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: <CAL_JsqJ-05tB7QSjmGvFLbKFGmzezJhukDGS3fP9GFtp2=BWOA@mail.gmail.com>
Date: Fri, 8 Nov 2024 10:03:31 -0600
From: Rob Herring <robh@...nel.org>
To: Herve Codina <herve.codina@...tlin.com>
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, 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

and the?

> This leads to the use of the default address cells and size cells values
> which are defined in the code to 1 for address cells and 1 for size cells

Missing period.

>
> According to the devicetree specification and the OpenFirmware standard
> (IEEE 1275-1994) the default value for #address-cells should be 2.
>
> Also, according to the devicetree specification, the #address-cells and
> the #size-cells are required properties in the root node.
>
> Modern implementation should have the #address-cells and the #size-cells
> properties set and should not rely on default values.
>
> On x86, this root empty node is used and the code default values are
> used.
>
> In preparation of the support for device-tree overlay on PCI devices
> feature on x86 (i.e. the creation of the PCI root bus device-tree node),
> the default value for #address-cells needs to be updated. Indeed, on
> x86_64, addresses are on 64bits and the upper part of an address is
> needed for correct address translations. On x86_32 having the default
> value updated does not lead to issues while the uppert part of a 64bits

upper

> address is zero.
>
> Changing the default value for all architectures may break device-tree
> compatibility. Indeed, existing dts file without the #address-cells
> property set in the root node will not be compatible with this
> modification.
>
> Instead of updating default values, add required #address-cells and

and?

>
> Signed-off-by: Herve Codina <herve.codina@...tlin.com>
> ---
>  drivers/of/empty_root.dts | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/of/empty_root.dts b/drivers/of/empty_root.dts
> index cf9e97a60f48..5017579f34dc 100644
> --- a/drivers/of/empty_root.dts
> +++ b/drivers/of/empty_root.dts
> @@ -2,5 +2,11 @@
>  /dts-v1/;
>
>  / {
> -
> +       /*
> +        * #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...

There's also dtc's idea of default which IIRC is 2 and 1 like OpenFirmware.

> +        */
> +       #address-cells = <0x02>;
> +       #size-cells = <0x01>;

I think we should just do 2 cells for size.

Rob