[PATCHv2 net] net: emac: mal: fix W1C write race in ICINTSTAT clearing

[Rosen Penev <rosenp@gmail.com>]

The ICINTSTAT register is write-1-to-clear (W1C).  The read-modify-write
pattern in both mal_txeob() and mal_rxeob() can lose interrupts: if a bit
that should not be cleared is already asserted when mfdcri() reads the
register, it is included in the read value, retained by the bitwise OR, and
then written back as 1 - inadvertently clearing a pending but unhandled
interrupt.

Fix by writing only the specific bit to clear (ICINTSTAT_ICTX for TXEOB,
ICINTSTAT_ICRX for RXEOB).  W1C semantics guarantee that writing 0 to the
other bits has no effect.

Tested on Cisco Meraki MX60. No issues seen.

Fixes: 1d3bb996481e ("Device tree aware EMAC driver")
Assisted-by: opencode:big-pickle
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 v2: mention that this was tested.
 drivers/net/ethernet/ibm/emac/mal.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/ibm/emac/mal.c b/drivers/net/ethernet/ibm/emac/mal.c
index 74526002d52b..e88e4a1ddcd4 100644
--- a/drivers/net/ethernet/ibm/emac/mal.c
+++ b/drivers/net/ethernet/ibm/emac/mal.c
@@ -282,8 +282,7 @@ static irqreturn_t mal_txeob(int irq, void *dev_instance)
 
 #ifdef CONFIG_PPC_DCR_NATIVE
 	if (mal_has_feature(mal, MAL_FTR_CLEAR_ICINTSTAT))
-		mtdcri(SDR0, DCRN_SDR_ICINTSTAT,
-				(mfdcri(SDR0, DCRN_SDR_ICINTSTAT) | ICINTSTAT_ICTX));
+		mtdcri(SDR0, DCRN_SDR_ICINTSTAT, ICINTSTAT_ICTX);
 #endif
 
 	return IRQ_HANDLED;
@@ -302,8 +301,7 @@ static irqreturn_t mal_rxeob(int irq, void *dev_instance)
 
 #ifdef CONFIG_PPC_DCR_NATIVE
 	if (mal_has_feature(mal, MAL_FTR_CLEAR_ICINTSTAT))
-		mtdcri(SDR0, DCRN_SDR_ICINTSTAT,
-				(mfdcri(SDR0, DCRN_SDR_ICINTSTAT) | ICINTSTAT_ICRX));
+		mtdcri(SDR0, DCRN_SDR_ICINTSTAT, ICINTSTAT_ICRX);
 #endif
 
 	return IRQ_HANDLED;
-- 
2.55.0