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Date:   Fri, 17 Jul 2020 08:49:35 +0100
From:   Jonathan McDowell <noodles@...th.li>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Matthew Hagan <mnhagan88@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>, linux@...linux.org.uk,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        John Crispin <john@...ozen.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH 2/2] dt-bindings: net: dsa: qca8k: Add PORT0_PAD_CTRL
 properties

On Fri, Jul 17, 2020 at 01:38:22AM +0300, Vladimir Oltean wrote:
> On Thu, Jul 16, 2020 at 03:09:25PM -0700, Jakub Kicinski wrote:
> > On Mon, 13 Jul 2020 21:50:26 +0100 Matthew Hagan wrote:
> > > +- qca,sgmii-rxclk-falling-edge:	If present, sets receive clock phase to
> > > +				falling edge.
> > > +- qca,sgmii-txclk-falling-edge:	If present, sets transmit clock phase to
> > > +				falling edge.
> > 
> > These are not something that other vendors may implement and therefore
> > something we may want to make generic? Andrew?
> > 
> 
> It was asked before whether this device uses source-synchronous clock
> for SGMII or if it recovers the clock from the data stream. Just "pass"
> was given for a response.
> 
> https://patchwork.ozlabs.org/project/netdev/patch/8ddd76e484e1bedd12c87ea0810826b60e004a65.1591380105.git.noodles@earth.li/
> 
> One can, in principle, tell easily by examining schematics. If the SGMII
> is only connected via RX_P, RX_N, TX_P, TX_N (and optionally there might
> be external reference clocks for the SERDES lanes, but these are not
> part of the data connection itself), then the clock is recovered from
> the serial data stream, and we have no idea what "SGMII delays" are.
> 
> If the schematic shows 2 extra clock signals, one in each transmit
> direction, then this is, in Russell King's words, "a new world of RGMII
> delay pain but for SGMII". In principle I would fully expect clock skews
> to be necessary for any high-speed protocol with source-synchronous
> clocking. The problem, really, is that we aren't ready to deal with this
> properly. We aren't distinguishing "SGMII with clock" from "SGMII
> without clock" in any way. We have no idea who else is using such a
> thing. Depending on the magnitude of this new world, it may be wise to
> let these bindings go in as-is, or do something more kernel-wide...

I don't have the schematic for the device I've been working with, but
the switch data sheet just shows 2 differential pairs (input/output) for
the SerDes Interface (whereas the RGMII interfaces *are* listed with
their clocks).

J.

-- 
I just Fedexed my soul to hell. I'm *real* clever.

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