Re: [PATCH net-next v2 0/7] net: devmem: size TX net_iovs from the dmabuf layout

[Mina Almasry <almasrymina@google.com>]

On Tue, Nov 3, 2026 at 3:51 PM Stanislav Fomichev <sdf.kernel@gmail.com> wrote:
>
> TX bindings use PAGE_SIZE net_iovs, so a frag covers at most a page.
> A 64KB skb needs 16 frags, and MAX_SKB_FRAGS caps devmem skbs at about
> 68KB, which makes BIG TCP ineffective for devmem TX.
>
> This series picks the TX net_iov size from the dmabuf sg table instead:
> the largest power of 2, up to 2MB, which aligns every DMA segment. With
> a 2MB hugepage-backed udmabuf a BIG TCP skb takes a few frags. There's
> no UAPI change since TX addresses are dmabuf offsets.
>
> TCP still bounds skbs by size_goal, so frags get larger, skbs don't.
> A single frag can now be as large as the skb, which matters for
> NETMEM_TX drivers with tso_max_size above 64KB:
> - mlx5: 32-bit length per data segment
> - gve: splits frags into 16KB descriptors
> - fbnic: keeps the default 64KB tso_max_size
> - bnxt: 16-bit BD length, devmem TX frags are capped at 32KB by a new
>   per-device limit

Reviewed-by: Mina Almasry <almasrymina@google.com>