Re: [PATCH] sparc64: Use relaxed ordering for DMA_FROM_DEVICE mappings on sun4v

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 13.10.2026 15:37, Stian Halseth wrote:
> sun4v IOMMU and ATU mappings carry a relaxed ordering attribute, but
> dma_4v_map_phys() and dma_4v_map_sg() set it only when the caller
> passes DMA_ATTR_WEAK_ORDERING. Some network drivers do, storage drivers
> do not. Without it every device write into memory is strictly ordered,
> which on a T7-1 (M7) caps DMA reads at about 280 MiB/s per PCIe device
> while writes run at full speed.
>
> Set the attribute for every DMA_FROM_DEVICE streaming mapping. Devices
> report completion through coherent memory or an interrupt, which stay
> strictly ordered, and a strictly ordered write does not pass the writes
> before it.
>
> T7-1, fio 1M sequential read, QD32:
>
>                       before        after
>   NVMe (PM1725, x4)   281 MiB/s     3169 MiB/s
>   SATA SSD (mpt3sas)  278 MiB/s      502 MiB/s
>
> Closes: https://protect2.fireeye.com/v1/url?k=e7e38a98-865a22d7-e7e201d7-000babe84d41-bac19a2569e412a9&q=1&e=ae0c8a5d-455c-422c-8bfa-d99f41e1f1b6&u=https%3A%2F%2Fgithub.com%2Fsparclinux%2Fissues%2Fissues%2F114
> Signed-off-by: Stian Halseth <stian@itx.no>
> ---
> Tested on a T7-1 (M7) and a T4-1:
>
> - T7: fresh RAID1 on two NVMe drives, 192 GB fio crc32c write+verify
>   and md check: no errors, mismatch_cnt 0.
> - T7 and T4: file checksums on the SAS boot mirrors identical to a
>   strictly ordered kernel; no new messages in dmesg.
> - 4k random read and sequential writes unchanged.
>
>  arch/sparc/kernel/pci_sun4v.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>

I assume this will go via sparc tree.


> diff --git a/arch/sparc/kernel/pci_sun4v.c b/arch/sparc/kernel/pci_sun4v.c
> index 61f14b4c8f90d..4dd17b65dd6cc 100644
> --- a/arch/sparc/kernel/pci_sun4v.c
> +++ b/arch/sparc/kernel/pci_sun4v.c
> @@ -403,7 +403,7 @@ static dma_addr_t dma_4v_map_phys(struct device *dev, phys_addr_t phys,
>  	if (direction != DMA_TO_DEVICE)
>  		prot |= HV_PCI_MAP_ATTR_WRITE;
>  
> -	if (attrs & DMA_ATTR_WEAK_ORDERING)
> +	if (direction == DMA_FROM_DEVICE || (attrs & DMA_ATTR_WEAK_ORDERING))
>  		prot |= HV_PCI_MAP_ATTR_RELAXED_ORDER;
>  
>  	local_irq_save(flags);
> @@ -503,7 +503,7 @@ static int dma_4v_map_sg(struct device *dev, struct scatterlist *sglist,
>  	if (direction != DMA_TO_DEVICE)
>  		prot |= HV_PCI_MAP_ATTR_WRITE;
>  
> -	if (attrs & DMA_ATTR_WEAK_ORDERING)
> +	if (direction == DMA_FROM_DEVICE || (attrs & DMA_ATTR_WEAK_ORDERING))
>  		prot |= HV_PCI_MAP_ATTR_RELAXED_ORDER;
>  
>  	outs = s = segstart = &sglist[0];

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland