Date:   Thu, 30 Apr 2020 12:38:25 -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 12:31 PM, Michael Walle wrote:
> Hi Florian,
> 
> Am 2020-04-30 20:34, schrieb Florian Fainelli:
>> On 4/30/20 10:48 AM, Michael Walle wrote:
>>> Hi Andrew,
>>>
>>> Am 2020-04-29 18:32, schrieb Andrew Lunn:
>>>> On Wed, Apr 29, 2020 at 06:02:13PM +0200, Michael Walle wrote:
>>>>> Hi Andrew,
>>>>>
>>>>> > Add infrastructure in ethtool and phylib support for triggering a
>>>>> > cable test and reporting the results. The Marvell 1G PHY driver is
>>>>> > then extended to make use of this infrastructure.
>>>>>
>>>>> I'm currently trying this with the AR8031 PHY. With this PHY, you
>>>>> have to select the pair which you want to start the test on. So
>>>>> you'd have to start the test four times in a row for a normal
>>>>> gigabit cable. Right now, I don't see a way how to do that
>>>>> efficiently if there is no interrupt. One could start another test
>>>>> in the get_status() polling if the former was completed
>>>>> successfully. But then you'd have to wait at least four polling
>>>>> intervals to get the final result (given a cable with four pairs).
>>>>>
>>>>> Any other ideas?
>>>>
>>>> Hi Michael
>>>>
>>>> Nice to see some more PHYs getting support for this.
>>>>
>>>> It is important that the start function returns quickly. However, the
>>>> get status function can block. So you could do all the work in the
>>>> first call to get status, polling for completion at a faster rate,
>>>> etc.
>>>
>>> Ok. I do have one problem. TDR works fine for the AR8031 and the
>>> BCM54140 as long as there is no link partner, i.e. open cable,
>>> shorted pairs etc. But as soon as there is a link partner and a
>>> link, both PHYs return garbage. As far as I understand TDR, there
>>> must not be a link, correct? The link partner may send data or
>>> link pulses. No how do you silence the local NIC or even the peer?
>>
>> Michael do you use the enhanced cable diagnostics (ECD) or the simple
>> cable diagnostics?
> 
> 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".
> 
> I don't know what simple cable diagnostics should be, I guess the
> BCM54140 doesn't support it or its not documented. Actually, ECD
> has very little documentation in general.

Yes, there is very little information, even internally. My understanding
is that diagnostics at run at auto-neg, so you need to break the link,
and when the link comes back up is when you should have per-pair
results. There are also some caveats, like the link parnter must also
have auto-negotiation on for the diagnostics to work.

> 
>> Having tried to get older Broadcom PHYs to work with
>> cable diagnostics, you need to calibrate the PHY prior to running
>> diagnostics and you need to soft reset it.
> 
> What do you mean by calibrate it?

You need to tune the AFE and DSP of the PHY in order for it to report
accurate cable lengths. I would not think that you have access to that,
and what I got access to is not really well documented, which is why I
have not had a chance to submit those changes yet.

How accurate are your results when there is no link?
-- 
Florian

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