[U-Boot-Custodians] SPL / TPL

[Masahiro Yamada]

Hi Bin,


On Wed, Oct 9, 2019 at 12:31 AM Bin Meng <bmeng.cn at gmail.com> wrote:
>
> Hi,
>
> Simon is on vacation, hence I hoped to get the clarification from
> other maintainers.
>
> As I see in U-Boot currently only NXP PowerPC uses TPL in the NAND
> configurations, and some ARM boards use TPL.
>
> It looks in x86 we are using TPL in a different boot flow and this
> confused me a lot in the past.
> In x86 we expect TPL to load SPL then U-Boot proper. (TPL => SPL =>
> U-Boot proper)
>
> However in README.TPL, it says TPL is the 3rd program loader, to be
> loaded by SPL. So if my read is correct, the expected boot flow is SPL
> => TPL => U-Boot proper.
>
> I believe only one boot flow is the correct way. So which one is correct?



When the TPL support was first developed,
it worked as  SPL -> TPL -> U-Boot proper.

SPL is a short of Secondary Program Loader,
and TPL was Tertiary Program Loader.

For the platforms using this 3 stage bootstrap,
its SPL is really small (and implemented in a rather ad-hoc way)
and its TPL tends to be as large as the SPL of other non-TPL platforms.

I noticed some problems in the SPL framework
because TPL corresponds to SPL of the 2-stage-boot platforms.

As a result, we end up with duplicating SPL configuraion
like CONFIG_SPL_FOO, CONFIG_SPL_BAR,
to CONFIG_TPL_FOO, CONFIG_TPL_BAR.


I pointed out TPL -> SPL -> U-Boot proper
would make more sense.

TPL (let's say Tiny Program Loader) is a really small
implementation for that platform,
and SPL can re-use the common SPL framework.

This requires flipping the boot order.

I do not know how much consensus we got,
or documented somewhere, though.




> Regards,
> Bin
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-- 
Best Regards
Masahiro Yamada