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

[Lucas Stach <l.stach@pengutronix.de>]

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);
 	writel(int_events, fep->hwp + FEC_IEVENT);
 	fec_enet_collect_events(fep, int_events);
 
-- 
2.11.0