netdev - Re: [PATCH 7/7] can: m_can: workaround for transmit data less than 4 bytes

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Date:	Tue, 04 Nov 2014 14:13:30 +0100
From:	Oliver Hartkopp <socketcan@...tkopp.net>
To:	Marc Kleine-Budde <mkl@...gutronix.de>,
	Dong Aisheng <b29396@...escale.com>
CC:	linux-can@...r.kernel.org, wg@...ndegger.com,
	varkabhadram@...il.com, netdev@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 7/7] can: m_can: workaround for transmit data less than
 4 bytes



On 04.11.2014 11:33, Marc Kleine-Budde wrote:
> On 11/04/2014 10:27 AM, Dong Aisheng wrote:


>> +       /* We meet an IC issue that we have to write the full 8
>
> At least on the *insert SoC name here*, an issue with the Message RAM
> was discovered. Sending CAN frames with dlc less than 4 bytes will lead
> to bit errors, when the first 8 bytes of the Message RAM have not been
> initialized (i.e. written to). To work around this issue, the first 8
> bytes are initialized here.

Yes. Also put the current IP revision (3.0.x) into the comment.
Did inform the Bosch guys from this issue - or is it already in some errata sheet?

>
>> +        * bytes (whatever value for the second word) in Message RAM to
>> +        * avoid bit error for transmit data less than 4 bytes at the first
>> +        * time. By initializing the first 8 bytes of tx buffer before using
>> +        * it can avoid such issue.
>> +        */
>> +       m_can_fifo_write(priv, 0, M_CAN_FIFO_DATA(0), 0x0);
>> +       m_can_fifo_write(priv, 0, M_CAN_FIFO_DATA(1), 0x0);
>> +
>>          m_can_config_endisable(priv, false);
>>   }
>
> Can you trigger the issue when sending CAN-FD frames with dlc > 8 && dlc
> < 64?

Just a nitpick:

DLC can just be 0 .. 15

and the length (struct canfd_frame.len) can be from 0 .. 64

See:

http://lxr.free-electrons.com/source/include/uapi/linux/can.h#L83

That's the reason for all these helpers

http://lxr.free-electrons.com/source/drivers/net/can/dev.c#L36

that hide the evil "DLC" from userspace now and make 'len' a usable loop 
variable as we were able to use the former dlc for classic CAN :-)

Regards,
Oliver

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