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

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Message-ID: <a972cec1-3386-3ca3-ace3-175e1cabd286@hartkopp.net>
Date:   Thu, 19 Oct 2017 13:14:55 +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

On 10/19/2017 11:13 AM, Marc Kleine-Budde wrote:
> On 10/19/2017 07:07 AM, Sekhar Nori 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

>>>
>>> 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'.

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

Regards,
Oliver