linux-kernel - Re: [Intel-wired-lan] [PATCH iwl-net v1] igc: fix race condition in TX timestamp read for register 0

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Message-ID: <11e02598-3fc8-4350-91b7-5fc587a8cf6b@molgen.mpg.de>
Date: Thu, 18 Sep 2025 18:03:27 +0200
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: Chwee-Lin Choong <chwee.lin.choong@...el.com>
Cc: Tony Nguyen <anthony.l.nguyen@...el.com>,
 Przemek Kitszel <przemyslaw.kitszel@...el.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S . Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Richard Cochran <richardcochran@...il.com>,
 Vinicius Costa Gomes <vinicius.gomes@...el.com>,
 intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Avi Shalev <avi.shalev@...el.com>,
 Song Yoong Siang <yoong.siang.song@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH iwl-net v1] igc: fix race condition in
 TX timestamp read for register 0

Dear Chwee-Lin,

Thank you for your patch.

Am 18.09.25 um 20:38 schrieb Chwee-Lin Choong:
> 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.

Reading the TXTT mask is `rd32(IGC_TSYNCTXCTL)`, correct?

> 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.

The unit shouldn’t hang, even if valid timestamps are dropped?

Do you have a reproducer?

> Fixes: c789ad7cbebc ("igc: Work around HW bug causing missing timestamps")
> Suggested-by: Avi Shalev <avi.shalev@...el.com>
> Signed-off-by: Song Yoong Siang <yoong.siang.song@...el.com>
> Signed-off-by: Chwee-Lin Choong <chwee.lin.choong@...el.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);
>   

Kind regards,

Paul