netdev - Re: [PATCH v4 1/1] can: dev: add software tx timestamps

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Message-ID: <f9ebb060-f190-79af-d57a-d5394390d222@pengutronix.de>
Date:   Tue, 12 Jan 2021 11:03:00 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        linux-can@...r.kernel.org,
        Jeroen Hofstee <jhofstee@...tronenergy.com>,
        Richard Cochran <richardcochran@...il.com>
Cc:     Wolfgang Grandegger <wg@...ndegger.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "open list : NETWORKING DRIVERS" <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 1/1] can: dev: add software tx timestamps

On 1/12/21 10:54 AM, Vincent Mailhol wrote:
> Call skb_tx_timestamp() within can_put_echo_skb() so that a software
> tx timestamp gets attached on the skb.
> 
> There two main reasons to include this call in can_put_echo_skb():
> 
>   * It easily allow to enable the tx timestamp on all devices with
>     just one small change.
> 
>   * According to Documentation/networking/timestamping.rst, the tx
>     timestamps should be generated in the device driver as close as
>     possible, but always prior to passing the packet to the network
>     interface. During the call to can_put_echo_skb(), the skb gets
>     cloned meaning that the driver should not dereference the skb
>     variable anymore after can_put_echo_skb() returns. This makes
>     can_put_echo_skb() the very last place we can use the skb without
>     having to access the echo_skb[] array.
> 
> Remark: by default, skb_tx_timestamp() does nothing. It needs to be
> activated by passing the SOF_TIMESTAMPING_TX_SOFTWARE flag either
> through socket options or control messages.
> 
> References:
> 
>  * Support for the error queue in CAN RAW sockets (which is needed for
>    tx timestamps) was introduced in:
>    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=eb88531bdbfaafb827192d1fc6c5a3fcc4fadd96
> 
>   * Put the call to skb_tx_timestamp() just before adding it to the
>     array: https://lkml.org/lkml/2021/1/10/54
> 
>   * About Tx hardware timestamps
>     https://lore.kernel.org/linux-can/20210111171152.GB11715@hoboy.vegasvil.org/
> 
> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>

Applied to linux-can-next/testing

tnx,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
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