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

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Date:   Thu, 19 Oct 2017 20:35:07 +0200
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Marc Kleine-Budde <mkl@...gutronix.de>,
        Sekhar Nori <nsekhar@...com>,
        Franklin S Cooper Jr <fcooper@...com>,
        Mario Hüttel <mario.huettel@....net>,
        "Yang, Wenyou" <Wenyou.Yang@...rochip.com>, wg@...ndegger.com,
        quentin.schulz@...e-electrons.com, edumazet@...gle.com,
        linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Wenyou Yang <wenyou.yang@...el.com>,
        Dong Aisheng <b29396@...escale.com>,
        "Quadros, Roger" <rogerq@...com>
Subject: Re: [RFC PATCH] can: m_can: Support higher speed CAN-FD bitrates

Hi Marc,

On 10/19/2017 01:26 PM, Marc Kleine-Budde wrote:
> On 10/19/2017 01:14 PM, Oliver Hartkopp wrote:
>>>>>> Since we have a netlink socket interface to configure sample point, I
>>>>>> wonder if that should be extended to configure SSP too (or at least the
>>>>>> offset part of SSP)?
>>
>> +1 too
> 
> The struct can_bittiming in defined in uapi, so we have to keep ABI
> compatibility in mind.
> 

Oh, this is fortunately NO problem ;-)

struct can_bittiming {
         __u32 bitrate;          /* Bit-rate in bits/second */
         __u32 sample_point;     /* Sample point in one-tenth of a 
percent */
         __u32 tq;               /* Time quanta (TQ) in nanoseconds */
         __u32 prop_seg;         /* Propagation segment in TQs */
         __u32 phase_seg1;       /* Phase buffer segment 1 in TQs */
         __u32 phase_seg2;       /* Phase buffer segment 2 in TQs */
         __u32 sjw;              /* Synchronisation jump width in TQs */
         __u32 brp;              /* Bit-rate prescaler */
};

So we have two of these: One for the arbitration bitrate and one 
sample_point for the data bitrate -> the 'secondary' SP -> SSP

:-)

We already have this 'dsample-point' implemented in the ip tool:

$ ip link set vcan0 type can help
Usage: ip link set DEVICE type can
	[ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
	[ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
  	  phase-seg2 PHASE-SEG2 [ sjw SJW ] ]

	[ dbitrate BITRATE [ dsample-point SAMPLE-POINT] ] |  <<-- here!
	[ dtq TQ dprop-seg PROP_SEG dphase-seg1 PHASE-SEG1
  	  dphase-seg2 PHASE-SEG2 [ dsjw SJW ] ]

But AFAIK m_can is not using that value in m_can_set_bittiming().

>>> If good default values are transceiver and board specific, they can go
>>> into the DT. We need a generic (this means driver agnostic) binding for
>>> this. If this table needs to be tweaked for special purpose, then we can
>>> add a netlink interface for this as well. >
>>> Comments?
>>
>> By now we calculate reasonable default values (e.g. for SP and SJW), you
>> can override by setting alternative values via netlink configuration.
>>
>> I would tend to stay on this approach and not hide these things in DTs -
>> just because of someone wants to initialize his specific interface 'easier'.
> 
> If the values are not board specific, then it makes no sense to put them
> into the DT.

When they are NOT(?) board specific?

Thinking about non-SoC CAN adapters with PCI and USB pushing the SSP to 
the DT looks wrong to me.

Best,
Oliver