Re: [net-next v9 07/10] net: bnxt: Implement software USO

Jakub Kicinski
2026-04-08T18:06:54Z

On Wed, 8 Apr 2026 10:04:55 -0700 Joe Damato wrote:
> > This is the check I added. AI says this is wrong and netdev_queues.h says:
> >
> > * @get_desc must be a formula or a function call, it must always
> > * return up-to-date information when evaluated!
> >
> > which I obviously failed to do, so I'm pretty sure I got this wrong.
>
> So, there's two options to fix this that I can think of. I am leaning torward
> option 2, but if there are any strong opinions (or other options that I am
> missing) please let me know:
>
> 1. Allocate the maximum number of slots per ring and eliminate this check
> entirely. I figured this would be disliked because it potentially wastes
> memory. The driver would need ring_size / 3 slots, and if we assume the
> maximum is 2048 and the slot size is 256b, that works out to 175kb per
> ring. Of course, this only affects NICs with SW USO and the buffer isn't
> allocated for NICS with HW USO.
>
> This is probably simpler, but costs more memory than the existing design.
>
> 2. Or, keep the smaller buffer that we have now (BNXT_SW_USO_MAX_SEGS (64)
> * 256b = 16kb per ring) and fix the try_stop like this:
>
> +static inline u16 bnxt_inline_avail(struct bnxt_tx_ring_info *txr)
> +{
> + return BNXT_SW_USO_MAX_SEGS -
> + (u16)(txr->tx_inline_prod - READ_ONCE(txr->tx_inline_cons));
> +}
> +
>
> - slots = txr->tx_inline_prod - txr->tx_inline_cons;
> - slots = BNXT_SW_USO_MAX_SEGS - slots;
> -
> - if (unlikely(slots < num_segs)) {
> - netif_txq_try_stop(txq, slots, num_segs);
> + if (unlikely(bnxt_inline_avail(txr) < num_segs)) {
> + netif_txq_try_stop(txq, bnxt_inline_avail(txr), num_segs);

I think option 2 makes sense. The point (which I think you got) is that
the condition must be evaluated after the memory barrier.

Since the condition is repeated in your latest snippet - you can
probably use netif_txq_maybe_stop() ?