netdev - Re: [RFC 00/10] add support for fwnode in i2c mux system and sfp

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Message-ID: <YhfQ+MCYxrdDqN9J@piout.net>
Date:   Thu, 24 Feb 2022 19:39:52 +0100
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Clément Léger <clement.leger@...tlin.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Daniel Scally <djrscally@...il.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Wolfram Sang <wsa@...nel.org>, Peter Rosin <peda@...ntia.se>,
        Russell King <linux@...linux.org.uk>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, linux-i2c@...r.kernel.org,
        netdev@...r.kernel.org,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [RFC 00/10] add support for fwnode in i2c mux system and sfp

On 24/02/2022 20:14:51+0200, Sakari Ailus wrote:
> Hi Mark,
> 
> On Thu, Feb 24, 2022 at 03:33:12PM +0000, Mark Brown wrote:
> > On Thu, Feb 24, 2022 at 03:58:04PM +0100, Hans de Goede wrote:
> > 
> > > As Mark already mentioned the regulator subsystem has shown to
> > > be a bit problematic here, but you don't seem to need that?
> > 
> > I believe clocks are also potentially problematic for similar reasons
> > (ACPI wants to handle those as part of the device level power management
> > and/or should have native abstractions for them, and I think we also
> > have board file provisions that work well for them and are less error
> > prone than translating into an abstract data structure).
> 
> Per ACPI spec, what corresponds to clocks and regulators in DT is handled
> through power resources. This is generally how things work in ACPI based
> systems but there are cases out there where regulators and/or clocks are
> exposed to software directly. This concerns e.g. camera sensors and lens
> voice coils on some systems while rest of the devices in the system are
> powered on and off the usual ACPI way.
> 
> So controlling regulators or clocks directly on an ACPI based system
> wouldn't be exactly something new. All you need to do in that case is to
> ensure that there's exactly one way regulators and clocks are controlled
> for a given device. For software nodes this is a non-issue.
> 
> This does have the limitation that a clock or a regulator is either
> controlled through power resources or relevant drivers, but that tends to
> be the case in practice. But I presume it wouldn't be different with board
> files.
> 

In this use case, we don't need anything that is actually described in
ACPI as all the clocks we need to control are on the device itself so I
don't think this is relevant here.

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com