linux-kernel - Re: [PATCH net-next 2/2] net: phy: Add ability to debug RGMII connections

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Message-ID: <20191018160116.GD24810@lunn.ch>
Date:   Fri, 18 Oct 2019 18:01:16 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        open list <linux-kernel@...r.kernel.org>,
        Heiner Kallweit <hkallweit1@...il.com>,
        bcm-kernel-feedback-list@...adcom.com, cphealy@...il.com,
        Jose Abreu <joabreu@...opsys.com>
Subject: Re: [PATCH net-next 2/2] net: phy: Add ability to debug RGMII
 connections

> I don't think you are following the big picture of what I am saying. I
> was trying to follow Florian's intention (first make sure I understand
> it) and suggest that the FCS checking code in the patch he submitted
> is not doing what it was intended to. I am getting apparent FCS
> mismatches reported by the program, when I know full well that the MAC
> I am testing on would have dropped those frames were they really
> invalid.

I think this FCS check is not needed. If we feed the MAC random data,
something like 1 in 65535 will have a valid FCS and get passed up.
I've not seen this happen with Ethernet, but i have seen other network
technologies wrong decoding noise on the line and passing up frames
with around 1 in 65536 probability.

But then having the correct Ethertype is another 1 in 65536. So it
seem pretty improbably we do receiver a packet in this method which is
bad. So i would say, any packet received here is a good packet, and
indicate the RGMII mode works. If we don't receive a packet, the mode
is very probably bad.

	 Andrew