linux-kernel - Re: [PATCH 0/7] MIPS DMA coherence fixes

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Date:   Wed, 22 Feb 2023 13:04:47 +0000
From:   Jiaxun Yang <jiaxun.yang@...goat.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        mpe@...erman.id.au, paul.walmsley@...ive.com, palmer@...belt.com,
        Rob Herring <robh+dt@...nel.org>, m.szyprowski@...sung.com
Subject: Re: [PATCH 0/7] MIPS DMA coherence fixes



> 2023年2月22日 12:55，Robin Murphy <robin.murphy@....com> 写道：
> 
> On 2023-02-21 19:55, Jiaxun Yang wrote:
>>> 2023年2月21日 19:46，Robin Murphy <robin.murphy@....com> 写道：
>>> 
>>> On 2023-02-21 18:15, Jiaxun Yang wrote:
>>>>> 2023年2月21日 17:54，Christoph Hellwig <hch@....de> 写道：
>>>>> 
>>>>> Can you explain the motivation here?  Also why riscv patches are at
>>>>> the end of a mips fіxes series?
>>>> Ah sorry for any confusion.
>>>> So the main purpose of this patch is to fix MIPS’s broken per-device coherency.
>>>> To be more precise, we want to be able to control the default coherency for all devices probed from
>>>> devicetree in early boot code.
>>> 
>>> Including the patch which actually does that would be helpful. As it is, patches 4-7 here just appear to be moving an option around for no practical effect.
>> Well the affect is default coherency of devicetree probed devices are now following dma_default_coherent
>> instead of a static Kconfig option. For MIPS platform, dma_default_coherent will be determined by boot code.
> 
> "Will be" is the issue I'm getting at. We can't review some future promise of a patch, we can only review actual patches. And it's hard to meaningfully review preparatory patches for some change without the full context of that change.

Actually this already present in current MIPS platform code.

arch/mips/mti-malta is setting dma_default_coherent on boot, and it’s devicetree does not explicitly specify coherency.


Thanks
- Jiaxun



> 
> Thanks,
> Robin.