Re: [PATCH 1/1] riscv: set ARCH_DMA_DEFAULT_COHERENT if RISCV_DMA_NONCOHERENT is not set

[Jiaxun Yang]

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