[PATCH iwl-net v1] igc: fix race condition in TX timestamp read for register 0

[Chwee-Lin Choong <chwee.lin.choong@intel.com>]

The current HW bug workaround checks the TXTT_0 ready bit first,
then reads LOW -> HIGH -> LOW from register 0 to detect if a
timestamp was captured.

This sequence has a race: if a new timestamp is latched after
reading the TXTT mask but before the first LOW read, both old
and new timestamp match, causing the driver to drop a valid
timestamp.

Fix by reading the LOW register first, then the TXTT mask,
so a newly latched timestamp will always be detected.

This fix also prevents TX unit hangs observed under heavy
timestamping load.

Fixes: c789ad7cbebc ("igc: Work around HW bug causing missing timestamps")
Suggested-by: Avi Shalev <avi.shalev@intel.com>
Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
Signed-off-by: Chwee-Lin Choong <chwee.lin.choong@intel.com>
---
 drivers/net/ethernet/intel/igc/igc_ptp.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/intel/igc/igc_ptp.c b/drivers/net/ethernet/intel/igc/igc_ptp.c
index b7b46d863bee..930486b02fc1 100644
--- a/drivers/net/ethernet/intel/igc/igc_ptp.c
+++ b/drivers/net/ethernet/intel/igc/igc_ptp.c
@@ -774,10 +774,17 @@ static void igc_ptp_tx_reg_to_stamp(struct igc_adapter *adapter,
 static void igc_ptp_tx_hwtstamp(struct igc_adapter *adapter)
 {
 	struct igc_hw *hw = &adapter->hw;
+	u32 txstmpl_old;
 	u64 regval;
 	u32 mask;
 	int i;
 
+	/* Read the "low" register 0 first to establish a baseline value.
+	 * This avoids a race where a new timestamp could be latched
+	 * after checking the TXTT mask.
+	 */
+	txstmpl_old = rd32(IGC_TXSTMPL);
+
 	mask = rd32(IGC_TSYNCTXCTL) & IGC_TSYNCTXCTL_TXTT_ANY;
 	if (mask & IGC_TSYNCTXCTL_TXTT_0) {
 		regval = rd32(IGC_TXSTMPL);
@@ -801,9 +808,8 @@ static void igc_ptp_tx_hwtstamp(struct igc_adapter *adapter)
 		 * timestamp was captured, we can read the "high"
 		 * register again.
 		 */
-		u32 txstmpl_old, txstmpl_new;
+		u32 txstmpl_new;
 
-		txstmpl_old = rd32(IGC_TXSTMPL);
 		rd32(IGC_TXSTMPH);
 		txstmpl_new = rd32(IGC_TXSTMPL);
 
-- 
2.42.0