From: Chwee-Lin Choong The current HW bug workaround checks the TXTT_0 ready bit first, then reads TXSTMPL_0 twice (before and after reading TXSTMPH_0) to detect whether a new timestamp was captured by timestamp register 0 during the workaround. This sequence has a race: if a new timestamp is captured after checking the TXTT_0 bit but before the first TXSTMPL_0 read, the detection fails because both the "old" and "new" values come from the same timestamp. Fix by reading TXSTMPL_0 first to establish a baseline, then checking the TXTT_0 bit. This ensures any timestamp captured during the race window will be detected. Old sequence: 1. Check TXTT_0 ready bit 2. Read TXSTMPL_0 (baseline) 3. Read TXSTMPH_0 (interrupt workaround) 4. Read TXSTMPL_0 (detect changes vs baseline) New sequence: 1. Read TXSTMPL_0 (baseline) 2. Check TXTT_0 ready bit 3. Read TXSTMPH_0 (interrupt workaround) 4. Read TXSTMPL_0 (detect changes vs baseline) Fixes: c789ad7cbebc ("igc: Work around HW bug causing missing timestamps") Suggested-by: Avi Shalev Reviewed-by: Aleksandr Loktionov Co-developed-by: Song Yoong Siang Signed-off-by: Song Yoong Siang Signed-off-by: Chwee-Lin Choong Tested-by: Avigail Dahan Signed-off-by: Tony Nguyen --- drivers/net/ethernet/intel/igc/igc_ptp.c | 43 ++++++++++++++---------- 1 file changed, 25 insertions(+), 18 deletions(-) diff --git a/drivers/net/ethernet/intel/igc/igc_ptp.c b/drivers/net/ethernet/intel/igc/igc_ptp.c index b7b46d863bee..7aae83c108fd 100644 --- a/drivers/net/ethernet/intel/igc/igc_ptp.c +++ b/drivers/net/ethernet/intel/igc/igc_ptp.c @@ -774,36 +774,43 @@ 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; + /* Establish baseline of TXSTMPL_0 before checking TXTT_0. + * This baseline is used to detect if a new timestamp arrives in + * register 0 during the hardware bug workaround below. + */ + txstmpl_old = rd32(IGC_TXSTMPL); + mask = rd32(IGC_TSYNCTXCTL) & IGC_TSYNCTXCTL_TXTT_ANY; if (mask & IGC_TSYNCTXCTL_TXTT_0) { regval = rd32(IGC_TXSTMPL); regval |= (u64)rd32(IGC_TXSTMPH) << 32; } else { - /* There's a bug in the hardware that could cause - * missing interrupts for TX timestamping. The issue - * is that for new interrupts to be triggered, the - * IGC_TXSTMPH_0 register must be read. + /* TXTT_0 not set - register 0 has no new timestamp initially. + * + * Hardware bug: Future timestamp interrupts won't fire unless + * TXSTMPH_0 is read, even if the timestamp was captured in + * registers 1-3. * - * To avoid discarding a valid timestamp that just - * happened at the "wrong" time, we need to confirm - * that there was no timestamp captured, we do that by - * assuming that no two timestamps in sequence have - * the same nanosecond value. + * Workaround: Read TXSTMPH_0 here to enable future interrupts. + * However, this read clears TXTT_0. If a timestamp arrives in + * register 0 after checking TXTT_0 but before this read, it + * would be lost. * - * So, we read the "low" register, read the "high" - * register (to latch a new timestamp) and read the - * "low" register again, if "old" and "new" versions - * of the "low" register are different, a valid - * timestamp was captured, we can read the "high" - * register again. + * To detect this race: We saved a baseline read of TXSTMPL_0 + * before TXTT_0 check. After performing the workaround read of + * TXSTMPH_0, we read TXSTMPL_0 again. Since consecutive + * timestamps never share the same nanosecond value, a change + * between the baseline and new TXSTMPL_0 indicates a timestamp + * arrived during the race window. If so, read the complete + * timestamp. */ - u32 txstmpl_old, txstmpl_new; + u32 txstmpl_new; - txstmpl_old = rd32(IGC_TXSTMPL); rd32(IGC_TXSTMPH); txstmpl_new = rd32(IGC_TXSTMPL); @@ -818,7 +825,7 @@ static void igc_ptp_tx_hwtstamp(struct igc_adapter *adapter) done: /* Now that the problematic first register was handled, we can - * use retrieve the timestamps from the other registers + * retrieve the timestamps from the other registers * (starting from '1') with less complications. */ for (i = 1; i < IGC_MAX_TX_TSTAMP_REGS; i++) { -- 2.47.1