linux-kernel - Re: [RFC PATCH 0/4] Add ACPI support for pinctrl configuration

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Date:	Wed, 6 Apr 2016 11:49:00 +0100
From:	Graeme Gregory <gg@...mlogic.co.uk>
To:	ahs3@...hat.com
Cc:	Charles Garcia-Tobin <charles.garcia-tobin@....com>,
	Mark Rutland <mark.rutland@....com>,
	Irina Tirdea <irina.tirdea@...el.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Octavian Purdila <octavian.purdila@...el.com>,
	Cristina Ciocan <cristina.ciocan@...el.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/4] Add ACPI support for pinctrl configuration

On Tue, Apr 05, 2016 at 06:00:31PM -0600, Al Stone wrote:
> On 04/05/2016 02:56 AM, Charles Garcia-Tobin wrote:
> > 
> > 
> > On 04/04/2016 23:52, "Mark Rutland" <mark.rutland@....com> wrote:
> > 
> >> Hi,
> >>
> >> On Thu, Mar 31, 2016 at 02:44:41PM +0300, Irina Tirdea wrote:
> >>> This is a proposal for adding ACPI support for pin controller
> >>> configuration.
> >>>
> >>> It has been developed to enable the MinnowBoard and IoT community
> >>> by providing an easy way to specify pin multiplexing and
> >>> pin configuration.
> >>>
> >>> This proposal is based on using _DSD properties to specify device
> >>> states and configuration nodes and it follows closely the device
> >>> tree model. Device states are defined using the Device Properties
> >>> format and the configuration nodes are defined using the
> >>> Hierarchical Properties Extension format. The generic properties
> >>> for the configuration nodes are the same as the ones for device
> >>> tree, while pincontroller drivers can also define custom ones.
> >>
> >>From a look of the Documentation addition, and of the current uses of
> >> pinctrl-names in device tree bindings, one reason for requiring multiple
> >> pinctrl states is power management. Given that, I'm somewhat concerned by
> >> this,
> >> as it goes against the usual ACPI model of abstracting this sort of thing
> >> behind power control methods.
> >>
> >> To the best of my knowledge, that goes against the ASWG's expectations on
> >> how
> >> _DSD will be used (per [1]). Charles, please correct me if that document
> >> is no
> >> longer representative.
> > 
> > It is though latest version was posted by Rafael a bit later:
> > https://lists.acpica.org/pipermail/dsd/2015-December/000027.html
> > 
> > 
> > In addition the core rules requiring that existing ACPI paradigms are not
> > subverted through DSD (basically the concern you express) are also
> > documented in the main DSD documentation itself:
> > http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UU
> > ID.pdf (section 2.3)
> 
> I have to echo Mark's concern: wholesale importation of portions of current
> DT bindings simply because it's expedient is one of the things I've been hoping
> to avoid.  These patches seem to be just that.
> 
> And while the latest version mentioned above describes a bit of a review
> process to handle this case, I don't recall the kernel community at large
> agreeing to it, nor to it having been implemented.  If I missed that part,
> my apologies; please let me know where it was decided.  If I haven't, then
> perhaps this needs to be the first test case of that process.
> 

My concern over this is similar to Mark/Als this looks like a quick hack
to "solve" an issue that has been worked on in Linux for at least 10
years now. And thats how to describe a non descoverable card that is
plugged into random busses on a SoC.

The issue I see with a quick hack of DT pinctrl into _DSD is that this
does not take into account the power model of ACPI. As far as I can see
the core code has no manner to tell a pin/function allocated through
_DSD actually has effects on the power of other devices. So the core
could end up powering down devices when it should not. This is very
relevent to your intended use of IoT devices where power control is key.

Why has there been no attempt in ASWG to make these sort of features a
1st class citizen of ACPI so they can interact correctly with the other
features?

Thanks

Graeme