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

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Date:   Sat, 24 Jun 2023 01:26:12 +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, 12:22:35 CEST schrieb Heiko Stübner:
> 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 :-) .

ok, so now I know the documentation is wrong.

	before, vxrm 0x0, vxsat 0x0
writing the 0x700 to fcsr
	after, vxrm 0x3, vxsat 0x1

Essentially the link between the CSRs really is there - oh fun.
So we're back at your original concern - sadly.

I guess I need to figure out how to not have this stuff break
because relying on the fpu parts to handle feels not correct
at first glance.


Heiko