netdev - Re: [PATCH] can: don't count arbitration lose as an error

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Date:   Wed, 2 Dec 2020 15:35:22 +0100
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Jeroen Hofstee <jhofstee@...tronenergy.com>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        linux-can@...r.kernel.org
Cc:     Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        "open list:NETWORKING DRIVERS" <netdev@...r.kernel.org>,
        "moderated list:ARM/Allwinner sunXi SoC support" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] can: don't count arbitration lose as an error

Hi Jeroen,

On 27.11.20 12:09, Jeroen Hofstee wrote:
> On 11/27/20 11:30 AM, Marc Kleine-Budde wrote:
>> On 11/27/20 10:59 AM, Jeroen Hofstee wrote:
>>> Losing arbitration is normal in a CAN-bus network, it means that a
>>> higher priority frame is being send and the pending message will be
>>> retried later. Hence most driver only increment arbitration_lost, but
>>> the sja1000 and sun4i driver also incremeant tx_error, causing errors
>>> to be reported on a normal functioning CAN-bus. So stop counting them
>>> as errors.
>> Sounds plausible.
>>
>>> For completeness, the Kvaser USB hybra also increments the tx_error
>>> on arbitration lose, but it does so in single shot. Since in that
>>> case the message is not retried, that behaviour is kept.
>> You mean only in one shot mode?
> 
> Yes, well at least the function is called kvaser_usb_hydra_one_shot_fail.
> 
> 
>>   What about one shot mode on the sja1000 cores?
> 
> 
> That is a good question. I guess it will be counted as error by:
> 
>          if (isrc & IRQ_TI) {
>              /* transmission buffer released */
>              if (priv->can.ctrlmode & CAN_CTRLMODE_ONE_SHOT &&
>                  !(status & SR_TCS)) {
>                  stats->tx_errors++;

I can confirm with CAN_CTRLMODE_ONE_SHOT active and the patch ("can: 
sja1000: sja1000_err(): don't count arbitration lose as an error")

https://git.kernel.org/pub/scm/linux/kernel/git/mkl/linux-can.git/commit/?h=testing&id=bd0ccb92efb09c7da5b55162b283b42a93539ed7

I now get ONE(!) increment for tx_errors and ONE increment in the 
arbitration-lost counter.

Before the above patch I had TWO tx_errors for each arbitration lost case.

>                  can_free_echo_skb(dev, 0);
>              } else {
>                  /* transmission complete */
>                  stats->tx_bytes +=
>                      priv->read_reg(priv, SJA1000_FI) & 0xf;
>                  stats->tx_packets++;
>                  can_get_echo_skb(dev, 0);
>              }
>              netif_wake_queue(dev);
>              can_led_event(dev, CAN_LED_EVENT_TX);
>          }
> 
>  From the datasheet, Transmit Interrupt:
> 
> "set; this bit is set whenever the transmit bufferstatus
> changes from ‘0-to-1’ (released) and the TIE bit is set
> within the interrupt enable register".
> 
> I cannot test it though, since I don't have a sja1000.

Do we agree that in one-shot mode both the tx_errors and the 
arbitration_lost counters are increased in the arbitration-lost case?

At least this would fit to the Kvaser USB behaviour.

And btw. I wondered if we should remove the check for 
CAN_CTRLMODE_ONE_SHOT here, as we ALWAYS should count a tx_error and 
drop the echo_skb when we have a TX-interrupt and TX-complete flag is zero.

So replace:

if (priv->can.ctrlmode & CAN_CTRLMODE_ONE_SHOT &&
                   !(status & SR_TCS)) {

with:

if (!(status & SR_TCS)) {

Any suggestions?

Best regards,
Oliver