Re: [PATCH net-next 4/4] eth: mpnic: add a NAPI depletion check

[Daniel Zahka]

On Fri Oct 2, 2026 at 8:53 PM EDT, Harshitha Ramamurthy wrote:
> On Thu, Oct 1, 2026 at 9:41 AM Daniel Zahka <daniel.zahka@xxxxxxxxx> wrote:
[...]
> +void mpnic_napi_depletion_check(struct mpnic_net *mpn)
>> +{
>> +       int i, j, t;
>> +
>> +       for (i = 0; i < mpn->num_napi; i++) {
>> +               struct mpnic_napi_vector *nv = mpn->napi[i];
>> +
>> +               for (t = nv->txt_count, j = 0; j < nv->rxt_count; j++, t++) {
>> +                       /* Check if BDs posted covers a max sized frame
>> +                        *  + 1 BD held by RDE as a spare
>> +                        *  + 1 BD of extra safety margin
>> +                        */
>
> Curious how the "+ 1 BD of extra safety margin" was decided. I
> understand the "+ 1 BD held by RDE as a spare" is because of HW
> behaviour but wondering about the extra buffer descriptor - if it came
> from testing or guarding against something specific.
>

The extra +1 isn't strictly necessary. The real lower bound is
MPNIC_RX_HPQ_DROP_THRS + 1, which was derived from the design of the
device and our napi poll implementation. I added an extra BD count
because the actual bound doesn't matter as long as its conservative.
Perhaps that's a bit superstitious.

>> +                       if (mpnic_desc_used(&nv->qt[t].sub0) <
>> +                           MPNIC_RX_HPQ_DROP_THRS + 2 ||
>> +                           mpnic_desc_used(&nv->qt[t].sub1) <
>> +                           MPNIC_RX_PPQ_DROP_THRS + 2) {
>> +                               mpnic_nv_irq_trigger(nv);
>> +                               break;
>> +                       }
>> +               }
>> +       }
>> +
>> +       mpnic_wrfl(mpn->mpd);
>> +}
>> diff --git a/drivers/net/ethernet/meta/mpnic/mpnic_txrx.h b/drivers/net/ethernet/meta/mpnic/mpnic_txrx.h
>> index 9397010557eb..767d87a36c35 100644
>> --- a/drivers/net/ethernet/meta/mpnic/mpnic_txrx.h
>> +++ b/drivers/net/ethernet/meta/mpnic/mpnic_txrx.h
>> @@ -148,6 +148,7 @@ int mpnic_set_netif_queues(struct mpnic_net *mpn);
>>  void mpnic_reset_netif_queues(struct mpnic_net *mpn);
>>  void mpnic_napi_enable(struct mpnic_net *mpn);
>>  void mpnic_napi_disable(struct mpnic_net *mpn);
>> +void mpnic_napi_depletion_check(struct mpnic_net *mpn);
>>  void mpnic_enable(struct mpnic_net *mpn);
>>  void mpnic_disable(struct mpnic_net *mpn);
>>  void mpnic_wait_all_queues_idle(struct mpnic_dev *mpd);
>>
>> --
>> 2.52.0
>>
>>