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Date: Fri, 22 Dec 2023 15:45:34 +0000
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Subject: Re: [PATCH 1/1] riscv: set ARCH_DMA_DEFAULT_COHERENT if
 RISCV_DMA_NONCOHERENT is not set
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To: Maxim Kochetkov <fido_max@inbox.ru>, Conor Dooley <conor@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, linux-riscv@lists.infradead.org,
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 aou@eecs.berkeley.edu, palmer@dabbelt.com, paul.walmsley@sifive.com
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 <20231222-outburst-spoiling-75082a7826dd@spud>
 <be10eb21-a0ef-416e-9c0c-d53326996fba@inbox.ru>
From: Jiaxun Yang <jiaxun.yang@flygoat.com>
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在 2023/12/22 15:38, Maxim Kochetkov 写道:
>
>
> On 22.12.2023 17:54, Conor Dooley wrote:
>
>>> etc..) do not have this feature. These devices will use value from
>>> device_initialize(). And we have no possibility to change
>>> dma_default_coherent value by disabling ARCH_DMA_DEFAULT_COHERENT.
>>> Moreover, changing dma_default_coherent from false to true may cause
>>> regression for other devices.
>>
>> How can there be a regression when dma has been coherent by default for
>> the RISC-V kernel from day 1?
>
> Before ARCH_DMA_DEFAULT_COHERENT patch dma_default_coherent was used 
> unassigned as "false" in device_initialize():
> ..........
> #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)
>     dev->dma_coherent = dma_default_coherent;
> #endif
> ..........
> And now it becomes "true". It may change behavior of other non-DT 
> drivers.
I don't see any problem here, default is default.
Actually leaving those device with  dev->dma_coherent = false is risky, 
because
we can't guarantee underlying cache flush functions are here.

If a non-dt device do need to override it, it should be done in 
arch_setup_dma_ops.

Thanks
- Jiaxun