Re: [PATCH v7 1/6] riscv: mm: dma-noncoherent: Switch using function pointers for cache management

From: Conor Dooley <conor.dooley@microchip.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "Christoph Hellwig" <hch@infradead.org>,
	Prabhakar <prabhakar.csengg@gmail.com>,
	"Geert Uytterhoeven" <geert+renesas@glider.be>,
	"Heiko Stübner" <heiko@sntech.de>, guoren <guoren@kernel.org>,
	"Andrew Jones" <ajones@ventanamicro.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Samuel Holland" <samuel@sholland.org>,
	linux-riscv@lists.infradead.org,
	"Rob Herring" <robh+dt@kernel.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	"Biju Das" <biju.das.jz@bp.renesas.com>,
	"Lad, Prabhakar" <prabhakar.mahadev-lad.rj@bp.renesas.com>
Subject: Re: [PATCH v7 1/6] riscv: mm: dma-noncoherent: Switch using function pointers for cache management
Date: Tue, 4 Apr 2023 07:59:49 +0100	[thread overview]
Message-ID: <20230404-5cd527745ab99524c5dde42c@wendy> (raw)
In-Reply-To: <377d2841-18b5-4f61-b675-3a7c2e0db3b2@app.fastmail.com>

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On Tue, Apr 04, 2023 at 08:50:16AM +0200, Arnd Bergmann wrote:
> On Tue, Apr 4, 2023, at 07:29, Christoph Hellwig wrote:
> > On Thu, Mar 30, 2023 at 09:42:12PM +0100, Prabhakar wrote:
> >> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> >> 
> >> Currently, selecting which CMOs to use on a given platform is done using
> >> and ALTERNATIVE_X() macro. This was manageable when there were just two
> >> CMO implementations, but now that there are more and more platforms coming
> >> needing custom CMOs, the use of the ALTERNATIVE_X() macro is unmanageable.
> >> 
> >> To avoid such issues this patch switches to use of function pointers
> >> instead of ALTERNATIVE_X() macro for cache management (the only drawback
> >> being performance over the previous approach).
> >> 
> >> void (*clean_range)(unsigned long addr, unsigned long size);
> >> void (*inv_range)(unsigned long addr, unsigned long size);
> >> void (*flush_range)(unsigned long addr, unsigned long size);
> >> 
> >
> > NAK.  Function pointers for somthing high performance as cache
> > maintainance is a complete no-go.
> 
> As we already discussed, this is now planned to use a direct
> branch to the zicbom version when the function pointer is NULL,
> which should be a fast predicted branch on all standard implementations
> and only be slow on the nonstandard ones, which I think is a reasonable
> compromise.
> 
> I'm also not sure I'd actually consider this a fast path, since
> 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.
> 
> I suppose an alternative would be to use the linux/static_call.h
> infrastructure to avoid the overhead of indirect branches altogether.
> Right now, this has no riscv specific implementation though, so
> using it just turns into a regular indirect branch until someone
> implements static_call.

Someone actually posted an implementation for that the other day:
https://patchwork.kernel.org/project/linux-riscv/patch/tencent_A8A256967B654625AEE1DB222514B0613B07@qq.com/

I haven't looked at it at all, other than noticing the build issues
outside of for !rv64 || !mmu, so I have no idea what state it is
actually in.

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