Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 4 Apr 2023 07:59:49 +0100
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 =?iso-8859-1?Q?St=FCbner?= <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
Message-ID: <20230404-5cd527745ab99524c5dde42c@wendy>
References: <20230330204217.47666-1-prabhakar.mahadev-lad.rj@bp.renesas.com>
 <20230330204217.47666-2-prabhakar.mahadev-lad.rj@bp.renesas.com>
 <ZCu1tFrQCVe2sgNz@infradead.org>
 <377d2841-18b5-4f61-b675-3a7c2e0db3b2@app.fastmail.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha256;
        protocol="application/pgp-signature"; boundary="ZREIQbKvtp6hD4WG"
Content-Disposition: inline
In-Reply-To: <377d2841-18b5-4f61-b675-3a7c2e0db3b2@app.fastmail.com>
Precedence: bulk

--ZREIQbKvtp6hD4WG
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

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>
> >>=20
> >> Currently, selecting which CMOs to use on a given platform is done usi=
ng
> >> and ALTERNATIVE_X() macro. This was manageable when there were just two
> >> CMO implementations, but now that there are more and more platforms co=
ming
> >> needing custom CMOs, the use of the ALTERNATIVE_X() macro is unmanagea=
ble.
> >>=20
> >> To avoid such issues this patch switches to use of function pointers
> >> instead of ALTERNATIVE_X() macro for cache management (the only drawba=
ck
> >> being performance over the previous approach).
> >>=20
> >> 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);
> >>=20
> >
> > NAK.  Function pointers for somthing high performance as cache
> > maintainance is a complete no-go.
>=20
> 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.
>=20
> 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.
>=20
> 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_A8A256967B65=
4625AEE1DB222514B0613B07@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.


--ZREIQbKvtp6hD4WG
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----

iHUEABYIAB0WIQRh246EGq/8RLhDjO14tDGHoIJi0gUCZCvK5QAKCRB4tDGHoIJi
0u41AQC5LlNjFCo7KuWt5ZFwh4+c+I5y0gSMhl//kBtKQMsMKgD+NN9jEGHmflZR
INGsvjP7dlUKwSSpdNO7N3SeKqmFVwo=
=rvG3
-----END PGP SIGNATURE-----

--ZREIQbKvtp6hD4WG--