netdev - RE: [PATCH net-next RFC 0/9] net: dsa: PTP timestamping for mv88e6xxx

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Date:   Fri, 29 Sep 2017 15:34:05 +0000
From:   Brandon Streiff <brandon.streiff@...com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        Richard Cochran <richardcochran@...il.com>,
        Erik Hons <erik.hons@...com>
Subject: RE: [PATCH net-next RFC 0/9] net: dsa: PTP timestamping for mv88e6xxx

> From: Andrew Lunn [mailto:andrew@...n.ch]
> Sent: Thursday, September 28, 2017 12:36 PM
>
> I assume ptp already has the core code to use pinctrl and Linux
> standard GPIOs? What does the device tree binding look like? How do
> you specify the GPIOs to use?
> 
> What we want to avoid is defining an ABI now, otherwise it is going to
> be hard to swap to pinctrl later.

A ptp_clock_info has an array of struct ptp_pin_desc which defines "pins" with a name ("Hardware specific human readable pin name"), an index, and a bitmask of valid functions. The ptp_pin_desc structure is shared with usermode for the PTP_PIN_GETFUNC and PTP_PIN_SETFUNC ioctls. The pins are also exposed in sysfs (see Documentation/ABI/testing/sysfs-ptp). The underlying implementation for configuring the hardware is left up to the PHC driver. I don't see any drivers today that use the PHC pin API as a layer over pinctrl/gpiochip, but there's no reason that that couldn't be the case.

For mv88e6xxx, we name the pins using the pattern "mv88e6xxx_gpio%d"; this appears to be in line with current practice (igb_ptp.c uses "SDP%d", mlx5 driver uses "mlx5_pps%d"). Usermode code appears to be expected to determine which pin it needs to use. (Our current userspace code, for instance, knows that it needs to find "mv88e6xxx_gpio8".)

For mv88e6xxx, Device Tree does feel like a better option here for declaring names, functions, and pin usages. Not all platforms that use the PTP API use Device Tree though.

-- brandon