Re: [RFC PATCH 0/2] AT8031 PHY timestamping support
From: Michael Walle
Date: Wed Feb 26 2020 - 06:30:53 EST
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@xxxxxxx>:
>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