Re: [RFC PATCH] can: m_can: Support higher speed CAN-FD bitrates

From: Mario HÃttel
Date: Wed Sep 20 2017 - 17:38:08 EST

On 09/20/2017 10:19 PM, Franklin S Cooper Jr wrote:
> Hi Wenyou,
> On 09/17/2017 10:47 PM, Yang, Wenyou wrote:
>> On 2017/9/14 13:06, Sekhar Nori wrote:
>>> On Thursday 14 September 2017 03:28 AM, Franklin S Cooper Jr wrote:
>>>> On 08/18/2017 02:39 PM, Franklin S Cooper Jr wrote:
>>>>> During test transmitting using CAN-FD at high bitrates (4 Mbps) only
>>>>> resulted in errors. Scoping the signals I noticed that only a single
>>>>> bit
>>>>> was being transmitted and with a bit more investigation realized the
>>>>> actual
>>>>> MCAN IP would go back to initialization mode automatically.
>>>>> It appears this issue is due to the MCAN needing to use the Transmitter
>>>>> Delay Compensation Mode as defined in the MCAN User's Guide. When this
>>>>> mode is used the User's Guide indicates that the Transmitter Delay
>>>>> Compensation Offset register should be set. The document mentions
>>>>> that this
>>>>> register should be set to (1/dbitrate)/2*(Func Clk Freq).
>>>>> Additional CAN-CIA's "Bit Time Requirements for CAN FD" document
>>>>> indicates
>>>>> that this TDC mode is only needed for data bit rates above 2.5 Mbps.
>>>>> Therefore, only enable this mode and only set TDCO when the data bit
>>>>> rate
>>>>> is above 2.5 Mbps.
>>>>> Signed-off-by: Franklin S Cooper Jr <fcooper@xxxxxx>
>>>>> ---
>>>>> I'm pretty surprised that this hasn't been implemented already since
>>>>> the primary purpose of CAN-FD is to go beyond 1 Mbps and the MCAN IP
>>>>> supports up to 10 Mbps.
>>>>> So it will be nice to get comments from users of this driver to
>>>>> understand
>>>>> if they have been able to use CAN-FD beyond 2.5 Mbps without this
>>>>> patch.
>>>>> If they haven't what did they do to get around it if they needed higher
>>>>> speeds.
>>>>> Meanwhile I plan on testing this using a more "realistic" CAN bus to
>>>>> insure
>>>>> everything still works at 5 Mbps which is the max speed of my CAN
>>>>> transceiver.
>>>> ping. Anyone has any thoughts on this?
>>> I added Dong who authored the m_can driver and Wenyou who added the only
>>> in-kernel user of the driver for any help.
>> I tested it on SAMA5D2 Xplained board both with and without this patch,
>> both work with the 4M bps data bit rate.
> Thank you for testing this out. Its interesting that you have been able
> to use higher speeds without this patch. What is the CAN transceiver
> being used on the SAMA5D2 Xplained board? I tried looking at the
> schematic but it seems the CAN signals are used on an extension board
> which I can't find the schematic for. Also do you mind sharing your test
> setup? Were you doing a short point to point test?
> Thank You,
> Franklin
Hello Franklin,

your patch definitely makes sense.

I forgot the TDC in my patches because it was not present in the
previous driver versions and because I didn't encounter any
problems when testing it myself.

The error is highly dependent on the hardware (transceiver) setup.
So it is definitely possible that some people don't encounter errors
without your patch.

Could you clarify what you meant with
> Scoping the signals I noticed that only a single bit was being transmitted

Do you mean one data bit (high bit rate)Â or did the core already fail
in the arbitration phase?

There is also another aspect that can lead to errors:

If the CAN clock 'cclk' is above the frequency of the interface/logic
clock 'hclk', the clock domain crossing of the CAN messages can't
work properly. However, I will throw this topic as an extra e-mail into
the round.



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