linux-kernel - Re: [PATCH v2 5/6] of: Add #address-cells/#size-cells in the device-tree root empty node

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Message-ID: <20241108183958.0f136e01@bootlin.com>
Date: Fri, 8 Nov 2024 18:39:58 +0100
From: Herve Codina <herve.codina@...tlin.com>
To: Rob Herring <robh@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, Saravana Kannan <saravanak@...gle.com>, Bjorn Helgaas
 <bhelgaas@...gle.com>, Lizhi Hou <lizhi.hou@....com>,
 linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
 linux-pci@...r.kernel.org, Allan Nielsen <allan.nielsen@...rochip.com>,
 Horatiu Vultur <horatiu.vultur@...rochip.com>, Steen Hegelund
 <steen.hegelund@...rochip.com>, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v2 5/6] of: Add #address-cells/#size-cells in the
 device-tree root empty node

On Fri, 8 Nov 2024 11:24:36 -0600
Rob Herring <robh@...nel.org> wrote:

> On Fri, Nov 8, 2024 at 10:29 AM Herve Codina <herve.codina@...tlin.com> wrote:
> >
> > Hi Rob,
> >
> > On Fri, 8 Nov 2024 10:03:31 -0600
> > Rob Herring <robh@...nel.org> wrote:
> >  
> > > On Fri, Nov 8, 2024 at 8:36 AM Herve Codina <herve.codina@...tlin.com> wrote:  
> > > >
> > > > On systems where ACPI is enabled or when a device-tree is not passed to
> > > > the kernel by the bootloader, a device-tree root empty node is created.
> > > > This device-tree root empty node doesn't have the #address-cells and the  
> 
> > > > +       /*
> > > > +        * #address-cells/#size-cells are required properties at root node
> > > > +        * according to the devicetree specification. Use same values as default
> > > > +        * values mentioned for #address-cells/#size-cells properties.  
> > >
> > > Which default? We have multiple...  
> >
> > I will reword:
> >   Use values mentioned in the devicetree specification as default values
> >   for #address-cells and #size-cells properties  
> 
> My point was that "default" is meaningless because there are multiple
> sources of what's default.

I see thanks.
I will update the code comment.

> 
> > >
> > > There's also dtc's idea of default which IIRC is 2 and 1 like OpenFirmware.  
> >
> > I can re-add this part in the commit log:
> >   The device tree compiler already uses 2 as default value for address cells
> >   and 1 for size cells. The powerpc PROM code also use 2 as default value
> >   for #address-cells and 1 for #size-cells. Modern implementation should
> >   have the #address-cells and the #size-cells properties set and should
> >   not rely on default values.
> >
> > In your opinion, does it make sense?
> >  
> > >  
> > > > +        */
> > > > +       #address-cells = <0x02>;
> > > > +       #size-cells = <0x01>;  
> > >
> > > I think we should just do 2 cells for size.  
> >
> > Why using 2 for #size-cells?
> >
> > I understand that allows to have size defined on 64bits but is that needed?
> > How to justify this value here?  
> 
> Most systems are 64-bit today. And *all* ACPI based systems are. Not
> that the DT has to match 32 vs 64 bit CPU, most of the time it does.
> 
> It also doesn't actually change anything for you because you're going
> to have "pci" nodes and the "ranges" there takes #size-cells from the
> pci node, not the parent.
> 

Right.
I will set:
  #address-cells = <0x02>;
  #size-cells = <0x02>;

Best regards,
Hervé