[PATCH 1/2] net: fec: don't ack masked interrupt events

The FEC doesn't have a real interrupt status register, that takes
into account the mask status of the IRQ. The driver reads the raw
interrupt event register, which also reports events for masked
IRQs.

The driver needs to apply the current mask itself, to avoid acking
IRQs that are currently masked, as NAPI relies on the masking to
hide the IRQs. The current behavior of just acking all interrupts
regardless of their mask status opens the driver up the "rotting
packet" race-window, as described in the original NAPI-HOWTO, which
has been observed in the wild.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
---
 drivers/net/ethernet/freescale/fec_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
index 610573855213..0b70c07eb703 100644
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c

@@ -1584,7 +1584,8 @@ fec_enet_interrupt(int irq, void *dev_id)
 	uint int_events;
 	irqreturn_t ret = IRQ_NONE;
 
-	int_events = readl(fep->hwp + FEC_IEVENT);
+	int_events = readl_relaxed(fep->hwp + FEC_IEVENT) &
+	             readl_relaxed(fep->hwp + FEC_IMASK);

fep->work_rx and fep->work_tx are both non-zero, as long as the NAPI
softirq hasn't finished its work. So if the current IRQ does not signal any RX or
TX completion, but some unrelated event, the path to schedule the NAPI
context is still entered.

The handler works correctly as in this case napi_schedule_prep() will reject
the scheduling attempt, but the flow can still be optimized by not trying to
schedule if the IRQ doesn't signal RX or TX completion.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
---
drivers/net/ethernet/freescale/fec_main.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/freescale/fec_main.c

b/drivers/net/ethernet/freescale/fec_main.c
index 0b70c07eb703..2043e140e9bd 100644

--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c

@@ -1587,14 +1587,14 @@ fec_enet_interrupt(int irq, void *dev_id)

	int_events = readl_relaxed(fep->hwp + FEC_IEVENT) &
	             readl_relaxed(fep->hwp + FEC_IMASK);
	writel(int_events, fep->hwp + FEC_IEVENT);
-	fec_enet_collect_events(fep, int_events);

-	if ((fep->work_tx || fep->work_rx) && fep->link) {
+	if ((int_events & (FEC_ENET_RXF | FEC_ENET_TXF)) && fep->link) {
		ret = IRQ_HANDLED;

		if (napi_schedule_prep(&fep->napi)) {
			/* Disable the NAPI interrupts */
			writel(FEC_NAPI_IMASK, fep->hwp + FEC_IMASK);
+			fec_enet_collect_events(fep, int_events);
			__napi_schedule(&fep->napi);
		}
	}
--
2.11.0