netdev - Re: [PATCH 09/15] net: phy: delay PHY driver probe until PHY registration

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Message-ID: <CAMRc=McBxJdujCyjQF3NA=bCWHF1dx8xJ1Nc2snmqukvJ_VyoQ@mail.gmail.com>
Date:   Wed, 24 Jun 2020 15:48:50 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Mark Brown <broonie@...nel.org>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
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        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH 09/15] net: phy: delay PHY driver probe until PHY registration

śr., 24 cze 2020 o 11:43 Mark Brown <broonie@...nel.org> napisał(a):
>
> On Tue, Jun 23, 2020 at 12:49:15PM -0700, Florian Fainelli wrote:
> > On 6/22/20 6:51 AM, Mark Brown wrote:
>
> > > If the bus includes power management for the devices on the bus the
> > > controller is generally responsible for that rather than the devices,
> > > the devices access this via facilities provided by the bus if needed.
> > > If the device is enumerated by firmware prior to being physically
> > > enumerable then the bus will generally instantiate the device model
> > > device and then arrange to wait for the physical device to appear and
> > > get joined up with the device model device, typically in such situations
> > > the physical device might appear and disappear dynamically at runtime
> > > based on what the driver is doing anyway.
>
> > In premise there is nothing that prevents the MDIO bus from taking care
> > of the regulators, resets, prior to probing the PHY driver, what is
> > complicated here is that we do need to issue a read of the actual PHY to
> > know its 32-bit unique identifier and match it with an appropriate
> > driver. The way that we have worked around this with if you do not wish
> > such a hardware access to be made, is to provide an Ethernet PHY node
> > compatible string that encodes that 32-bit OUI directly. In premise the
> > same challenges exist with PCI devices/endpoints as well as USB, would
> > they have reset or regulator typically attached to them.
>
> That all sounds very normal and is covered by both cases I describe?
>
> > > We could use a pre-probe stage in the device model for hotpluggable
> > > buses in embedded contexts where you might need to bring things out of
> > > reset or power them up before they'll appear on the bus for enumeration
> > > but buses have mostly handled that at their level.
>
> > That sounds like a better solution, are there any subsystems currently
> > implementing that, or would this be a generic Linux device driver model
> > addition that needs to be done?
>
> Like I say I'm suggesting doing something at the device model level.

I didn't expect to open such a can of worms...

This has evolved into several new concepts being proposed vs my
use-case which is relatively simple. The former will probably take
several months of development, reviews and discussions and it will
block supporting the phy supply on pumpkin boards upstream. I would
prefer not to redo what other MAC drivers do (phy-supply property on
the MAC node, controlling it from the MAC driver itself) if we've
already established it's wrong.

Is there any compromise we could reach to add support for a basic,
common use-case of a single regulator supplying a PHY that needs
enabling before reading its ID short-term (just like we currently
support a single reset or reset-gpios property for PHYs) and
introducing a whole new concept to the device model for more advanced
(but currently mostly hypothetical) cases long-term?

Bart