linux-kernel - Re: [PATCH v2 1/2] riscv: errata: thead: use riscv_nonstd_cache

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Date:   Mon, 2 Oct 2023 11:55:31 +0100
From:   Conor Dooley <conor@...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>, Guo Ren <guoren@...nel.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Emil Renner Berthing <emil.renner.berthing@...onical.com>
Subject: Re: [PATCH v2 1/2] riscv: errata: thead: use riscv_nonstd_cache_ops
 for CMO

On Sun, Oct 01, 2023 at 06:34:32PM +0800, Jisheng Zhang 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."
> 
> Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> Tested-by: Emil Renner Berthing <emil.renner.berthing@...onical.com>

Reviewed-by: Conor Dooley <conor.dooley@...rochip.com>

Thanks,
Conor.

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