netdev - Re: [PATCH net-next 5/7] bnxt_en: Get the RX packet timestamp.

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Date:   Fri, 28 May 2021 18:39:08 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Michael Chan <michael.chan@...adcom.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, gospo@...adcom.com,
        richardcochran@...il.com, pavan.chebbi@...adcom.com,
        edwin.peer@...adcom.com
Subject: Re: [PATCH net-next 5/7] bnxt_en: Get the RX packet timestamp.

On Fri, 28 May 2021 20:53:19 -0400 Michael Chan wrote:
> +	struct bnxt_ptp_cfg *ptp = bp->ptp_cfg;
> +	u64 time;
> +
> +	if (!ptp)
> +		return -ENODEV;
> +
> +	time = READ_ONCE(ptp->old_time);

READ_ONCE() on a u64? That's not gonna prevent tearing the read on 32
bit architectures, right?

> +	*ts = (time & BNXT_HI_TIMER_MASK) | pkt_ts;
> +	if (pkt_ts < (time & BNXT_LO_TIMER_MASK))
> +		*ts += BNXT_LO_TIMER_MASK + 1;

The stamp is from the MAC, I hope, or otherwise packet which could have
been sitting on the ring for some approximation of eternity. You can
easily see a packet stamp older than the value stashed in ptp->old_time
if you run soon after the refresh.