linux-kernel - Re: [PATCH v5 2/4] devicetree: bindings: Document Krait CPU/L1 EDAC

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Message-ID: <20140116183326.GG25540@e102568-lin.cambridge.arm.com>
Date:	Thu, 16 Jan 2014 18:33:26 +0000
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
	Mark Rutland <Mark.Rutland@....com>,
	Kumar Gala <galak@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v5 2/4] devicetree: bindings: Document Krait CPU/L1 EDAC

On Thu, Jan 16, 2014 at 06:05:05PM +0000, Stephen Boyd wrote:
> On 01/16, Lorenzo Pieralisi wrote:
> > On Thu, Jan 16, 2014 at 01:38:40AM +0000, Stephen Boyd wrote:
> > > On 01/15, Stephen Boyd wrote:
> > > > 
> > > > Ah sorry, I forgot to put the compatible property here like in
> > > > the dts change. I'll do that in the next revision. Yes we need a
> > > > compatible property here to match the platform driver.
> > > > 
> > > 
> > > This is the replacement patch
> > > 
> > > -----8<------
> > > From: Stephen Boyd <sboyd@...eaurora.org>
> > > Subject: [PATCH v9] devicetree: bindings: Document Krait CPU/L1 EDAC
> > > 
> > > The Krait CPU/L1 error reporting device is made up a per-CPU
> > > interrupt. While we're here, document the next-level-cache
> > > property that's used by the Krait EDAC driver.
> > > 
> > > Cc: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> > > Cc: Mark Rutland <mark.rutland@....com>
> > > Cc: Kumar Gala <galak@...eaurora.org>
> > > Cc: <devicetree@...r.kernel.org>
> > > Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> > > ---
> > >  Documentation/devicetree/bindings/arm/cpus.txt | 58 ++++++++++++++++++++++++++
> > >  1 file changed, 58 insertions(+)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt
> > > index 91304353eea4..03a529e791c4 100644
> > > --- a/Documentation/devicetree/bindings/arm/cpus.txt
> > > +++ b/Documentation/devicetree/bindings/arm/cpus.txt
> > > @@ -62,6 +62,20 @@ nodes to be present and contain the properties described below.
> > >  		Value type: <u32>
> > >  		Definition: must be set to 0
> > >  
> > > +	- compatible
> > > +		Usage: optional
> > > +		Value type: <string>
> > > +		Definition: should be one of the compatible strings listed
> > > +			    in the cpu node compatible property. This property
> > > +			    shall only be present if all the cpu nodes have the
> > > +			    same compatible property.
> > 
> > Do we really want to do that ? I am not sure. A cpus node is supposed to
> > be a container node, we should not define this binding just because we
> > know the kernel creates a platform device for it then.
> 
> This is just copying more of the ePAPR spec into this document.
> It just so happens that having a compatible field here allows a
> platform device to be created. I don't see why that's a problem.

I do not see why you cannot define a node like pmu or arch-timer and stick
a compatible property in there. cpus node does not represent a device, and
must not be created as a platform device, that's my opinion.

What would you do for big.LITTLE systems ? We are going to create two
cpus node because we need two platform devices ? I really think there
must be a better way to implement this, but I will let DT maintainers
make a decision.

> > interrupts is a cpu node property and I think it should be kept as such.
> > 
> > I know it will be duplicated and I know you can't rely on a platform
> > device for probing (since if I am not mistaken, removing a compatible
> > string from cpus prevents its platform device creation), but that's an issue
> > related to how the kernel works, you should not define DT bindings to solve
> > that IMHO.
> 
> The interrupts property is also common for all cpus so it seems
> fine to collapse the value down into a PPI specifier indicating
> that all CPUs get the interrupt, similar to how we compress the
> information about the compatible string.

I think it is nicer to create a device node (as I said, like a pmu or an
arch-timer) and define interrupts there along with a proper compatible
property. This would serve the same purpose without adding properties in
the cpus node.

cpu-edac {
	compatible = "qcom,cpu-edac";
	interrupts = <...>;
};

Thanks,
Lorenzo

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