linux-kernel - Re: [RFC v5.1 9/9] [DON'T APPLY] cache: sifive-ccache: add cache flushing capability

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Message-Id: <43aee000-5b89-4d94-98d2-b37b1a18a83e@app.fastmail.com>
Date:   Wed, 04 Jan 2023 11:19:44 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Conor Dooley" <conor@...nel.org>
Cc:     "Palmer Dabbelt" <palmer@...belt.com>,
        Prabhakar <prabhakar.csengg@...il.com>,
        "Conor.Dooley" <conor.dooley@...rochip.com>,
        "Andrew Jones" <ajones@...tanamicro.com>,
        "Albert Ou" <aou@...s.berkeley.edu>,
        "Anup Patel" <apatel@...tanamicro.com>,
        "Atish Patra" <atishp@...osinc.com>,
        "Biju Das" <biju.das.jz@...renesas.com>,
        devicetree@...r.kernel.org,
        "Geert Uytterhoeven" <geert@...ux-m68k.org>,
        guoren <guoren@...nel.org>,
        "Christoph Hellwig" <hch@...radead.org>,
        Heiko Stübner <heiko@...ech.de>,
        "Jisheng Zhang" <jszhang@...nel.org>,
        krzysztof.kozlowski+dt@...aro.org, linux-kernel@...r.kernel.org,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        linux-riscv@...ts.infradead.org,
        "Magnus Damm" <magnus.damm@...il.com>,
        "Nathan Chancellor" <nathan@...nel.org>,
        "Paul Walmsley" <paul.walmsley@...ive.com>,
        "Philipp Tomsich" <philipp.tomsich@...ll.eu>,
        "Lad, Prabhakar" <prabhakar.mahadev-lad.rj@...renesas.com>,
        "Rob Herring" <robh+dt@...nel.org>,
        "Samuel Holland" <samuel@...lland.org>, soc@...nel.org,
        "Daire McNamara" <daire.mcnamara@...rochip.com>
Subject: Re: [RFC v5.1 9/9] [DON'T APPLY] cache: sifive-ccache: add cache flushing
 capability

On Wed, Jan 4, 2023, at 10:23, Conor Dooley wrote:
>>Right, no need to touch the existing file as part of this series,
>>it probably just gets in the way of defining a good interface here.
>
> Sure. Can leave it where it was & I'll sort it out later when it's 
> errata etc get added.
>
> Btw, would you mind pointing out where you wanted to have that if/else 
> you mentioned on IRC?

I meant replacing both of the runtime patching indirections in
arch_sync_dma_for_device(). At the moment, this function calls
ALT_CMO_OP(), which is patched to either call the ZICBOM or the
THEAD variant, and if I read this right you add a third case
there with another level of indirection using static_branch.

I would try to replace both of these indirections and instead
handle it all from C code in arch_sync_dma_for_device() directly,
for the purpose of readability and maintainability.

static inline void dma_cache_clean(void *vaddr, size_t size)
{
        if (!cache_maint_ops.clean)
               zicbom_cache_clean(vaddr, size, riscv_cbom_block_size);
        else
               cache_maint_ops.clean(vaddr, size, riscv_cbom_block_size);
}

void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
                              enum dma_data_direction dir)
{
        void *vaddr = phys_to_virt(paddr);

        switch (dir) {
        case DMA_TO_DEVICE:
        case DMA_FROM_DEVICE:
                dma_cache_clean(vaddr, size);
                break;
        case DMA_BIDIRECTIONAL:
                dma_cache_flush(vaddr, size);
                break;
        default:
                break;
        }
}

which then makes it very clear what the actual code path
is, while leaving the zicbom case free of indirect function
calls. You can still use a static_branch() to optimize the
conditional, but I would try to avoid any extra indirection
levels or errata checks.

     Arnd