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

[Rick Chen]

Hi Tom

> 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?

Because RISC-V Linux runs in S-Mode, it needs OpenSBI which runs in
M-mode to handle SBI calls.
So it can not boot Linux (OS) directly from U-Boot SPL but passing
back to openSBI to do this.

Rick

>
> --
> Tom