linux-kernel - Re: [PATCH RESEND] can: j1939: do not wait 250ms if the same addr was already claimed

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Message-ID: <20220510042609.GA10669@pengutronix.de>
Date:   Tue, 10 May 2022 06:26:09 +0200
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Devid Antonio Filoni <devid.filoni@...uetechnologies.com>,
        Robin van der Gracht <robin@...tonic.nl>,
        kernel@...gutronix.de, linux-can@...r.kernel.org,
        Oleksij Rempel <linux@...pel-privat.de>,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maxime Jayat <maxime.jayat@...ile-devices.fr>,
        kbuild test robot <lkp@...el.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] can: j1939: do not wait 250ms if the same addr
 was already claimed

Hi,

On Mon, May 09, 2022 at 09:04:06PM +0200, Kurt Van Dijck wrote:
> On ma, 09 mei 2022 19:03:03 +0200, Devid Antonio Filoni wrote:
> > This is not explicitly stated in SAE J1939-21 and some tools used for
> > ISO-11783 certification do not expect this wait.

It will be interesting to know which certification tool do not expect it and
what explanation is used if it fails?

> IMHO, the current behaviour is not explicitely stated, but nor is the opposite.
> And if I'm not mistaken, this introduces a 250msec delay.
> 
> 1. If you want to avoid the 250msec gap, you should avoid to contest the same address.
> 
> 2. It's a balance between predictability and flexibility, but if you try to accomplish both,
> as your patch suggests, there is slight time-window until the current owner responds,
> in which it may be confusing which node has the address. It depends on how much history
> you have collected on the bus.
> 
> I'm sure that this problem decreases with increasing processing power on the nodes,
> but bigger internal queues also increase this window.
> 
> It would certainly help if you describe how the current implementation fails.
> 
> Would decreasing the dead time to 50msec help in such case.
> 
> Kind regards,
> Kurt
> 

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