netdev - Re: [PATCH net-next v1 0/9] Ethernet Cable test support

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Message-ID: <c52e7043-cbd5-8fa0-96e6-e29e783d3a5f@gmail.com>
Date:   Thu, 30 Apr 2020 13:19:10 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Michael Walle <michael@...le.cc>
Cc:     Andrew Lunn <andrew@...n.ch>, cphealy@...il.com,
        davem@...emloft.net, hkallweit1@...il.com, mkubecek@...e.cz,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next v1 0/9] Ethernet Cable test support

On 4/30/20 1:13 PM, Michael Walle wrote:
> Am 2020-04-30 22:04, schrieb Florian Fainelli:
>> On 4/30/20 12:41 PM, Andrew Lunn wrote:
>>>> ECD. The registers looks exactly like the one from the Marvell PHYs,
>>>> which makes me wonder if both have the same building block or if one
>>>> imitated the registers of the other. There are subtle differences
>>>> like one bit in the broadcom PHY is "break link" and is self-clearing,
>>>> while the bit on the Marvell PHY is described as "perform diagnostics
>>>> on link break".
>>>
>>> Should we be sharing code between the two drivers?
>>
>> Yes, I am amazed how how identical they are, nearly on a bit level
>> identical, the coincidence is uncanny. The expansion registers are also
>> 0x10 - 0x15 just in the reverse order,
> 
> At what PHY are you looking? I've just found some datasheets where they
> are at 0xC0 to 0xC5.

BCM54810 because that's what I have on my desk right now, but 0x10 would
be the offset relative to the expansion register space, which would
translate into this:

https://github.com/ffainelli/linux/commits/broadcom-cable-tests

(sorry for the mess it is a patchwork of tests on various platforms,
based on an earlier branch from Andrew).

> 
>> you know, so as to make it not
>> too obvious this looks about the same ;) I wonder if we managed to find
>> something here.
>>
>>>
>>>> What do you mean by calibrate it?
>>>
>>> Some of the Marvell documentation talks about calibrating for losses
>>> on the PCB. Run a diagnostics with no cable plugged in, and get the
>>> cable length to the 'fault'. This gives you the distance to the RJ45
>>> socket. You should then subtract that from all subsequent results.
>>> But since this is board design specific, i decided to ignore it. I
>>> suppose it could be stuffed into a DT property, but i got the feeling
>>> it is not worth it, given the measurement granularity of 80cm.
>>
>> OK, accuracy is different here, they are said to be +/- 5m accurate for
>> cable faults and +/- 10m accurate for good cables.
> 
> Accuracy != granularity. But yes, if one digit correspond to 80cm it
> doesn't really make sense to remove the PCB trace error if you assume
> that it might add just one digit at most.

One of the test racks that I use has very short cables, but I guess it
does not matter if they get reported as 80cm or 160cm...
-- 
Florian