netdev - Re: [RFC PATCH 0/2] AT8031 PHY timestamping support

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Message-ID: <fa823a08fa6d50c57ca03bdc58bf4921@walle.cc>
Date:   Wed, 26 Feb 2020 12:30:48 +0100
From:   Michael Walle <michael@...le.cc>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [RFC PATCH 0/2] AT8031 PHY timestamping support

Am 2020-02-26 03:54, schrieb Richard Cochran:
> On Wed, Feb 26, 2020 at 01:07:26AM +0100, Michael Walle wrote:
>> Am 26. Februar 2020 00:50:40 MEZ schrieb Andrew Lunn <andrew@...n.ch>:
>> >That sounds fundamentally broken.
> 
> Right.  It can't work unless the PHY latches the time stamp.

To make things worse, it only has one slot for RX and one slot for TX
timestamps.

>> This might be the case, but the datasheet (some older revision can
>> be found on the internet, maybe you find something) doesn't mention
>> it. Nor does the PTP "guide" (I don't know the exact name, I'd have
>> to check at work) of this PHY. Besides the timestamp there's also
>> the sequence number and the source port id which would need to be
>> read atomically together with the timestamp.
> 
> Maybe the part is not intended to be used at all in this way?
> 
> AFAICT, PHYs like this are meant to feed a "PTP frame detected" pulse
> into the time stamping unit on the attached MAC.  The interrupt serves
> to allow the SW to gather the matching fields from the frame.

But then there would need to be such a hardware pin, correct? Unless
you'd misuse the INT# for it. Also, why should the PHY then have a PHC
which can be adjusted.

>> That sounds fundamentally broken. Which would be odd. Sometimes there
>> is a way to take a snapshot of the value. Reading the first word could
>> trigger this snapshot. Or the last word, or some status register. One
>> would hope the datasheet would talk about this.
> 
> This might be the case, but the datasheet (some older revision can
> be found on the internet, maybe you find something) doesn't mention
> it. Nor does the PTP "guide" (I don't know the exact name, I'd have
> to check at work).

BTW, the name of the document is "AR8031 1588v2 Precision Time Protocol,
Application Note, 80-Y0618-15 Rev. A", which describes a use case
where the RTC (ie PHC) is in the AR8031. I don't argue that the PHY is
not broken, only that Atheros at least intended to have that use case.

-michael