netdev - Re: [PATCH V2 1/4] can: m_can: update to support CAN FD features

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Message-ID: <545A23DA.7030303@pengutronix.de>
Date:	Wed, 05 Nov 2014 14:19:22 +0100
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	Oliver Hartkopp <socketcan@...tkopp.net>,
	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 V2 1/4] can: m_can: update to support CAN FD features

On 11/05/2014 02:10 PM, Oliver Hartkopp wrote:
> On 05.11.2014 12:26, Dong Aisheng wrote:
>> On Wed, Nov 05, 2014 at 11:12:24AM +0100, Oliver Hartkopp wrote:
>>> On 05.11.2014 08:58, Dong Aisheng wrote:
> 
> 
>>> Unfortunately No. Here it becomes complicated due to the fact that
>>> the revision 3.0.x does not support per-frame switching for FD/BRS
>>> ...
>>
>> I'm not sure i got your point.
>>  From m_can spec, it allows switch CAN mode by setting CMR bit.
>>
>> Bits 11:10 CMR[1:0]: CAN Mode Request
>> A change of the CAN operation mode is requested by writing to this bit
>> field. After change to the
>> requested operation mode the bit field is reset to “00” and the status
>> flags FDBS and FDO are set
>> accordingly. In case the requested CAN operation mode is not enabled,
>> the value written to CMR is
>> retained until it is overwritten by the next mode change request. In
>> case CME = “01”/”10”/”11” a
>> change to CAN operation according to ISO 11898-1 is always possible.
>> Default is CAN operation
>> according to ISO11898-1.
>> 00 unchanged
>> 01 Request CAN FD operation
>> 10 Request CAN FD operation with bit rate switching
>> 11 Request CAN operation according ISO11898-1
>>
>> So what's the difference between this function and the per-frame
>> switching
>> you mentioned?
>>
>>>
>>> When (priv->can.ctrlmode & CAN_CTRLMODE_FD) is true this *only*
>>> tells us, that the controller is _capable_ to send either CAN or CAN
>>> FD frames.
>>>
>>> It does not configure the controller into one of these specific
>>> settings!
>>>
>>> Additionally: AFAIK when writing to the CCCR you have to make sure
>>> that there's currently no ongoing transfer.
>>>
>>
>> I don't know about it before.
>> By searching m_can user manual v302 again, i still did not find this
>> limitation. Can you point me if you know where it is?
>>
>> BTW, since we only use one Tx Buffer for transmission currently, so we
>> should not meet that case that CAN mode is switched during transfer.
>> So the issue you concern may not happen.
> 
> Yes. You are right. Having a FIFO with a size of 1 will help here :-)

Errrr....sorry...no.

Taking an easy route here but making it x times harder to extend the
driver to make use of the FIFO is not an option.

Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
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