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

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Message-ID: <0099eefa-32c8-a818-761c-667239d9ec3b@pengutronix.de>
Date:   Thu, 19 Oct 2017 13:26:55 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Oliver Hartkopp <socketcan@...tkopp.net>,
        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

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.

>>>> Sekhar is right that ideally the user should be able to set the SSP at
>>>> runtime. However, my issue is that based on my experience CAN users
>>>> expect the driver to just work the majority of times. For unique use
>>>> cases where the driver calculated values don't work then the user should
>>>> be able to override it. This should only be done for a very small
>>>> percentage of CAN users. Unless you allow DT to provide a default SSP
>>>> many users of MCAN may find that the default SSP doesn't work and must
>>>> always use runtime overrides to get anything to work. I don't think that
>>>> is a good user experience which is why I don't like the idea.
>>>
>>> Fair enough. But not quite sure if CAN users expect CAN-FD to "just
>>> work" without doing any bittiming related setup.
>>
>>  From my point of view I'd rather buy a board from a HW vendor where
>> CAN-FD works, rather than a board where I have to tweak the bit-timing
>> for a simple CAN-FD test setup.
>>
>> As far as I see for the m_can driver it's a single tuple: "bitrate > 2.5
>> MBit/s -> 50%". Do we need an array of tuples in general?
>>
>> 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.

> One tool, one place is IMHO still the best option.

Marc

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