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

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Message-ID: <39c21e55-4aeb-4935-ab8a-759d97662ec3@gmail.com>
Date: Fri, 9 May 2025 00:30:54 +0300
From: Tariq Toukan <ttoukan.linux@...il.com>
To: Stanislav Fomichev <stfomichev@...il.com>,
 Jason Xing <kerneljasonxing@...il.com>
Cc: 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 08/05/2025 19:22, Stanislav Fomichev wrote:
> 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.

I see.
Patch LGTM, except for the function name nit, pointed out in an earlier 
comment.
We could remove the "hw" from function name mlx5e_tx_skb_update_hwts_flags.