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@xxxxxxxxxxxx> wrote: > > > > This document describes the constraints and requirements of the early > > boot process in a RISC-V kernel. > > > > Szigned-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> > > --- > > 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@xxxxxxxxxxxx> > > +: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
