FAILED: patch "[PATCH] can: rcar_canfd: fix possible IRQ storm on high load" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From d4b890aec4bea7334ca2ca56fd3b12fb48a00cd1 Mon Sep 17 00:00:00 2001
From: Nikita Yushchenko <[email protected]>
Date: Wed, 26 Jun 2019 16:08:48 +0300
Subject: [PATCH] can: rcar_canfd: fix possible IRQ storm on high load

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.

index 05410008aa6b..de34a4b82d4a 100644
@@ -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;
 }