netdev - Re: [PATCH net-next v2 2/5] ptp: Request cycles for TX timestamp

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Date:   Sat, 9 Apr 2022 23:47:51 -0700
From:   Richard Cochran <richardcochran@...il.com>
To:     Gerhard Engleder <gerhard@...leder-embedded.com>
Cc:     vinicius.gomes@...el.com, yangbo.lu@....com, davem@...emloft.net,
        kuba@...nel.org, mlichvar@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 2/5] ptp: Request cycles for TX timestamp

On Sun, Apr 03, 2022 at 07:55:41PM +0200, Gerhard Engleder wrote:
> The free running cycle counter of physical clocks called cycles shall be
> used for hardware timestamps to enable synchronisation.
> 
> Introduce new flag SKBTX_HW_TSTAMP_USE_CYCLES, which signals driver to
> provide a TX timestamp based on cycles if cycles are supported.
> 
> Signed-off-by: Gerhard Engleder <gerhard@...leder-embedded.com>
> ---
>  include/linux/skbuff.h |  3 +++
>  net/core/skbuff.c      |  2 ++
>  net/socket.c           | 11 ++++++++++-
>  3 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 3a30cae8b0a5..aeb3ed4d6cf8 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -578,6 +578,9 @@ enum {
>  	/* device driver is going to provide hardware time stamp */
>  	SKBTX_IN_PROGRESS = 1 << 2,
>  
> +	/* generate hardware time stamp based on cycles if supported */
> +	SKBTX_HW_TSTAMP_USE_CYCLES = 1 << 3,
> +
>  	/* generate wifi status information (where possible) */
>  	SKBTX_WIFI_STATUS = 1 << 4,
>  
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 10bde7c6db44..c0f8f1341c3f 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4847,6 +4847,8 @@ void __skb_tstamp_tx(struct sk_buff *orig_skb,
>  		skb_shinfo(skb)->tx_flags |= skb_shinfo(orig_skb)->tx_flags &
>  					     SKBTX_ANY_TSTAMP;
>  		skb_shinfo(skb)->tskey = skb_shinfo(orig_skb)->tskey;
> +	} else {
> +		skb_shinfo(skb)->tx_flags &= ~SKBTX_HW_TSTAMP_USE_CYCLES;

Why is this needed?

>  	}
>  
>  	if (hwtstamps)

Thanks,
Richard