linux-kernel - Re: [RFC] MIPS: BCM63XX: add empty Device Trees for all supported boards

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Date:	Wed, 14 Nov 2012 13:15:26 +0100
From:	Jonas Gorski <jonas.gorski@...il.com>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	linux-mips@...ux-mips.org, Ralf Baechle <ralf@...ux-mips.org>,
	John Crispin <blogic@...nwrt.org>,
	Maxime Bizon <mbizon@...ebox.fr>,
	Florian Fainelli <florian@...nwrt.org>,
	Kevin Cernekee <cernekee@...il.com>,
	devicetree-discuss@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] MIPS: BCM63XX: add empty Device Trees for all supported boards

On 13 November 2012 06:12, Stephen Warren <swarren@...dotorg.org> wrote:
> On 11/11/2012 05:50 AM, Jonas Gorski wrote:
>> Add empty board files for all boards supported by the legacy board
>> support.
>
>> diff --git a/arch/mips/bcm63xx/dts/96328avng.dts b/arch/mips/bcm63xx/dts/96328avng.dts
>
>> +/ {
>> +     model = "96328avng";
>> +     compatible = "96328avng";
>
> The board should be compatible with both the board name and the SoC on
> the board. I know that right now the MIPS code is choosing the DT to use
> based on the board name, but I think it's more typical to pass an
> explicit DT to the kernel, and then choose the kernel support to execute
> based on the compatible value (certainly this is the case on ARM and I
> assume other architectures too). That would require the DT content to
> include the SoC name in the compatible property, so that the kernel
> support didn't then need to contain a table of all supported board names.

I'll add the SoC name to the compatible line.

>> +     ubus@...00000 {
>> +
>> +     };
>
> Do you need to include this empty node in each file? I guess it gets
> added to in the next patch so it's not a big deal though.

It's just there so it is already present when adding blocks to it. It
is/was mainly for making reordering patches easier.

>
>> diff --git a/arch/mips/bcm63xx/dts/Kconfig b/arch/mips/bcm63xx/dts/Kconfig
>
>> +config BOARD_96328AVNG
>> +     bool "96328avng reference board"
>> +     select BCM63XX_CPU_6328
>
> Why not simply compile all DTs whenever the SoC support is enabled? I
> suppose you're trying to avoid packing all the DTs into the kernel
> image. Does it make more sense to amend the MIPS kernel boot process so
> that a single user-/firmware-selected DT is passed to the kernel, rather
> than packing the DTs into the kernel and selecting one?

The plan is to add support for an externally attached DT (but not
present yet), and eventually add support for a bootloader passed DT,
but since I don't know yet how these will work, I didn't want to add
something based on guesses.

My reasoning for allowing (de-)selecting each board is to dampen the
bloat from the dtbs - after these few blocks the combined dtbs are
already four times as large as the old board setup code including all
boards. Especially older devices are constrained to 4 or even 2 MB
flash, so every kB counts there.


Jonas
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