netdev - Re: [PATCH 2/2] net: ethernet: nb8800: Fix RGMII TX clock delay setup

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Date:   Tue, 25 Jul 2017 00:30:45 +0200
From:   Mason <slash.tmp@...e.fr>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Mans Rullgard <mans@...sr.com>,
        Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        Andrew Lunn <andrew@...n.ch>,
        Martin Blumenstingl <martin.blumenstingl@...il.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Fabio Estevam <fabio.estevam@....com>,
        Zefir Kurtisi <zefir.kurtisi@...atec.com>,
        Timur Tabi <timur@...eaurora.org>,
        Daniel Mack <zonque@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "David S. Miller" <davem@...emloft.net>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>
Subject: Re: [PATCH 2/2] net: ethernet: nb8800: Fix RGMII TX clock delay setup

On 24/07/2017 23:49, Florian Fainelli wrote:
> On 07/24/2017 02:21 PM, Mason wrote:
>> On 20/07/2017 14:33, Mason wrote:
>>
>>> As [Florian] pointed out, the spec states that the
>>> "Data to Clock input Skew (at Receiver)"
>>> must be within [ 1.0, 2.6 ] ns.
>>>
>>> I understand that 2 ns is 1/4 of a 125 MHz period,
>>> but it's not clear to me why the above interval is
>>> centered at 1.8 instead of 2.0 ns.
>>>
>>> Also, the AR8035 PHY offers 4 possible TX clock delays:
>>> { 0.25, 1.3, 2.4, 3.4 } according to their doc.
>>> The two extremes are outside the interval, when would
>>> they be useful? In case the transmitter adds "bad" skew?
>>>
>>> Why doesn't the PHY support 1.8/2.0? Is it perhaps
>>> unable to, because of PLL limitations?
>>
>> I haven't yet found answers for these questions.
>>
>> - Why is the interval centered at 1.8 instead of 2.0 ns?
> 
> Presumably because this is almost the middle of the available range and
> it still provides a value that is within the specification...

I was talking about the RGMII spec.
If - theoretically - the best results are achieved
by having a 2 ns skew between clock and data,
it seems odd for the RGMII spec to define an
interval of [ 1.0, 2.6 ] ns for acceptable values.
I would have expected [ 1.2, 2.8 ] ns.

>> - What use are 0.25 ns and 3.4 ns skew?
> 
> Accounting for extreme PCB traces lengths possibly, or just exposing the
> raw values that the HW supports by increments of 0.25 ns, just because
> the HW supports it.

The AR8035 doesn't support increments of 0.25 ns,
it supports just 4 values: 0.25, 1.3, 2.4, 3.4
Two of which are outside the acceptable range
defined in the RGMII spec. Odd.
Giving it more thought, I don't think trace length
factors in, unless the data and clock lines have
very different length (signal propagation).

>> - Why doesn't the PHY support a "recommended" value like 1.8 ns?
>>
>> Does anyone have pointers to good resources?
> 
> The PHY datasheet and the RGMII specification really ought to be the
> starting points, there is not much more to it. Maybe go ask your support
> person at Qualcomm/Atheros about their PHY design?

Sadly, I rarely have access to support for the blocks
we use. I had to download the datasheet off the internet.
But I was only asking out of personal curiosity, since
this is outside my field. I don't think any customer
has complained about the default settings.

Regards.