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

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Message-ID: <20141105134625.GG4007@shlinux1.ap.freescale.net>
Date:	Wed, 5 Nov 2014 21:46:26 +0800
From:	Dong Aisheng <b29396@...escale.com>
To:	Marc Kleine-Budde <mkl@...gutronix.de>
CC:	Oliver Hartkopp <socketcan@...tkopp.net>,
	<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 Wed, Nov 05, 2014 at 02:19:22PM +0100, Marc Kleine-Budde wrote:
> 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.
> 

Hmm, this way is just following the original approach the current driver
used. It's initial work and won't make things complicated.

Extend the driver to use FIFO for TX is another story and based on
my understanding it may be a bit complicated to do CAN FD mode switch on this
case due to hw limitation that the revision 3.0.x does not support per-frame
switching for FD/BRS as Oliver pointed out.
(e.g. how to switch FD MODE for each frame on Tx FIFO?)
Probably that's why the 3.1.x version will add the FD/BRS bit controller
in Tx Buffer to fix this issue.

Anyway, that's future work and we can discuss it when adding FIFO support
for Tx function.

Regards
Dong Aisheng

> Marc
> 
> -- 
> Pengutronix e.K.                  | Marc Kleine-Budde           |
> Industrial Linux Solutions        | Phone: +49-231-2826-924     |
> Vertretung West/Dortmund          | Fax:   +49-5121-206917-5555 |
> Amtsgericht Hildesheim, HRA 2686  | http://www.pengutronix.de   |
> 


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