linux-kernel - Re: [PATCH net-next] net/mlx5: support software TX timestamp

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Message-ID: <aBzaK8I0hA31ba_4@mini-arch>
Date: Thu, 8 May 2025 09:22:03 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: Tariq Toukan <ttoukan.linux@...il.com>, netdev@...r.kernel.org,
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, saeedm@...dia.com, tariqt@...dia.com,
	andrew+netdev@...n.ch, linux-rdma@...r.kernel.org,
	linux-kernel@...r.kernel.org, leon@...nel.org,
	Carolina Jubran <cjubran@...dia.com>
Subject: Re: [PATCH net-next] net/mlx5: support software TX timestamp

On 05/08, Jason Xing wrote:
> Hi Tariq,
> 
> On Thu, May 8, 2025 at 2:30 PM Tariq Toukan <ttoukan.linux@...il.com> wrote:
> >
> >
> >
> > On 07/05/2025 0:55, Stanislav Fomichev wrote:
> > > Having a software timestamp (along with existing hardware one) is
> > > useful to trace how the packets flow through the stack.
> > > mlx5e_tx_skb_update_hwts_flags is called from tx paths
> > > to setup HW timestamp; extend it to add software one as well.
> > >
> > > Signed-off-by: Stanislav Fomichev <stfomichev@...il.com>
> > > ---
> > >   drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c | 1 +
> > >   drivers/net/ethernet/mellanox/mlx5/core/en_tx.c      | 1 +
> > >   2 files changed, 2 insertions(+)
> > >
> > > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c b/drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c
> > > index fdf9e9bb99ac..e399d7a3d6cb 100644
> > > --- a/drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c
> > > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c
> > > @@ -1689,6 +1689,7 @@ int mlx5e_ethtool_get_ts_info(struct mlx5e_priv *priv,
> > >               return 0;
> > >
> > >       info->so_timestamping = SOF_TIMESTAMPING_TX_HARDWARE |
> > > +                             SOF_TIMESTAMPING_TX_SOFTWARE |
> > >                               SOF_TIMESTAMPING_RX_HARDWARE |
> > >                               SOF_TIMESTAMPING_RAW_HARDWARE;
> > >
> > > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
> > > index 4fd853d19e31..f6dd26ad29e5 100644
> > > --- a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
> > > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
> > > @@ -341,6 +341,7 @@ static void mlx5e_tx_skb_update_hwts_flags(struct sk_buff *skb)
> > >   {
> > >       if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP))
> > >               skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
> > > +     skb_tx_timestamp(skb);
> >
> > Doesn't this interfere with skb_tstamp_tx call in the completion flow
> > (mlx5e_consume_skb)?
> 
> skb_tstamp_tx() only takes care of hardware timestamp in this case.
> 
> >
> > What happens if both flags (SKBTX_SW_TSTAMP / SKBTX_HW_TSTAMP) are set
> > Is it possible?
> 
> If only these two are set, only hardware timestamp will be passed to
> the userspace because of the SOF_TIMESTAMPING_OPT_TX_SWHW limit in
> __skb_tstamp_tx().
> 
> If users expect to see both timestamps, then
> SOF_TIMESTAMPING_OPT_TX_SWHW has to be set.

Right, skb_tx_timestamp does nothing and bails out if it detects
SKBTX_IN_PROGRESS. And skb_tstamp_tx in mlx5e_consume_skb handles
only (and will trigger only for) HW tstamp case.