netdev - Re: [RFC] option to use proper skew timings for Micrel KSZ9021

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Message-ID: <f2dff077-8a0b-2164-404f-d9bf4ee40d76@mev.co.uk>
Date:   Fri, 11 Nov 2022 19:03:08 +0000
From:   Ian Abbott <abbotti@....co.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org
Subject: Re: [RFC] option to use proper skew timings for Micrel KSZ9021

On 11/11/2022 17:53, Andrew Lunn wrote:
>>> And since you are adding more foot guns, please validate the values in
>>> DT as strictly as possible, without breaking the existing binding.
>>
>> Yes, some min/max clamping of skew values would be good.  The code for
>> KSZ9131 does that already.
> 
> I would want much more strict checking than that. The old and the new
> values probably don't intersect. So if you see an old value while
> micrel,skew-equals-real-picoseconds is in force, fail the probe with
> -EINVAL. It looks like the old binding silently preforms rounding to
> the nearest delay. So you probably should not do the opposite, error
> out for a new value when micrel,skew-equals-real-picoseconds is not in
> force. But you can add range checks. A negative value is clearly wrong
> for the old values and should be -EINVAL. You just need to watch out
> for that the current code reads the values as u32, not s32, so you
> won't actually see a negative value.

I'm not sure how to tell old values and new values apart (except for 
negative new values).  A divisibility test won't work for values that 
are divisible by 600 (lcm(120, 200)).

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