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There are 3 big benefits to suballocating a single big DMA buffer for command submission: 1. Avoid hammering CMA. The old way of allocating and freeing a DMA buffer for each submission was hitting some of the real slow pathes in CMA, as this allocator was not designed for a concurrent small buffers load. 2. Less TLB flushes on IOMMUv2. If a new command buffer is mapped into the GPU address space the MMU TLBs need to be flushed. By having one big buffer statically mapped to the GPU, a lot of those flushes can be avoided. 3. No funky workarounds for GC3000. The FE TLB flush on GC3000 isn't reliable. To work around that we tried to lay out the cmdbufs in the GPU address space in a way to avoid this issue. This hasn't always worked if the address space is crowded. A single statically mapped buffer avoids the erratum completely. Signed-off-by: Lucas Stach <l.stach@pengutronix.de> Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com> |
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