Re: [PATCH v8 net-next] octeontx2-af: switch qmem from coherent DMA alloc to streaming DMA mapping

[Leon Romanovsky]

On Tue, Sep 08, 2026 at 12:04:11PM +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 memory with kmalloc(),
> then map it for device access with dma_map_single(). Add
> otx2_dma_alloc_coherent() and otx2_dma_free_coherent() helpers 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 kmalloc-backed
> memory 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.
> 
> 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>
> 
> ---
> v7 -> v8: Addressed Leon comments.
> - Replace __get_free_pages() and __GFP_COMP with kmalloc()
> - Drop GFP_DMA32 retry loop and dma_capable()/phys_to_dma() mask probing
> - Use dma_map_single()/dma_unmap_single() instead of dma_map_page_attrs()
>   with DMA_ATTR_REQUIRE_COHERENT
> - Remove defensive parameter checks and dma_max_mapping_size() from the
>   allocator helper
> - Move MAX_PAGE_ORDER validation to qmem_alloc()
> - Retain dev_is_dma_coherent() guard in otx2_dma_alloc_coherent()
> 
> v6 -> v7: Addressed Sashiko comments.
> 	https://lore.kernel.org/netdev/178863855246.219967.10510865726694393307@xxxxxxxxxx/
> 
> v5 -> v6: Addressed review comments.
> 	https://lore.kernel.org/netdev/20260901015621.2708182-1-rkannoth@xxxxxxxxxxx/
> 
> v4 -> v5: Fixed compilation issues.
> 	https://lore.kernel.org/netdev/20260831024210.208447-1-rkannoth@xxxxxxxxxxx/
> 
> v3 -> v4: Fixed compilation issues.
> 	https://lore.kernel.org/netdev/apTpKcN_S1xIwRbZ@rkannoth-OptiPlex-7090/
> 
> v2 -> v3: Addressed sashiko comments.
> 	https://sashiko.dev/#/patchset/20260825045616.3723078-1-rkannoth%40marvell.com
> 
> v1 -> v2: Rewrote patch as per sashiko comment.
> ---
>  .../ethernet/marvell/octeontx2/af/common.h    | 45 ++++++++++++++++---
>  1 file changed, 39 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ethernet/marvell/octeontx2/af/common.h b/drivers/net/ethernet/marvell/octeontx2/af/common.h
> index 779413a383b7..0569b6d9f03b 100644
> --- a/drivers/net/ethernet/marvell/octeontx2/af/common.h
> +++ b/drivers/net/ethernet/marvell/octeontx2/af/common.h
> @@ -7,6 +7,10 @@
>  #ifndef COMMON_H
>  #define COMMON_H
>  
> +#include <linux/dma-mapping.h>
> +#include <linux/gfp.h>
> +#include <linux/mm.h>
> +
>  #include "rvu_struct.h"
>  
>  #define OTX2_ALIGN			128  /* Align to cacheline */
> @@ -44,6 +48,33 @@ struct qmem {
>  	u32		qsize;
>  };
>  
> +static inline void *otx2_dma_alloc_coherent(struct device *dev, size_t size,
> +					    dma_addr_t *dma_handle)
> +{
> +	dma_addr_t dma_addr;
> +	void *vaddr;
> +
> +	vaddr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);

This is kzalloc(size, GFP_KERNEL)

Thanks