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    SubjectRe: [PATCH 1/2] Documentation: riscv: Add early boot document
    Hey Alex,

    Many thanks for writing this up!

    On Mon, Jun 19, 2023 at 11:49:13AM +0200, Alexandre Ghiti wrote:
    > @Sunil V L Something about ACPI is more than welcome :)
    >
    > And thanks to @Björn Töpel and @Atish Kumar Patra for helping in
    > writing this document!
    >
    > On Mon, Jun 19, 2023 at 11:47 AM Alexandre Ghiti <alexghiti@rivosinc.com> wrote:
    > >
    > > This document describes the constraints and requirements of the early
    > > boot process in a RISC-V kernel.
    > >
    > > Szigned-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
    > > ---
    > > Documentation/riscv/boot-image-header.rst | 3 -
    > > Documentation/riscv/boot.rst | 181 ++++++++++++++++++++++
    > > Documentation/riscv/index.rst | 1 +
    > > 3 files changed, 182 insertions(+), 3 deletions(-)
    > > create mode 100644 Documentation/riscv/boot.rst
    > >
    > > diff --git a/Documentation/riscv/boot-image-header.rst b/Documentation/riscv/boot-image-header.rst
    > > index d7752533865f..a4a45310c4c4 100644
    > > --- a/Documentation/riscv/boot-image-header.rst
    > > +++ b/Documentation/riscv/boot-image-header.rst
    > > @@ -7,9 +7,6 @@ Boot image header in RISC-V Linux
    > >
    > > This document only describes the boot image header details for RISC-V Linux.
    > >
    > > -TODO:
    > > - Write a complete booting guide.
    > > -
    > > The following 64-byte header is present in decompressed Linux kernel image::
    > >
    > > u32 code0; /* Executable code */
    > > diff --git a/Documentation/riscv/boot.rst b/Documentation/riscv/boot.rst
    > > new file mode 100644
    > > index 000000000000..b02230818b79
    > > --- /dev/null
    > > +++ b/Documentation/riscv/boot.rst
    > > @@ -0,0 +1,181 @@
    > > +.. SPDX-License-Identifier: GPL-2.0
    > > +
    > > +=============================================
    > > +Early boot requirements/constraints on RISC-V
    > > +=============================================
    > > +
    > > +:Author: Alexandre Ghiti <alexghiti@rivosinc.com>
    > > +:Date: 23 May 2023
    > > +
    > > +This document describes what the RISC-V kernel expects from the previous stages
    > > +and the firmware, but also the constraints that any developer must have in mind
    > > +when touching the early boot process, e.g. before the final virtual mapping is
    > > +setup.
    > > +
    > > +Pre-kernel boot (Expectations from firmware)
    > > +============================================
    > > +
    > > +Registers state
    > > +---------------
    > > +
    > > +The RISC-V kernel expects:
    > > +
    > > + * `$a0` to contain the hartid of the current core.
    > > + * `$a1` to contain the address of the device tree in memory.
    > > +
    > > +CSR state
    > > +---------
    > > +
    > > +The RISC-V kernel expects:
    > > +
    > > + * `$satp = 0`: the MMU must be disabled.
    > > +
    > > +Reserved memory for resident firmware
    > > +-------------------------------------
    > > +
    > > +The RISC-V kernel expects the firmware to mark any resident memory with the
    > > +`no-map` flag, thus the kernel won't map those regions in the direct mapping
    > > +(avoiding issues with hibernation, speculative accesses and probably other
    > > +subsystems).
    > > +
    > > +Kernel location
    > > +---------------
    > > +
    > > +The RISC-V kernel expects to be placed at a PMD boundary (2MB for rv64 and 4MB
    > > +for rv32). Note though that the EFI stub will physically relocate the kernel if
    > > +that's not the case.
    > > +
    > > +Device-tree
    > > +-----------
    > > +
    > > +The RISC-V kernel always expects a device tree, it is:
    > > +
    In general, the firmware can pass either DT or ACPI (not both at the
    same time) to the OS. So, I think the statement that kernel always
    expects DT probably causes confusion. Can we mention some thing like
    below?

    The firmware can pass either 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 device" tree section below for
    details of the tiny DeviceTree.

    Thanks,
    Sunil

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