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

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Message-ID: <20141105020739.GB18982@shlinux1.ap.freescale.net>
Date:	Wed, 5 Nov 2014 10:07:40 +0800
From:	Dong Aisheng <b29396@...escale.com>
To:	Oliver Hartkopp <socketcan@...tkopp.net>
CC:	Marc Kleine-Budde <mkl@...gutronix.de>,
	<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 Tue, Nov 04, 2014 at 02:13:30PM +0100, Oliver Hartkopp wrote:
> 
> 
> 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?
> 

Good idea, will add it.
I will try if we can talk Bosch guys about this issue.

Regards
Dong Aisheng

> >
> >>+        * 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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