[PATCH] can: rcar_canfd: fix possible IRQ storm on high load

[Nikita Yushchenko <nikita.yoush@cogentembedded.com>]

We have observed rcar_canfd driver entering IRQ storm under high load,
with following scenario:
- rcar_canfd_global_interrupt() in entered due to Rx available,
- napi_schedule_prep() is called, and sets NAPIF_STATE_SCHED in state
- Rx fifo interrupts are masked,
- rcar_canfd_global_interrupt() is entered again, this time due to
  error interrupt (e.g. due to overflow),
- since scheduled napi poller has not yet executed, condition for calling
  napi_schedule_prep() from rcar_canfd_global_interrupt() remains true,
  thus napi_schedule_prep() gets called and sets NAPIF_STATE_MISSED flag
  in state,
- later, napi poller function rcar_canfd_rx_poll() gets executed, and
  calls napi_complete_done(),
- due to NAPIF_STATE_MISSED flag in state, this call does not clear
  NAPIF_STATE_SCHED flag from state,
- on return from napi_complete_done(), rcar_canfd_rx_poll() unmasks Rx
  interrutps,
- Rx interrupt happens, rcar_canfd_global_interrupt() gets called
  and calls napi_schedule_prep(),
- since NAPIF_STATE_SCHED is set in state at this time, this call
  returns false,
- due to that false return, rcar_canfd_global_interrupt() returns
  without masking Rx interrupt
- and this results into IRQ storm: unmasked Rx interrupt happens again
  and again is misprocessed in the same way.

This patch fixes that scenario by unmasking Rx interrupts only when
napi_complete_done() returns true, which means it has cleared
NAPIF_STATE_SCHED in state.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
---
 drivers/net/can/rcar/rcar_canfd.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/net/can/rcar/rcar_canfd.c b/drivers/net/can/rcar/rcar_canfd.c
index 05410008aa6b..de34a4b82d4a 100644
--- a/drivers/net/can/rcar/rcar_canfd.c
+++ b/drivers/net/can/rcar/rcar_canfd.c
@@ -1508,10 +1508,11 @@ static int rcar_canfd_rx_poll(struct napi_struct *napi, int quota)
 
 	/* All packets processed */
 	if (num_pkts < quota) {
-		napi_complete_done(napi, num_pkts);
-		/* Enable Rx FIFO interrupts */
-		rcar_canfd_set_bit(priv->base, RCANFD_RFCC(ridx),
-				   RCANFD_RFCC_RFIE);
+		if (napi_complete_done(napi, num_pkts)) {
+			/* Enable Rx FIFO interrupts */
+			rcar_canfd_set_bit(priv->base, RCANFD_RFCC(ridx),
+					   RCANFD_RFCC_RFIE);
+		}
 	}
 	return num_pkts;
 }
-- 
2.11.0