[U-Boot] RFC: postload: Loading some U-Boot code later

[Wolfgang Denk]

Dear Simon Glass,

In message <CAPnjgZ0Y7Lnu6KiOsm8Q4qFqNeZh0wg3txFFiTqhzPZUW8Weig at mail.gmail.com> you wrote:
> 
> I am looking to implement a feature where a cut-down U-Boot is
> initially loaded, which is enough for a normal boot. But then when an
> abnormal boot is required (e.g. from a USB stick or network), we can
> load more code to cope with it.
...
> - has this been done before, or something similar?

This has been done before, since the very earliest versions of U-Boot
(or PPCboot, as it was called at that times).  The feature is called
standalone applications.  There are _many_ systems around where you
can load (and run) manufacturing test code, hardware diagnosis code,
burn in test stuff, proprietary code to perform top secret actions
nobody ever may know, etc.

> - any hints, tips, dragon warnings?
> - does the above approach sound reasonable?

I understand what you want, and why you want it.  But I disagree with
the suggested implementation.

If you just need a simple environment, you can probably do well with
the existing standalone approach.

If you are looking for a more powerful solution, that is more tightly
integrated with the rest of the U-Boot code (**) then we should make
the jump and implement loadable modules.  This has been done before,
too - see Linux for a big example, or barebox for a smaller, more
U-Boot-like one.

(**) The separation and restriction of the standalone code is an
     intentional one, made not for technical but for political
     reasons - standalone applications are provided as a means to run
     closed-source, proprietary code.  This is something that has to
     hurt a bit.

Best regards,

Wolfgang Denk

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