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

[Jiaxun Yang]

在 2023/12/22 15:53, Maxim Kochetkov 写道:
> On 22.12.2023 18:45, Jiaxun Yang wrote:
> > 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.
>
> But arch_setup_dma_ops() is called only from of_dma_configure_id() and 
> acpi_dma_configure_id(). So it works only for DT and ACPI devices. What 
> about platform_device?

Ah I see, that's the problem, in MIPS's use case all DMA capable devices
are following platform's default coherency. For RISC-V we assume all 
device are enabled by ACPI or DT.

Perhaps you can override it in driver, but that will make drivers 
platform dependent.

I'll leave this question to Christoph.

Thanks
- Jiaxun