap still grows dynamically - it just does so over the span of a couple frames. In practice that means a tiny hit to startup time as we dynamically figure out the right size, incurring extra flushing at the start, without needing any "grow-on-page-fault" heroics. This should solve the problem completely for CSF/panthor. So it's only hardware that architecturally cannot do incremental rendering (older Mali: panfrost/lima) where we need this mess.
