Re: [PATCH v5 net] octeontx2-af: switch qmem from coherent DMA alloc to streaming DMA mapping

[Leon Romanovsky]

On Tue, Sep 01, 2026 at 07:26:21AM +0530, Ratheesh Kannoth 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.
> 
> cc: Geetha sowjanya <gakula@xxxxxxxxxxx>
> Fixes: 73d33dbc0723 ("octeontx2-af: Use DMA_ATTR_FORCE_CONTIGUOUS attribute in DMA alloc")
> Signed-off-by: Ratheesh Kannoth <rkannoth@xxxxxxxxxxx>
> 
> ---

<...>

> +	paddr = page_to_phys(virt_to_page(vaddr));
> +	dma_addr = dma_map_phys(dev, paddr, size, DMA_BIDIRECTIONAL,
> +				OTX2_DMA_COHERENT_ATTRS);

1. Don't redefine existing DMA attribute, e.g, use DMA_COHERENT_ATTRS
directly.
2. According to the Documentation/core-api/dma-attributes.rst, most likely
you don't need DMA_COHERENT_ATTRS too.
3. There is no need to use dma_map_phys() in your case as you are
supplying struct page backed memory and don't provide DMA_ATTR_MMIO,
use dma_map_page_attrs() instead.
4. Commit message is AI slop.

Thanks