netdev - Re: [PATCH net-next] net: sfp: cope with SFPs that set both LOS normal and LOS inverted

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Date:   Sun, 10 Jan 2021 17:48:15 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Russell King <rmk+kernel@...linux.org.uk>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next] net: sfp: cope with SFPs that set both LOS
 normal and LOS inverted

On Sun, Jan 10, 2021 at 10:58:32AM +0000, Russell King wrote:
> The SFP MSA defines two option bits in byte 65 to indicate how the
> Rx_LOS signal on SFP pin 8 behaves:
> 
> bit 2 - Loss of Signal implemented, signal inverted from standard
>         definition in SFP MSA (often called "Signal Detect").
> bit 1 - Loss of Signal implemented, signal as defined in SFP MSA
>         (often called "Rx_LOS").
> 
> Clearly, setting both bits results in a meaningless situation: it would
> mean that LOS is implemented in both the normal sense (1 = signal loss)
> and inverted sense (0 = signal loss).
> 
> Unfortunately, there are modules out there which set both bits, which
> will be initially interpret as "inverted" sense, and then, if the LOS
> signal changes state, we will toggle between LINK_UP and WAIT_LOS
> states.
> 
> Change our LOS handling to give well defined behaviour: only interpret
> these bits as meaningful if exactly one is set, otherwise treat it as
> if LOS is not implemented.
> 
> Signed-off-by: Russell King <rmk+kernel@...linux.org.uk>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew