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Date:   Fri, 6 Oct 2023 10:48:05 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Jisheng Zhang <jszhang@...nel.org>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] riscv: errata: thead: use riscv_nonstd_cache_ops
 for CMO

On Sun, Oct 1, 2023 at 6:46 PM Jisheng Zhang <jszhang@...nel.org> wrote:
>
> Previously, we use alternative mechanism to dynamically patch
> the CMO operations for THEAD C906/C910 during boot for performance
> reason. But as pointed out by Arnd, "there is already a significant
> cost in accessing the invalidated cache lines afterwards, which is
> likely going to be much higher than the cost of an indirect branch".
> And indeed, there's no performance difference with GMAC and EMMC per
> my test on Sipeed Lichee Pi 4A board.
>
> Use riscv_nonstd_cache_ops for THEAD C906/C910 CMO to simplify
> the alternative code, and to acchieve Arnd's goal -- "I think
> moving the THEAD ops at the same level as all nonstandard operations
> makes sense, but I'd still leave CMO as an explicit fast path that
> avoids the indirect branch. This seems like the right thing to do both
> for readability and for platforms on which the indirect branch has a
> noticeable overhead."
>
> To make bisect easy, I use two patches here: patch1 does the conversion
> which just mimics current CMO behavior via. riscv_nonstd_cache_ops, I
> assume no functionalities changes. patch2 uses T-HEAD PA based CMO
> instructions so that we don't need to covert PA to VA.
Sorry, I didn't see your second patch. If you have used PA, just
ignore my previous reply.

>
> Hi Guo,
>
> I didn't use wback_inv for wback as you suggested during v1 reviewing,
> this can be left as future optimizations.
Okay. I just don't want sg2042 & th1520 to have a difference here.

>
> Thanks
>
> since v1:
>   - collect Tested-by tag
>   - add patch2 to use T-HEAD PA based CMO instructions.
>
> Jisheng Zhang (2):
>   riscv: errata: thead: use riscv_nonstd_cache_ops for CMO
>   riscv: errata: thead: use pa based instructions for CMO
>
>  arch/riscv/Kconfig.errata            |  1 +
>  arch/riscv/errata/thead/errata.c     | 69 +++++++++++++++++++++++++++-
>  arch/riscv/include/asm/errata_list.h | 50 +++-----------------
>  3 files changed, 74 insertions(+), 46 deletions(-)
>
> --
> 2.40.1
>


-- 
Best Regards
 Guo Ren

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