linux-kernel - Re: [PATCH v4 5/7] riscv: mm: dma-noncoherent: Pass direction and operation to ALT_CMO

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Date:   Fri, 25 Nov 2022 12:49:06 -0600
From:   Samuel Holland <samuel@...lland.org>
To:     "Lad, Prabhakar" <prabhakar.csengg@...il.com>,
        Heiko Stübner <heiko@...ech.de>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Magnus Damm <magnus.damm@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Guo Ren <guoren@...nel.org>,
        Jisheng Zhang <jszhang@...nel.org>,
        Atish Patra <atishp@...osinc.com>,
        Anup Patel <apatel@...tanamicro.com>,
        Andrew Jones <ajones@...tanamicro.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Philipp Tomsich <philipp.tomsich@...ll.eu>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-renesas-soc@...r.kernel.org,
        Biju Das <biju.das.jz@...renesas.com>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH v4 5/7] riscv: mm: dma-noncoherent: Pass direction and
 operation to ALT_CMO_OP()

On 11/24/22 13:18, Lad, Prabhakar wrote:
> Hi Heiko,
> 
> Thank you for the review.
> 
> On Thu, Nov 24, 2022 at 6:29 PM Heiko Stübner <heiko@...ech.de> wrote:
>>
>> Am Donnerstag, 24. November 2022, 18:22:05 CET schrieb Prabhakar:
>>> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
>>>
>>> Pass direction and operation to ALT_CMO_OP() macro.
>>>
>>> This is in preparation for adding errata for the Andes CPU core.
>>
>> can you provide more explanation why that is necessary please?
>> I guess you want to use different cache operations for some cases?
>>
> Yes basically to call different cache operations based on the dir and
> operations (and also this allows to export just one function to handle
> the errata). I'll update the commit message in the next version.

This makes things less efficient, because it requires more instructions
and registers inside the alternative section, and your function
duplicates the logic from arch_sync_dma_for_device(). The alternative is
already passed the operation (clean/flush/invalidate) as a token, so you
can construct the function name with token pasting.

Regards,
Samuel