linux-kernel - Re: [PATCH V6 09/22] LoongArch: Add boot and setup routines

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Date:   Tue, 1 Mar 2022 12:17:03 +0800
From:   Huacai Chen <chenhuacai@...il.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Airlie <airlied@...ux.ie>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Yanteng Si <siyanteng@...ngson.cn>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-efi <linux-efi@...r.kernel.org>
Subject: Re: [PATCH V6 09/22] LoongArch: Add boot and setup routines

Hi, Ard & Arnd,

On Mon, Feb 28, 2022 at 7:35 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Mon, 28 Feb 2022 at 12:24, Arnd Bergmann <arnd@...db.de> wrote:
> >
> > On Mon, Feb 28, 2022 at 11:42 AM Huacai Chen <chenhuacai@...il.com> wrote:
> > > On Mon, Feb 28, 2022 at 4:52 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> > > > On Mon, 28 Feb 2022 at 09:38, Huacai Chen <chenhuacai@...il.com> wrote:
> > > > > >
> > > > > > RISC-V is a useful reference for the changes needed - this is the most
> > > > > > recent addition to the EFI stub, and avoids some legacy stuff that new
> > > > > > architectures have no need for.
> > > > > We still want to support the raw elf kernel (RISC-V also does),
> > > > > because LoongArch also has MCU and SoC and we want to support FDT (I
> > > > > think this is reasonable, because RISC-V also supports raw elf).
> > > > >
> > > >
> > > > That is fine. So perhaps the best course of action is to omit the
> > > > UEFI/ACPI parts entirely for now, and focus on the DT/embedded use
> > > > case. Once all the spec pieces are in place, the UEFI + ACPI changes
> > > > can be presented as a single coherent set.
> > > It seems that I made you confusing. :)
> > > There are big CPUs and small CPUs (MCU and SoC), big CPUs use
> > > UEFI+ACPI, while small CPUs use FDT.
> > > At present, the only matured LoongArch CPU is Loongson-3A5000 (big
> > > CPU) which uses UEFI+ACPI.
> > > We want to support raw elf because it can run on both ACPI firmware
> > > and FDT firmware, but at present we only have ACPI firmware.
> >
> > Can't you just use the UEFI protocol for kernel entry regardless
> > of the bootloader? It seems odd to use a different protocol for loading
> > grub and the kernel, especially if that means you end up having to
> > support both protocols inside of u-boot and grub, in order to chain-load
> > a uefi application like grub.
> >
>
> I think this would make sense. Now that the EFI stub has generic
> support for loading the initrd via a UEFI specific protocol (of which
> u-boot already carries an implementation), booting via UEFI only would
> mean that no Linux boot protocol would need to be defined outside of
> the kernel at all (i.e., where to load the kernel, where to put the
> command line, where to put the initrd, other arch specific rules etc
> etc) UEFI already supports both ACPI and DT boot

After one night thinking, I agree with Ard that we can use RISCV-style
fdt to support the raw elf kernel at present, and add efistub support
after new UEFI SPEC released.

If I'm right, it seems that RISC-V passes a0 (hartid) and a1 (fdt
pointer, which contains cmdline, initrd, etc.) to the raw elf kernel.
And in my opinion, the main drawback of current LoongArch method
(a0=argc a1=argv a2=bootparamsinterface pointer) is it uses a
non-standard method to pass kernel args and initrd. So, can the below
new solution be acceptable?

a0=bootparamsinterface pointer (the same as a2 in current method)
a1=fdt pointer (contains cmdline, initrd, etc., like RISC-V, I think
this is the standard method)

Huacai