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

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Message-ID: <9caef9bbfaed5c75e72e083db8a552fd@walle.cc>
Date:   Thu, 30 Apr 2020 21:31:03 +0200
From:   Michael Walle <michael@...le.cc>
To:     Florian Fainelli <f.fainelli@...il.com>
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

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.

> 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?

-michael