[Buildroot] Interested in board support for BeagleV-Ahead

[Kilian Zinnecker via buildroot <buildroot@buildroot.org>]

Hello,

I am interested in adding board support for the BeagleV-Ahead.

I already started trying to setup the board support, but at the moment it does 
not work yet, due to some gcc 12 and zicsr and zifencei RiscV extension issue 
(see below).

My first question is: As long as it does not yet work: Shall I already send 
(not yet working) patches to the mailing list, so that people can discuss 
about it and potentially help me? Maybe marked as RFC? Or shall I describe 
what I try as part of a normal email (like I do in this mail)?


Regarding board support, Yocto support already seems to exist:

https://git.beagleboard.org/beaglev-ahead/xuantie-yocto

Also a uboot and kernel:

https://git.beagleboard.org/beaglev-ahead/beaglev-ahead-u-boot
https://git.beagleboard.org/beaglev-ahead/beaglev-ahead-linux

I was able to successfully build the uboot, flash it onto an SD card and boot 
uboot. I then started to write a defconfig for the BeagleV-Ahead, starting from 
the already existing beaglev_defconfig. I used the following uboot settings:

BR2_TARGET_UBOOT=y
BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
BR2_TARGET_UBOOT_CUSTOM_GIT=y
BR2_TARGET_UBOOT_CUSTOM_REPO_URL="https://github.com/beagleboard/beaglev-ahead-u-boot.git"
BR2_TARGET_UBOOT_CUSTOM_REPO_VERSION="beaglev-v2020.01-1.1.2"
BR2_TARGET_UBOOT_BOARD_DEFCONFIG="light_beagle"
BR2_TARGET_UBOOT_CONFIG_FRAGMENT_FILES="board/beaglev-ahead/uboot.fragment"

The uboot fragment file overwrites a switch of the custom uboot defconfig, such 
that the custom uboot generates u-boot.dtb (without it the build fails):

CONFIG_OF_EMBED=n

For the kernel I use:

BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_10=y
BR2_LINUX_KERNEL=y
BR2_LINUX_KERNEL_CUSTOM_GIT=y
BR2_LINUX_KERNEL_CUSTOM_TARBALL=y
BR2_LINUX_KERNEL_CUSTOM_TARBALL_LOCATION="https://git.beagleboard.org/beaglev-ahead/beaglev-ahead-linux/-/archive/beaglev-v5.10.113-1.1.2/beaglev-ahead-linux-beaglev-v5.10.113-1.1.2.tar.gz"
BR2_LINUX_KERNEL_DEFCONFIG="light"
BR2_LINUX_KERNEL_DTS_SUPPORT=y
BR2_LINUX_KERNEL_INTREE_DTS_NAME="thead/light-beagle"
BR2_LINUX_KERNEL_INSTALL_TARGET=y
BR2_LINUX_KERNEL_NEEDS_HOST_OPENSSL=y


When trying to build, I currently encounter the following problems:

The SoC of the BeagleV-Ahead needs RiscV Vector Extension support enabled, 
else the build fails, because it cannot deal with the assembly instructions in 
some parts of the code. Hence, I believe one needs to activate support for 
this:

BR2_RISCV_ISA_CUSTOM_RVV=y

However, this enforces that gcc 12 or higher is used, see Config.in.riscv:

https://gitlab.com/buildroot.org/buildroot/-/blob/
b10ceec75520fab80f76081f50daaaab9df890f5/arch/Config.in.riscv#L69-72

It seems that the RiscV ISA was reorganized at some point, regarding which 
extension some microarch maintenance instructions belong to. Also it seems 
that this change was adopted in gcc 12 and that this caused some issues in the 
past, also within buildroot, see here:

https://lists.buildroot.org/pipermail/buildroot/2022-July/646795.html

The patch seems to be active in buildroot currently, see here:

https://gitlab.com/buildroot.org/buildroot/-/blob/
b10ceec75520fab80f76081f50daaaab9df890f5/arch/arch.mk.riscv#L38-39

However, when trying to build, I get lots of errors, that opcodes are not 
recognised and the zicsr and zifencei extension are required, e.g.:

"arch/riscv/cpu/mtrap.S:68: Error: unrecognized opcode `csrr a0,mcause', 
extension `zicsr' required"

I already tried using gcc 11 instead (omitting BR2_RISCV_ISA_CUSTOM_RVV and 
setting BR2_GCC_VERSION_11_X=y), however, as expected I get errors that vector 
instructions are unknown. Does anyone has an idea and can help me?

Best regards,
Kilian


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