linux-kernel - Re: [PATCH v2] of/fdt: Allow memory node and root to have different size/address cells

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Date:	Thu, 01 Oct 2015 14:21:59 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	Rob Herring <robh+dt@...nel.org>,
	kbuild test robot <lkp@...el.com>,
	"kbuild-all@...org" <kbuild-all@...org>,
	Grant Likely <grant.likely@...aro.org>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH v2] of/fdt: Allow memory node and root to have different size/address cells

On Thursday 01 October 2015 17:48:21 Vineet Gupta wrote:
> Currently memory node parsing uses root "#size-cells", "#address-cells"
> This doesn't work correctly when memory address/size is different or
> greater than root's.
> 
> e.g. ARC 32-bit systems implementing physical adressing extension and
> say 4GB of memory. All peripherals mappings stay within the 4GB (so root
> address/size cells remain 1 each), only the memory node address/size
> cells needs to specify greater than 32-bits as below
> 
>         memory {
>                 device_type = "memory";
>                 reg = <0x0 0x80000000 0x1 0x00000000>;  /* 4 GB */
>                 #address-cells = <2>;
>                 #size-cells = <2>;
>         };
> 
> This patch lets me boot a ARC system with PAE40 + 4GB of memory specified
> as above and fails to boot otherwise as memory parsing doesn't populate
> right base, size.
> 

This looks wrong: the #address-cells property in a device node is used
to parse the reg property of its child nodes, not the node itself.

The only way to list memory like this is to put #size-cells=<2>
into the root node. All lower bus nodes can then use the
normal #address-cells/#size-cells again and use a ranges property
to convert the register ranges so you don't need to update all
nodes.

	Arnd
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