linux-kernel - Re: [PATCH RFC 0/2] RISC-V: T-Head vector handling

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Date:   Tue, 14 Mar 2023 22:29:41 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     heiko@...ech.de
CC:     linux-riscv@...ts.infradead.org, samuel@...lland.org,
        guoren@...nel.org, christoph.muellner@...ll.eu, heiko@...ech.de,
        Conor Dooley <conor.dooley@...rochip.com>,
        linux-kernel@...r.kernel.org, heiko.stuebner@...ll.eu
Subject:     Re: [PATCH RFC 0/2] RISC-V: T-Head vector handling

On Tue, 28 Feb 2023 13:54:33 PST (-0800), heiko@...ech.de wrote:
> From: Heiko Stuebner <heiko.stuebner@...ll.eu>
>
> As is widely known the T-Head C9xx cores used for example in the
> Allwinner D1 implement an older non-ratified variant of the vector spec.
>
> While userspace will probably have a lot more problems implementing
> support for both, on the kernel side the needed changes are actually
> somewhat small'ish and can be handled via alternatives somewhat nicely.
>
> With this patchset I could run the same userspace program (picked from
> some riscv-vector-test repository) that does some vector additions on
> both qemu and a d1-nezha board. On both platforms it ran sucessfully and
> even produced the same results.
>
>
> As can be seen in the todo list, there are 2 places where the changed
> SR_VS location still needs to be handled in the next revision
> (assembly + ALTERNATIVES + constants + probably stringify resulted in
>  some grey hair so far already)
>
>
> ToDo:
> - follow along with the base vector patchset
> - handle SR_VS access in _save_context and _secondary_start_sbi
>
>
> Heiko Stuebner (2):
>   RISC-V: define the elements of the VCSR vector CSR
>   RISC-V: add T-Head vector errata handling
>
>  arch/riscv/Kconfig.erratas           |  13 +++
>  arch/riscv/errata/thead/errata.c     |  32 ++++++
>  arch/riscv/include/asm/csr.h         |  31 +++++-
>  arch/riscv/include/asm/errata_list.h |  62 +++++++++++-
>  arch/riscv/include/asm/vector.h      | 139 +++++++++++++++++++++++++--
>  5 files changed, 261 insertions(+), 16 deletions(-)

I have no opposition to calling the T-Head vector stuff an errata 
against V, the RISC-V folks have already made it quite apparent that 
anything goes here.  I would like to get the standard V uABI sorted out 
first, though, as there's still a lot of moving pieces there.  It's kind 
of hard here as T-Head got thrown under the bus, but I'm not sure what 
else to do about it.