linux-kernel - Re: [PATCH] can: netlink: Fix TDCO calculation using the old data bittiming

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Message-ID: <d4b3bc0b-95e3-470e-9993-a9f01a18364f@hartkopp.net>
Date: Wed, 21 Feb 2024 15:38:06 +0100
From: Oliver Hartkopp <socketcan@...tkopp.net>
To: Marc Kleine-Budde <mkl@...gutronix.de>
Cc: Vincent MAILHOL <mailhol.vincent@...adoo.fr>,
 Maxime Jayat <maxime.jayat@...ile-devices.fr>,
 Wolfgang Grandegger <wg@...ndegger.com>, linux-can@...r.kernel.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] can: netlink: Fix TDCO calculation using the old data
 bittiming

Hello Marc,

On 21.02.24 15:24, Marc Kleine-Budde wrote:
> On 21.02.2024 15:14:02, Oliver Hartkopp wrote:
>> I have an old PCAN USB adapter (Classical CAN) which uses the pcan_usb
>> driver and wanted to set a 50kbit/s bitrate:
>>
>> ip link set can0 up txqueuelen 500 type can bitrate 50000 sjw 4
>>
>> First it complained about the SJW having a higher value than some phase-seg
>> value which was 2.
>>
>> Error: sjw: 4 greater than phase-seg2: 2.
>>
>> I always thought the driver automatically adapts the SJW value to the
>> highest possible and SJW=4 could always be set. Did this change at a certain
>> point?
> 
> Yes, that changed with b5a3d0864ee7 ("can: bittiming: can_sjw_check():
> check that SJW is not longer than either Phase Buffer Segment")
> 
> See discussion in https://lore.kernel.org/all/20220907103845.3929288-3-mkl@pengutronix.de/

Ok, thanks.

>> Anyway, then I reduced the given SJW value and the ip command did not give
>> any error message.
>>
>> But finally there was not CAN traffic possible with my "always working
>> setup".
>>
>> I'm running 6.8.0-rc4-00433-g92a355464776 from Linus' tree.
>>
>> Reverting this patch fixed my issue.
> 
> But what has the tdco calculation to do with non CAN-FD controllers?

Hm, I was using a Debian 6.1 kernel in between, which worked.

Then I booted my 6.8.0-rc4-00433-g92a355464776 again, reverted the patch 
and loaded the changed can-dev module by hand - which then also worked.

To double check now I was unloading the patched version and using the 
can-dev from /lib/modules again, also works ¯\_(ツ)_/¯

So, I'll have to make more investigations what really went wrong here.
I also was astonished about such CAN FD dependency.

Will come back with more info about my experienced issue.
So far please drop my blame for this patch.

Best regards,
Oliver