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

On Fri Oct 2, 2026 at 8:53 PM EDT, Harshitha Ramamurthy wrote:

On Thu, Oct 1, 2026 at 9:41 AM Daniel Zahka [off-list ref] wrote:

[...]

+void mpnic_napi_depletion_check(struct mpnic_net *mpn)

quoted

+{
+       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.

quoted

+                       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