netdev - Re: [PATCH v2 2/3] netlink: add master/slave configuration support

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Message-ID: <20200607203940.7ayqe4otoop7mpuw@lion.mk-sys.cz>
Date:   Sun, 7 Jun 2020 22:39:40 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        "John W. Linville" <linville@...driver.com>,
        David Jander <david@...tonic.nl>, kernel@...gutronix.de,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        Russell King <linux@...linux.org.uk>, mkl@...gutronix.de,
        Marek Vasut <marex@...x.de>,
        Christian Herber <christian.herber@....com>,
        Amit Cohen <amitc@...lanox.com>,
        Petr Machata <petrm@...lanox.com>
Subject: Re: [PATCH v2 2/3] netlink: add master/slave configuration support

On Thu, May 28, 2020 at 01:54:13PM +0200, Oleksij Rempel wrote:
> This UAPI is needed for BroadR-Reach 100BASE-T1 devices. Due to lack of
> auto-negotiation support, we needed to be able to configure the
> MASTER-SLAVE role of the port manually or from an application in user
> space.
> 
> The same UAPI can be used for 1000BASE-T or MultiGBASE-T devices to
> force MASTER or SLAVE role. See IEEE 802.3-2018:
> 22.2.4.3.7 MASTER-SLAVE control register (Register 9)
> 22.2.4.3.8 MASTER-SLAVE status register (Register 10)
> 40.5.2 MASTER-SLAVE configuration resolution
> 45.2.1.185.1 MASTER-SLAVE config value (1.2100.14)
> 45.2.7.10 MultiGBASE-T AN control 1 register (Register 7.32)
> 
> The MASTER-SLAVE role affects the clock configuration:
> 
> -------------------------------------------------------------------------------
> When the  PHY is configured as MASTER, the PMA Transmit function shall
> source TX_TCLK from a local clock source. When configured as SLAVE, the
> PMA Transmit function shall source TX_TCLK from the clock recovered from
> data stream provided by MASTER.
> 
> iMX6Q                     KSZ9031                XXX
> ------\                /-----------\        /------------\
>       |                |           |        |            |
>  MAC  |<----RGMII----->| PHY Slave |<------>| PHY Master |
>       |<--- 125 MHz ---+-<------/  |        | \          |
> ------/                \-----------/        \------------/
>                                                ^
>                                                 \-TX_TCLK
> 
> -------------------------------------------------------------------------------
> 
> Since some clock or link related issues are only reproducible in a
> specific MASTER-SLAVE-role, MAC and PHY configuration, it is beneficial
> to provide generic (not 100BASE-T1 specific) interface to the user space
> for configuration flexibility and trouble shooting.
> 
> Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> ---

The patch looks good from technical point of view but I don't like the
inconsistency of user interface:

1. Similar to what we discussed earlier on kernel side, you use "Port
mode" in "ethtool <dev>" output but the corresponding command line
argument for "ethtool -s <dev> ..." is "master-slave". Even if it's
documented, it's rather confusing for users.

2. The values for "master-slave" parameter are "master-preferred" and
"master-force" (and the same for "slave"). Please use the same form for
both, i.e. either "prefer / force" or "preferred / forced". Also, it
would be friendlier to users to make the values consistent with
"ethtool <dev>" output, e.g. if it says "preferred Master", setting
should use something as close as possible, i.e. "preferred-master".

Michal

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