[U-Boot-Custodians] SPL / TPL

[Joe Hershberger]

Hi Bin,

On Tue, Oct 8, 2019 at 10:14 PM Bin Meng <bmeng.cn at gmail.com> wrote:
>
> Hi Masahiro,
>
> On Wed, Oct 9, 2019 at 12:50 AM Masahiro Yamada
> <yamada.masahiro at socionext.com> wrote:
> >
> > 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.
> >
>
> Thank you very much for the information. This clears my confusion.
>
> >
> > 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.
>
> I think you wanted to say "TPL can re-use the common SPL framework"?

That's not how I read his comment. I think he was saying that
regardless if you have a very stripped first stage, the SPL is always
sharing the same framework that immediately preceeds U-Boot proper.

> >
> > This requires flipping the boot order.
> >
> > I do not know how much consensus we got,
> > or documented somewhere, though.
> >
>
> It looks to me that we never get in agreement as we have the document
> README.TPL describes one boot flow and some other platforms implement
> another way.

It would be nice to make this consistent and update the docs to match.

-Joe

> Regards,
> Bin
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