linux-kernel - Re: [PATCH v3 1/1] can: dev: add software tx timestamps

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Message-ID: <69d878d4-78cc-82da-e4eb-b494ebdcd2b1@pengutronix.de>
Date:   Mon, 11 Jan 2021 09:10:43 +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>
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>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] can: dev: add software tx timestamps

On 1/10/21 1:49 PM, 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.
> 
> Remarks:
> 
>   * 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.
> 
>   * The hardware rx timestamp of a local loopback message is the
>     hardware tx timestamp. This means that there are no needs to
>     implement SOF_TIMESTAMPING_TX_HARDWARE for CAN sockets.
> 
> 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
> 
> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>

Applied to linux-can-next/testing (ontop of my dev infrastructure cleanup).

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |



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