linux-kernel - Re: [PATCH net-next 6/6] net: dsa: sja1105: parse {rx,tx}-internal-delay-ps properties for RGMII delays

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Date:   Fri, 15 Oct 2021 10:15:41 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <vladimir.oltean@....com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Rob Herring <robh+dt@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>, Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Prasanna Vengateshan <prasanna.vengateshan@...rochip.com>,
        Ansuel Smith <ansuelsmth@...il.com>,
        Alvin Šipraga <alsi@...g-olufsen.dk>
Subject: Re: [PATCH net-next 6/6] net: dsa: sja1105: parse
 {rx,tx}-internal-delay-ps properties for RGMII delays

On 10/13/21 3:23 PM, Vladimir Oltean wrote:
> This change does not fix any functional issue or address any real life
> use case that wasn't possible before. It is just a small step in the
> process of standardizing the way in which Ethernet MAC drivers may apply
> RGMII delays (traditionally these have been applied by PHYs, with no
> clear definition of what to do in the case of a fixed-link).
> 
> The sja1105 driver used to apply MAC-level RGMII delays on the RX data
> lines when in fixed-link mode and using a phy-mode of "rgmii-rxid" or
> "rgmii-id" and on the TX data lines when using "rgmii-txid" or "rgmii-id".
> But the standard definitions don't say anything about behaving
> differently when the port is in fixed-link vs when it isn't, and the new
> device tree bindings are about having a way of applying the delays in a
> way that is independent of the phy-mode and of the fixed-link property.
> 
> When the {rx,tx}-internal-delay-ps properties are present, use them,
> otherwise fall back to the old behavior and warn.
> 
> One other thing to note is that the SJA1105 hardware applies a delay
> value in degrees rather than in picoseconds (the delay in ps changes
> depending on the frequency of the RGMII clock - 125 MHz at 1G, 25 MHz at
> 100M, 2.5MHz at 10M). I assume that is fine, we calculate the phase
> shift of the internal delay lines assuming that the device tree meant
> gigabit, and we let the hardware scale those according to the link speed.
> 
> Link: https://patchwork.kernel.org/project/netdevbpf/patch/20210723173108.459770-6-prasanna.vengateshan@microchip.com/
> Link: https://patchwork.ozlabs.org/project/netdev/patch/20200616074955.GA9092@laureti-dev/#2461123
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Reviewed-by: Florian Fainelli <f.fainelli@...il.com>

FWIW, this is definitively a step in the right direction, and this is a
good way to deal with the phy-mode RGMII mess that has plagued us, thanks!
-- 
Florian