[RFC PATCH 0/3] riscv: Add riscv.fwsz kernel parameter to save memory

[Atish Patra]

On Tue, Nov 23, 2021 at 11:33 AM Heiko Stübner <heiko at sntech.de> wrote:
>
> Hi Guo,
>
> Am Dienstag, 23. November 2021, 02:57:14 CET schrieb guoren at kernel.org:
> > From: Guo Ren <guoren at linux.alibaba.com>
> >
> > The firmware of riscv (such as opensbi) occupy 2MB(64bit) /
> > 4MB(32bit) in Linux. It's very wasteful to small memory footprint
> > soc chip such as Allwinner D1s/F133. The kernel parameter gives a
> > chance to users to set the proper size of the firmware and get
> > more than 1.5MB of memory.
>
> is this kernel parameter approach a result of the T-Head Ice-SoC
> currently loading its openSBI from inside the main u-boot via extfs-load,
> directly before the kernel itself [0] ?

Looking at the defconfig[1], it may be U-Boot SPL not U-Boot proper. I
may be looking at the wrong config though.
If U-Boot SPL is actually used, you don't even need to manually load
OpenSBI "fw_jump" binary.

As Heiko pointed, you should just follow how U-Boot SPL works on
hifive unmatched (creating the FIT image)
The standard U-Boot SPL uses with fw_dynamic which provides all the
flexibility you want.

[1] https://github.com/T-head-Semi/u-boot/blob/main/configs/ice_evb_c910_defconfig
>
> Because that approach in general looks not ideal.
>
> Normally you want the main u-boot already running with less privileges
> so firmware like openSBI should've been already loaded before that.
> Even more true when you're employing methods to protect memory regions
> from less privileged access.
>
> A lot of socs set u-boot as opensbi payload, but for the example the D1
> mainline approach uses the Allwinner TOC1 image format to load both
> opensbi and the main uboot into memory from its 1st stage loader.
>
>
> Of course the best way would be to just mimic what a number of
> arm64 and also riscv socs do and use already existing u-boot utilities.
>
> U-Boot can create a FIT image containing both main u-boot, dtb and
> firmware images that all get loaded from SPL and placed at the correct
> addresses before having the SPL jump into opensbi and from there
> into u-boot [1] .
>
> And as Anup was writing, reserved-memory should then be the way
> to go to tell the kernel what regions to omit.
>
> And mainline u-boot has already the means to even take the reserved-memory
> from the devicetree used by opensbi and copy it to a new devicetree,
> if the second one is different.
>
>
> Heiko
>
>
> [0] https://github.com/T-head-Semi/u-boot/blob/main/include/configs/ice-c910.h#L46
> [1] see spl_invoke_opensbi() in common/spl/spl_opensbi.c
> [2] see riscv_board_reserved_mem_fixup() in arch/riscv/lib/fdt_fixup.c
>
> >
> > Guo Ren (3):
> >   riscv: Remove 2MB offset in the mm layout
> >   riscv: Add early_param to decrease firmware region
> >   riscv: Add riscv.fwsz kernel parameter
> >
> >  .../admin-guide/kernel-parameters.txt         |  3 +++
> >  arch/riscv/include/asm/page.h                 |  8 +++++++
> >  arch/riscv/kernel/head.S                      | 10 +++-----
> >  arch/riscv/kernel/vmlinux.lds.S               |  5 ++--
> >  arch/riscv/mm/init.c                          | 23 ++++++++++++++++---
> >  5 files changed, 36 insertions(+), 13 deletions(-)
> >
> >
>
>
>
>
>
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-- 
Regards,
Atish