[PATCH net] net: bgmac: don't process RX with a zero NAPI budget

[Rosen Penev]

netpoll calls the NAPI poll with a budget of 0 and expects only TX
completions to be handled. bgmac_poll() passes the budget straight to
bgmac_dma_rx_read(), which checks it only after handling a descriptor,
so one received frame is always processed and bgmac_poll() returns 1.
That trips the "exceeded budget in poll" WARN_ONCE() in poll_one_napi()
when netconsole is used, and passes a frame up the stack from netpoll
context.

Return right after cleaning TX when the budget is 0. NAPI stays
scheduled, so the pending RX frames are picked up by the next regular
poll.

Fixes: dd4544f05469 ("bgmac: driver for GBit MAC core on BCMA bus")
Assisted-by: LLM
Signed-off-by: Rosen Penev <rosenp@xxxxxxxxx>
---
 drivers/net/ethernet/broadcom/bgmac.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/ethernet/broadcom/bgmac.c b/drivers/net/ethernet/broadcom/bgmac.c
index 427a096e5ad4..b1cb973caee5 100644
--- a/drivers/net/ethernet/broadcom/bgmac.c
+++ b/drivers/net/ethernet/broadcom/bgmac.c
@@ -1154,6 +1154,11 @@ static int bgmac_poll(struct napi_struct *napi, int weight)
 	bgmac_write(bgmac, BGMAC_INT_STATUS, ~0);
 
 	bgmac_dma_tx_free(bgmac, &bgmac->tx_ring[0]);
+
+	/* A zero budget (netpoll) means only TX may be cleaned */
+	if (unlikely(!weight))
+		return 0;
+
 	handled += bgmac_dma_rx_read(bgmac, &bgmac->rx_ring[0], weight);
 
 	/* Poll again if more events arrived in the meantime */
-- 
2.56.0