linux-kernel - Re: [PATCH v3 net-next 20/24] net: dsa: sja1105: Error out if RGMII delays are requested in DT

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Date:   Sat, 13 Apr 2019 18:49:57 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     f.fainelli@...il.com, vivien.didelot@...il.com,
        davem@...emloft.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, georg.waibel@...sor-technik.de
Subject: Re: [PATCH v3 net-next 20/24] net: dsa: sja1105: Error out if RGMII
 delays are requested in DT

On Sat, Apr 13, 2019 at 04:28:18AM +0300, Vladimir Oltean wrote:
> Documentation/devicetree/bindings/net/ethernet.txt is confusing because
> it says what the MAC should not do, but not what it *should* do:
> 
>   * "rgmii-rxid" (RGMII with internal RX delay provided by the PHY, the MAC
>      should not add an RX delay in this case)
> 
> The gap in semantics is threefold:
> 1. Is it illegal for the MAC to apply the Rx internal delay by itself,
>    and simplify the phy_mode (mask off "rgmii-rxid" into "rgmii") before
>    passing it to of_phy_connect? The documentation would suggest yes.
> 1. For "rgmii-rxid", while the situation with the Rx clock skew is more
>    or less clear (needs to be added by the PHY), what should the MAC
>    driver do about the Tx delays? Is it an implicit wild card for the
>    MAC to apply delays in the Tx direction if it can? What if those were
>    already added as serpentine PCB traces, how could that be made more
>    obvious through DT bindings so that the MAC doesn't attempt to add
>    them twice and again potentially break the link?
> 3. If the interface is a fixed-link and therefore the PHY object is
>    fixed (a purely software entity that obviously cannot add clock
>    skew), what is the meaning of the above property?
> 
> So an interpretation of the RGMII bindings was chosen that hopefully
> does not contradict their intention but also makes them more applied.
> The SJA1105 driver understands to act upon "rgmii-*id" phy-mode bindings
> if the port is in the PHY role (either explicitly, or if it is a
> fixed-link). Otherwise it always passes the duty of setting up delays to
> the PHY driver.

That is a good interpretation. I always recommend the PHY does the
delay, because in general the PHY can, and often the MAC cannot.

> 
> Signed-off-by: Vladimir Oltean <olteanv@...il.com>
> Reviewed-by: Florian Fainelli <f.fainelli@...il.com>

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

    Andrew