Re: [PATCH V15 11/24] LoongArch: Add boot and setup routines

[WANG Xuerui]

On 6/3/22 18:02, Arnd Bergmann wrote:
> On Fri, Jun 3, 2022 at 11:27 AM WANG Xuerui <kernel@xxxxxxxxxx> wrote:
> > On 6/3/22 15:20, Huacai Chen wrote:
> > > Add basic boot, setup and reset routines for LoongArch. Now, LoongArch
> > > machines use UEFI-based firmware. The firmware passes configuration
> > > information to the kernel via ACPI and DMI/SMBIOS.
> > >
> > > Currently an existing interface between the kernel and the bootloader
> > > is implemented. Kernel gets 2 values from the bootloader, passed in
> > > registers a0 and a1; a0 is an "EFI boot flag" distinguishing UEFI and
> > > non-UEFI firmware, while a1 is a pointer to an FDT with systable,
> > > memmap, cmdline and initrd information.
> > >
> > > The standard UEFI boot protocol (EFISTUB) will be added later.
> > >
> > > Cc: linux-efi@xxxxxxxxxxxxxxx
> > > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > Reviewed-by: WANG Xuerui <git@xxxxxxxxxx>
> > > Reviewed-by: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>
> > > Co-developed-by: Yun Liu <liuyun@xxxxxxxxxxx>
> > > Signed-off-by: Yun Liu <liuyun@xxxxxxxxxxx>
> > > Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> > Would you please look at this patch, which has all the arch-independent
> > changes backed out, and Ack if it is fit for mainlining?
> >
> > I communicated a little with Huacai about the approach for supporting
> > alternative boot protocols down the road, and we agreed to carry the
> > respective changes downstream. And if needs truly arise for modifying
> > common EFI logic, we can do so in a non-rushed manner later.
> >
> > For the current status of the code, apparently it just accepts the
> > standard efistub-shape FDT pointer from (whatever booting the image),
> > and everything onwards are fully using the common code without
> > modification as you can see from the diffstat. I rebased my BPI support
> > patch on top of this (basically translating Loongson BPI data structures
> > into the expected FDT form), and can confirm the boot can progress to
> > the same point as before -- indeed the SVAM changes etc. are not
> > necessary for a working system, and the code remains working.
> I'm a bit lost here: Does this mean the v15 version is back to the old
> pre-efistub interface and allows booting with existing firmware, or
> is it now left out completely? I still see a kernel_entry() function
> in head.S, and I see references to loongson_sysconf, but I don't
> see if that is what gets passed in from the bootloader.
It's not the same interface as in some of the very early revisions; the 
earlier versions relied on "struct bootparamsinterface" or BPI, while 
it's the same FDT-based interface to initialize EFI from, as in 
arch/arm64 and arch/riscv I believe. No Loongson-specific things remain now.
> I really want to make sure that without the EFI stub, there is no
> other way to boot the kernel that would have to get maintained
> in the long run.
Yeah this is the case right now. No LoongArch bootloader that I know of 
can prepare the EFI stub-shaped FDT that the current code expects, and I 
don't know of any future Loongson plan to do that either (Loongson's 
previous in-house efforts all looked something different). So it's 
pretty safe to say the current code wouldn't get frozen once mainlined.
>          Arnd