linux-kernel - Re: [PATCH 1/1] riscv: set ARCH_DMA_DEFAULT_COHERENT if RISCV_DMA

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Date: Fri, 22 Dec 2023 18:38:22 +0300
From: Maxim Kochetkov <fido_max@...ox.ru>
To: Conor Dooley <conor@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Jiaxun Yang <jiaxun.yang@...goat.com>,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 robh@...nel.org, mpe@...erman.id.au, aou@...s.berkeley.edu,
 palmer@...belt.com, paul.walmsley@...ive.com
Subject: Re: [PATCH 1/1] riscv: set ARCH_DMA_DEFAULT_COHERENT if
 RISCV_DMA_NONCOHERENT is not set



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.