[PATCH 2/3] net: stmmac: Prevent indefinite RX stall on buffer exhaustion

[Sam Edwards <cfsworks@gmail.com>]

The stmmac driver handles interrupts in the usual NAPI way: an interrupt
arrives, the NAPI instance is scheduled and interrupts are masked, and
the actual work occurs in the NAPI polling function. Once no further
work remains, interrupts are unmasked and the NAPI instance is put to
sleep to await a future interrupt. In the receive case, the MAC only
sends the interrupt when a DMA operation completes; thus the driver must
make sure a usable RX DMA descriptor exists before expecting a future
interrupt.

The main receive loop in stmmac_rx() exits under one of 3 conditions:
1) It encounters a DMA descriptor with OWN=1, indicating that no further
   pending data exists. The MAC will use this descriptor for the next
   RX DMA operation, so the driver can expect a future interrupt.
2) It exhausts the NAPI budget. In this case, the driver doesn't know
   whether the MAC has any usable DMA descriptors. But when the driver
   consumes its full budget, that signals NAPI to keep polling, so the
   question is moot.
3) It runs out of (non-dirty) descriptors in the RX ring. In this case,
   the MAC will only have a usable descriptor if stmmac_rx_refill()
   succeeds (at least partially).

Currently, stmmac_rx() lacks any check against scenario #3 and
stmmac_rx_refill() failing: it will stop NAPI polling and unmask
interrupts to await an interrupt that will never arrive, stalling the
receive pipeline indefinitely.

Fix this by checking if stmmac_rx_dirty() returns its maximal value,
returning the full budget (which tells NAPI to keep polling) if so.

Cc: stable@vger.kernel.org
Signed-off-by: Sam Edwards <CFSworks@gmail.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index f98b070073c0..d18ee145f5ca 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -5593,6 +5593,7 @@ static int stmmac_rx_zc(struct stmmac_priv *priv, int limit, u32 queue)
  */
 static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 {
+	int budget = limit;
 	u32 rx_errors = 0, rx_dropped = 0, rx_bytes = 0, rx_packets = 0;
 	struct stmmac_rxq_stats *rxq_stats = &priv->xstats.rxq_stats[queue];
 	struct stmmac_rx_queue *rx_q = &priv->dma_conf.rx_queue[queue];
@@ -5870,6 +5871,12 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 	priv->xstats.rx_dropped += rx_dropped;
 	priv->xstats.rx_errors += rx_errors;
 
+	/* If the RX queue is completely dirty, we can't expect a future
+	 * interrupt; tell NAPI to keep polling.
+	 */
+	if (unlikely(stmmac_rx_dirty(priv, queue) == priv->dma_conf.dma_rx_size - 1))
+		return budget;
+
 	return count;
 }
 
-- 
2.52.0