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Subject: Re: [PATCH 3/3] of: address: Use dma_default_coherent to determine
 default coherency
From:   Jiaxun Yang <jiaxun.yang@flygoat.com>
In-Reply-To: <e9d882f4-d57d-f6b4-7ae0-bbfea0f5aa59@arm.com>
Date:   Wed, 22 Feb 2023 17:57:44 +0000
Cc:     "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        mpe@ellerman.id.au, paul.walmsley@sifive.com, palmer@dabbelt.com,
        Rob Herring <robh+dt@kernel.org>,
        Christoph Hellwig <hch@lst.de>, m.szyprowski@samsung.com,
        linux-riscv@lists.infradead.org
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Message-Id: <3E55E1F7-7A34-4165-9BF4-C2AAF6A6CFC1@flygoat.com>
References: <20230222133712.8079-1-jiaxun.yang@flygoat.com>
 <20230222133712.8079-4-jiaxun.yang@flygoat.com>
 <e9d882f4-d57d-f6b4-7ae0-bbfea0f5aa59@arm.com>
To:     Robin Murphy <robin.murphy@arm.com>
Precedence: bulk


> 2023=E5=B9=B42=E6=9C=8822=E6=97=A5 17:24=EF=BC=8CRobin Murphy =
<robin.murphy@arm.com> =E5=86=99=E9=81=93=EF=BC=9A

[...]

>=20
> AFAICS, all you should actually need is a single self-contained =
addition here, something like:
>=20
> + /*
> +  * DT-based MIPS doesn't use OF_DMA_DEFAULT_COHERENT, but
> +  * might override the system-wide default at runtime.
> +  */
> +#if defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE) || \
> + defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU) || \
> + defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL)
> + is_coherent =3D dma_default_coherent;
> +#endif

That makes more sense, thanks.
I=E2=80=99ll append CONFIG_MIPS as a condition here as well because it =
may break RISC-V whose dma_default_coherent
is not set to true from very beginning.

>=20
>>     node =3D of_node_get(np);
>> =20
>=20
> Then *after* that's fixed, we can do a more comprehensive refactoring =
to merge the two mechanisms properly. FWIW I think I'd prefer an =
approach closer to the first one, where config options control the =
initial value of dma_default_coherent rather than architectures having =
to override it unconditionally (and TBH I'd also like to have a generic =
config symbol for whether an arch supports per-device coherency or not).

Ok I=E2=80=99ll try to revert to the initial way.
Is there any reason that an arch can=E2=80=99t support per-device =
coherency?

Thanks
- Jiaxun


>=20
> Thanks,
> Robin.