[PATCH v5 2/3] Documentation: riscv: Add early boot document

[Alexandre Ghiti]

Hi Randy,

On Sat, Jul 22, 2023 at 10:19 PM Randy Dunlap <rdunlap at infradead.org> wrote:
>
> Hi,
>
> On 7/22/23 04:34, Alexandre Ghiti wrote:
> > This document describes the constraints and requirements of the early
> > boot process in a RISC-V kernel.
> >
> > Signed-off-by: Alexandre Ghiti <alexghiti at rivosinc.com>
> > Reviewed-by: Björn Töpel <bjorn at rivosinc.com>
> > Reviewed-by: Conor Dooley <conor.dooley at microchip.com>
> > Reviewed-by: Sunil V L <sunilvl at ventanamicro.com>
> > Reviewed-by: Andrew Jones <ajones at ventanamicro.com>
> > Reviewed-by: Palmer Dabbelt <palmer at rivosinc.com>
> > Reviewed-by: Atish Patra <atishp at rivosinc.com>
> > Reviewed-by: Song Shuai <songshuaishuai at tinylab.org>
> > Acked-by: Palmer Dabbelt <palmer at rivosinc.com>
> > ---
> > - Changes in v5:
> >   * Rebase on top of docs-next
> >
> >  Documentation/riscv/boot-image-header.rst |   3 -
> >  Documentation/riscv/boot.rst              | 169 ++++++++++++++++++++++
> >  Documentation/riscv/index.rst             |   1 +
> >  3 files changed, 170 insertions(+), 3 deletions(-)
> >  create mode 100644 Documentation/riscv/boot.rst
> >
>
> > diff --git a/Documentation/riscv/boot.rst b/Documentation/riscv/boot.rst
> > new file mode 100644
> > index 000000000000..f890ac442c91
> > --- /dev/null
> > +++ b/Documentation/riscv/boot.rst
> > @@ -0,0 +1,169 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +===============================================
> > +RISC-V Kernel Boot Requirements and Constraints
> > +===============================================
> > +
> > +:Author: Alexandre Ghiti <alexghiti at rivosinc.com>
> > +:Date: 23 May 2023
> > +
> > +This document describes what the RISC-V kernel expects from bootloaders and
> > +firmware, but also the constraints that any developer must have in mind when
>
> I would s/but/and/.

Ok I change that, thanks.

>
> > +touching the early boot process. For the purposes of this document, the
> > +``early boot process`` refers to any code that runs before the final virtual
> > +mapping is set up.
> > +
> > +Pre-kernel Requirements and Constraints
> > +=======================================
> > +
> > +The RISC-V kernel expects the following of bootloaders and platform firmware:
> > +
> > +Register state
> > +--------------
> > +
> > +The RISC-V kernel expects:
> > +
> > +  * ``$a0`` to contain the hartid of the current core.
> > +  * ``$a1`` to contain the address of the devicetree in memory.
> > +
> > +CSR state
> > +---------
> > +
> > +The RISC-V kernel expects:
> > +
> > +  * ``$satp = 0``: the MMU, if present, must be disabled.
> > +
> > +Reserved memory for resident firmware
> > +-------------------------------------
> > +
> > +The RISC-V kernel must not map any resident memory, or memory protected with
> > +PMPs, in the direct mapping, so the firmware must correctly mark those regions
> > +as per the devicetree specification and/or the UEFI specification.
> > +
> > +Kernel location
> > +---------------
> > +
> > +The RISC-V kernel expects to be placed at a PMD boundary (2MB aligned for rv64
> > +and 4MB aligned for rv32). Note that the EFI stub will physically relocate the
> > +kernel if that's not the case.
> > +
> > +Hardware description
> > +--------------------
> > +
> > +The firmware can pass either a devicetree or ACPI tables to the RISC-V kernel.
> > +
> > +The devicetree is either passed directly to the kernel from the previous stage
> > +using the ``$a1`` register, or when booting with UEFI, it can be passed using the
> > +EFI configuration table.
> > +
> > +The ACPI tables are passed to the kernel using the EFI configuration table. In
> > +this case, a tiny devicetree is still created by the EFI stub. Please refer to
> > +"EFI stub and devicetree" section below for details about this devicetree.
> > +
> > +Kernel entrance
> > +---------------
>
> How about "entry" instead of "entrance"?

I have to admit that I don't have the nuance between both words, if
'entry' is more appropriate, I'll change it.

>
> > +
> > +On SMP systems, there are 2 methods to enter the kernel:
> > +
> > +- ``RISCV_BOOT_SPINWAIT``: the firmware releases all harts in the kernel, one hart
> > +  wins a lottery and executes the early boot code while the other harts are
> > +  parked waiting for the initialization to finish. This method is mostly used to
> > +  support older firmwares without SBI HSM extension and M-mode RISC-V kernel.
> > +- ``Ordered booting``: the firmware releases only one hart that will execute the
> > +  initialization phase and then will start all other harts using the SBI HSM
> > +  extension. The ordered booting method is the preferred booting method for
> > +  booting the RISC-V kernel because it can support cpu hotplug and kexec.
>
> preferably s/cpu/CPU/

Done!

>
> > +
> > +UEFI
> > +----
>
> [snip]
>
> I can't say how correct the documentation is, but it is well-written
> and has no issues with punctuation, grammar, or spelling AFAICT, so
> you can take this if you want it:
>
> Reviewed-by: Randy Dunlap <rdunlap at infradead.org>

Thanks for the nice comments, @Conor Dooley (and others) really helped!

I'll respin a new version today,

Alex

>
> thanks.
> --
> ~Randy