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

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Message-Id: <c01ee919-9abb-4796-9ed4-9bb4565dc863@app.fastmail.com>
Date:   Thu, 22 Jun 2023 23:06:44 -0400
From:   "Stefan O'Rear" <sorear@...tmail.com>
To:     "Heiko Stuebner" <heiko@...ech.de>,
        "Palmer Dabbelt" <palmer@...belt.com>
Cc:     linux-riscv@...ts.infradead.org, samuel@...lland.org,
        guoren@...nel.org, christoph.muellner@...ll.eu,
        conor.dooley@...rochip.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 2/2] RISC-V: add T-Head vector errata handling

On Thu, Jun 22, 2023, at 4:35 PM, Heiko Stübner wrote:
> Am Donnerstag, 22. Juni 2023, 20:58:37 CEST schrieb Stefan O'Rear:
>> Are you aware of "3.7. Vector Fixed-Point Fields in fcsr" in
>> riscv-v-spec-0.7.1.pdf?
>
> oh wow, thanks a lot for that pointer, now I understand your concern.
>
> So in vector-0.7.1 fcsr[10:9] mirrors vxrm and fcsr[8] mirrors vxsat.
>
>
> On a positive note, the T-Head cores seem to not implement the full
> vector 0.7.1 specification after all, in the documentation I have [0]
> fcsr[31:8] are declared as "0" and uppermost bits are [7:5] for the "frm"
> field.

Given that the pdf you linked does not mention any vector CSRs, I am not
confident that it provides a complete and accurate description of vector
functionality in other registers for the C906 with vector extension.

Assuming that you have access to such a chip, I would be much happier with
the proposed "just a comment" approach if our understanding of the behavior
were confirmed on hardware (specifically: csr_write(CSR_FCSR, 0x700) should
not affect csr_read(CSR_VXRM) or csr_read(CSR_VXSAT)).

-s

> So I guess a code comment should suffice to explain :-)
>
>
> Regards
> Heiko
>
>
> [0] 
> https://github.com/T-head-Semi/openc906/blob/main/doc/%E7%8E%84%E9%93%81C906%E7%94%A8%E6%88%B7%E6%89%8B%E5%86%8C.pdf
> 16.3.1.3 浮点控制状态寄存器（FCSR） on page 334