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

Dong Aisheng b29396 at freescale.com
Wed Nov 5 04:47:09 PST 2014

On Wed, Nov 05, 2014 at 02:10:05PM +0100, 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 :-)
> >
> >Additionally it looks to me like m_can does allow set CMR bit at runtime
> >since the mode change will take affect when controller becomes idle.
> >See below:
> >
> >"A mode change requested by writing to CCCR.CMR will be executed next
> >time the CAN protocol controller FSM reaches idle phase between CAN frames.
> >Upon this event CCCR.CMR is reset to “00” and the status flags CCCR.FDBS
> >and CCCR.FDO are set accordingly. In case the requested
> >CAN operation mode is not enabled, the value written to CCCR.CMR is
> >retained until it is overwritten by the next mode change request.
> >Default is CAN operation according to ISO11898-1."
> Ah. I assumed we have to take care to set these bits in the idle state.
> So when thinking about the one and only tx buffer your current
> approach should work indeed. Sorry for my confusion.
> >
> >>I would suggest the following approach to make the revision 3.0.x
> >>act like a correct CAN FD capable controller:
> >>
> >>- create a helper function to switch FD and BRS while taking care of
> >>ongoing transmissions
> >>
> >>- create a variable that knows the current tx_mode for FD and BRS
> >>
> >>When we need to send a CAN frame which doesn't fit the settings in
> >>the tx_mode we need to switch the CCCR and update the tx_mode
> >>variable.
> >>
> >>This at least reduces the needed CCCR operations.
> >>
> >
> >Yes, i can do like that.
> >But what i see by doing that is only reduces the needed CCCR operations?
> >Any other benefits i missed?
> No. I just thought about a function that also takes care of the idle
> phase to switch the bits. But now you made it clear that this is not
> needed - so you can stay on your current implementation: Setting the
> CCCR each time before sending the frame.


> With the 3.1.x or 3.2.x this code will become obsolete then as the
> EDL and BRS seeting will get extra bits in the Tx Buffer.
> But that's a future point.

Got it.
Will update it in the future when the new IP doc is available.

> >And the test for current code showed it seemed work well on the Mode
> >switch among std frame/fd frame/brs frame.
> Fine.
> Btw. I would suggest to integrate patch [4/4] into [3/4].

Yes, will do it.

Dong Aisheng

> Best regards,
> Oliver

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