[PATCH] distro_bootcmd: Add support for loading user environment

[Jon Hunter]

On 31/01/2020 14:05, Tom Rini wrote:
> On Fri, Jan 31, 2020 at 01:24:21PM +0000, Jon Hunter wrote:
>>
>> On 31/01/2020 12:42, Soeren Moch wrote:
>>> On 31.01.20 11:59, Jon Hunter wrote:
>>>> U-Boot supports loading a user environment from a file in the
>>>> file-system. Therefore to make it easier for users to override the
>>>> default environment, add support to the 'distro_bootcmd' to look for
>>>> and load a user environment in a file called 'uEnv.txt' in the same
>>>> locations where an extlinux.conf or boot script might be found.
>>>
>>> We already have boot script support, which can easily be used to modify
>>> the environment. Do we really need to bloat the distro_boot machinery
>>> further with environment import, that is quite limited in contrast to
>>> boot scripts?
>>
>> If you are booting with an extlinux.conf file, as we do by default, then
>> if this file is found, this is always booted from before you have the
>> opportunity to run the boot.scr script.
>>
>> Furthermore if you did switch the order to boot from a boot.scr script
>> before the extlinux.conf, but you just wanted to do some simple
>> modifications of the environment before booting (ie. so the boot.scr
>> does not actually boot the system), then as the code is today you get a
>> 'echo SCRIPT FAILED: continuing...' message.
>>
>> Yes this could be changed, but just seems cleaner and simpler to add
>> support to make changes to the environment before the extlinux.conf is
>> loaded.
>>
>> Yes this does add more to the environment, but it is hardly significant
>> bloat, but if that is a concern then we could always disable this by
>> default and allow users to enable it.
> 
> This is something I think we had talked about and rejected doing
> initially.  Can you please expand on the use-case here, and why it's
> perhaps not better handled via PREBOOT on some platforms?  Thanks!

We have one specific use-case in a downstream version of u-boot where we
store some parameters in the u-boot environment for tweaking the DT
firmware on boot [0]. Now there are cases where we do not wish to modify
DT at all and so want to clear these variables and I would like a way to
automate the re-configuration of the u-boot environment without the user
having to manually set these just by the presence of a uEnv.txt file on
the disk.

Admittedly, this particular use-case/scenario is not directly applicable
to the mainline u-boot branch as there is no support for these
environment variables for Tegra (yet). However, it seemed that this
change would have some value in upstream u-boot as well to allow the
user to change any default environment variables without having to
re-compile and hence, I decided submit this change here.

For example, if we wanted to tweak any of the default environment
variables that are currently in 'include/configs/tegra210-common.h' we
could. Now you may say if we are not tweaking these very often, then why
not recompile, and we could, but given that we have the ability to load
environment parameters from a file on disk, it seems as shame that in
the current boot flow there is no way to make use of this extremely
useful feature.

Cheers!
Jon

[0]
https://nv-tegra.nvidia.com/gitweb/?p=3rdparty/u-boot.git;a=blob;f=include/configs/tegra210-common.h;h=81a3a2a447efacf34227847997fcc50639f54999;hb=refs/heads/l4t/l4t-r32.3.1-v2016.07#l59

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