[PATCH 1/4] riscv: spl: Introduce SPL_OPENSBI_OS_BOOT

[Rick Chen]

Hi Sean,

> On 12/12/22 10:03, Tom Rini wrote:
> > On Mon, Dec 12, 2022 at 02:45:10PM +0800, Rick Chen wrote:
> >> Hi Tom
> >>
> >>> On Fri, Dec 09, 2022 at 08:48:37AM -0500, Sean Anderson wrote:
> >>>> On 12/7/22 01:23, Rick Chen wrote:
> >>>>> In RISC-V, it only provide normal mode booting currently.
> >>>>> To speed up the booting process, here provide SPL_OPENSBI_OS_BOOT
> >>>>> to achieve this feature which will be call Fast-Boot mode. By
> >>>>
> >>>> Can you name this something different. We already have something called
> >>>> fastboot in-tree (the Android-derived protocol) and there's a Microsoft
> >>>> technology called fastboot (some kind of hibernation). "OS Boot" isn't
> >>>> very specific either, since we (almost always) boot an OS. Maybe "Eagle
> >>>> mode" by analogy to Falcon mode, which lets SPL directly boot an OS.
> >>>>
> >>>> (Is this substantially different from falcon mode anyway?)
> >>>
> >>> I was kind of wondering if this is different, really, from Falcon Mode.
> >>> Falcon Mode didn't initially have to factor in other-firmware as that's
> >>> not a hard requirement on arm32 like it is on arm64 or risc-v.  But my
> >>> first read of this was that it seems like the RISC-V specific side of
> >>> doing Falcon Mode and dealing with the prior stage needs correctly.
> >>>
> >>
> >> Yes. It is a little bit different from the Falcon mode (SPL_OS_BOOT=y).
> >> When I try to enable SPL_OS_BOOT, it will encounter that SYS_SPL_ARGS_ADDR and
> >>   jump_to_image_linux() shall be defined but they are un-necessary for RISC-V.
> >> Because the flow of OpenSBI and SPL_OS_BOOT are totally different code
> >> flow in board_init_r() of common/spl/spl.c.
> >> That is why I added a new symbol called SPL_OPENSBI_OS_BOOT for this
> >> RISC-V fast boot implementation.
> >
> > Those sound like fairly minor challenges for the same fundamental
> > concept. We have SYS_SPL_ARGS_ADDR for "where is the device tree to
> > pass along". We might need to do a little code re-factoring here. But
> > maybe also a little bit of explaining why we wouldn't be booting to the
> > OS directly but instead passing back to openSBI to do this? That's not
> > normally how RISC-V boots the OS, right? Or am I miss-understanding
> > something here?
> >
>
> The usual process is
>
> ROM -> SPL -> SBI -> U-Boot -> Linux

In RISC-V, Linux runs in S-mode, it needs the OpenSBI which plays as
M-mode to deal with SBI calls at run-time.
So the fast boot idea, I just want to bypass U-Boot and jump to Linux
directly and Linux still needs OpenSBI.
Hence SPL_OPENSBI can't be disabled here.

Rick

>
> Where SPL loads SBI and U-Boot and tells SBI "please run U-Boot after you are
> done booting". But the idea here is to load Linux instead of U-Boot. I think
> this is pretty similar to Falcon mode. Actually, when CONFIG_SPL_OPENSBI is
> enabled, I think it's reasonable for falcon mode to do exactly that. People
> who want the SPL -> Linux path can disable SPL_OPENSBI.
>
> --Sean