[U-Boot] [PATCH v2 1/1] board: raspberrypi: add serial and revision to the device tree

[Stephen Warren]

On 2/20/19 10:04 AM, Alexander Graf wrote:
> On 02/20/2019 05:59 PM, Stephen Warren wrote:
>> On 2/20/19 5:09 AM, Anton Gerasimov wrote:
>>> Raspberry Pi bootloader adds this node to fdt, but if u-boot script
>>> doesn't reuse the tree provided by it, this information is lost.
>>>
>>> Revision and serial are displayed in /proc/cpuinfo after boot.
>>
>> Are these properties documented in the upstream Linux kernel in 
>> Documentation/devicetree/bindings/? If so, this patch is fine. If not, 
>> we should not merge it, since we don't want to pollute the DT with 
>> non-standard properties (or: add this to the upstream kernel DT 
>> documentation, and then apply this patch once the doc update is 
>> approved upstream).
> 
> Hm, it almost looks like it's a downstream hack. Unfortunately as U-Boot 
> we're in this really weird place where people may want to use it to load 
> downstream kernels.

For similar things in L4T's[1] downstream fork of U-Boot, what I've done 
is this:

a) U-Boot implements the code to set various DT properties, or copy them 
from the DT provided by whatever-runs-before-U-Boot to the DT U-Boot 
provides to whatever-runs-after-U-Boot.

b) Whether U-Boot actually does this set/copy operation is determined by 
the value of an environment variable.

c) The default U-Boot environment enables all set/copy operations 
required by L4T.

d) If someone wants a "pure upstream" DT passed to the kernel, they can 
edit the environment to disable those operations, and save the environment.

This allows users to select which kind of DT they want passed to the kernel.

For example, see dt-edit.* at:
> http://nv-tegra.nvidia.com/gitweb/?p=3rdparty/u-boot.git;a=tree;f=arch/arm/mach-tegra;h=d9a17eec4f0271cdc3932f0cee8c39fb17197a0b;hb=l4t/l4t-r27.1
... and MEM_LAYOUT_ENV_SETTINGS in:
> http://nv-tegra.nvidia.com/gitweb/?p=3rdparty/u-boot.git;a=blob;f=include/configs/tegra210-common.h;h=156b280a00f37f811c96d0006fe2b87bdeb07e74;hb=l4t/l4t-r27.1

I'm pretty sure the value of fdt_copy_node_paths was longer in some 
release, but I don't remember which one, so can't quickly find it. And 
yes, the links above are about copying nodes between DTs, but you can 
imagine a similar flag env. var. for the feature in this current patch too.

[1] NVIDIA Linux for Tegra.