Re: [PATCH v4 net] octeontx2-af: switch qmem from coherent DMA alloc to streaming DMA mapping

[Ratheesh Kannoth]

On 2026-08-31 at 08:12:09, Ratheesh Kannoth (rkannoth@xxxxxxxxxxx) wrote:
> qmem_alloc() uses dma_alloc_attrs() with DMA_ATTR_FORCE_CONTIGUOUS, which
> allocates CPU-cache-coherent DMA memory and, with CMA enabled, draws from
> the CMA pool. qmem backs NIX/NPA queue contexts, admin queues, and LMTST
> regions (including CN10K LMTST areas that span page boundaries), so
> consumption grows with enabled interfaces and is hard to provision in CMA.
>
> Switch qmem to a streaming-DMA-style path: allocate physically contiguous
> compound pages from the buddy allocator via __get_free_pages(), then map
> them for device access with dma_map_phys() and dma_unmap_phys() using
> DMA_ATTR_REQUIRE_COHERENT. Add otx2_dma_alloc_coherent() and
> otx2_dma_free_coherent() helpers that enforce dev_is_dma_coherent(),
> retry with GFP_DMA32 when the physical range is outside the device DMA
> mask, and wire qmem_alloc()/qmem_free() through them instead of
> dma_alloc_attrs()/dma_free_attrs().
>
> This works on Octeon because the octeontx2 driver is written for
> DMA-coherent devices: Octeon platforms provide IO coherency (via SMMU), so
> the driver already uses streaming DMA APIs for packet data while
> deliberately skipping explicit CPU cache sync (DMA_ATTR_SKIP_CPU_SYNC).
> The same IO coherency lets qmem use a streaming map of buddy-allocated
> pages instead of a dedicated coherent allocator or CMA reservation. That
> is valid because the platform is DMA-coherent, not because omitting
> dma_sync_* magically makes memory coherent.
>
> Allocations requiring more than MAX_PAGE_ORDER pages are still rejected,
> since the buddy allocator cannot serve them without CMA.
>

Will address issues in v5

pw-bot: changes-requested