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

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Message-ID: <1961474.usQuhbGJ8B@diego>
Date:   Fri, 23 Jun 2023 12:22:35 +0200
From:   Heiko Stübner <heiko@...ech.de>
To:     Palmer Dabbelt <palmer@...belt.com>,
        Stefan O'Rear <sorear@...tmail.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

Am Freitag, 23. Juni 2023, 05:06:44 CEST schrieb Stefan O'Rear:
> 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)).

For one, you're right that I should definitly try to confirm this on hardware :-) .

But also the VXSAT and VXRM CSRs are actually documented in that pdf
on page 335


Heiko


> 
> -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
>