Re: [PATCH iwl-net v4] igc: fix race condition in TX timestamp read for register 0

[Vinicius Costa Gomes <vinicius.gomes@intel.com>]

Hi,

Chwee-Lin Choong <chwee.lin.choong@intel.com> writes:

> 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 <avi.shalev@intel.com>
> Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
> Co-developed-by: Song Yoong Siang <yoong.siang.song@intel.com>
> Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
> Signed-off-by: Chwee-Lin Choong <chwee.lin.choong@intel.com>

Patch looks good, my only concern is this report:

https://lore.kernel.org/intel-wired-lan/AS1PR10MB5675DBFE7CE5F2A9336ABFA4EBEAA@AS1PR10MB5675.EURPRD10.PROD.OUTLOOK.COM/

It's not clear to me how/why the different buffer utilization is
affecting this, but at least seems worth some investigation and
reporting back in that thread.

> ---
> v1: https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20250918183811.31270-1-chwee.lin.choong@intel.com/
> v2: https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20251127134927.2133-1-chwee.lin.choong@intel.com/
> v3: https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20251127151137.2883-1-chwee.lin.choong@intel.com/
>
> changelog:
> v1 -> v2 
> - Added detailed comments explaining the hardware bug workaround and race
>     detection mechanism
> v2 -> v3
> - Removed extra export file added by mistake	
> v3 -> v4
> - Added co-developer
> ---
>  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++) {
> -- 
> 2.43.0
>

-- 
Vinicius