Re: [V3 PATCH 1/5] ACPI / scan: Parse _CCA and setup device coherency

[Suravee Suthikulanit]

On 5/11/2015 8:20 PM, Rafael J. Wysocki wrote:
> On Monday, May 11, 2015 05:16:27 PM Catalin Marinas wrote:
> > On Fri, May 08, 2015 at 10:53:59PM +0200, Rafael J. Wysocki wrote:
> > > On Thursday, May 07, 2015 07:37:12 PM Suravee Suthikulpanit wrote:
> > > > diff --git a/drivers/acpi/Kconfig b/drivers/acpi/Kconfig
> > > > index ab2cbb5..7822149 100644
> > > > --- a/drivers/acpi/Kconfig
> > > > +++ b/drivers/acpi/Kconfig
> > > > @@ -54,6 +54,12 @@ config ACPI_GENERIC_GSI
> > > >   config ACPI_SYSTEM_POWER_STATES_SUPPORT
> > > >   	bool
> > > >
> > > > +config ACPI_CCA_REQUIRED
> > > > +	bool
> > > > +
> > > > +config ARM64_SUPPORT_ACPI_CCA_ZERO
> > >
> > > Hmm.  I guess the Arnd's idea what to simply use CONFIG_ARM64 directly instead
> > > of adding this new option.
> >
> > I agree.
> >
> > > > +static inline bool acpi_dma_is_supported(struct acpi_device *adev)
> > > > +{
> > > > +	/**
> > > > +	 * Currently, we mainly support _CCA=1 (i.e. is_coherent=1)
> > > > +	 * This should be equivalent to specifyig dma-coherent for
> > > > +	 * a device in OF.
> > > > +	 *
> > > > +	 * For the case when _CCA=0 (i.e. is_coherent=0 && cca_seen=1),
> > > > +	 * we would rely on arch-specific cache maintenance for
> > > > +	 * non-coherence DMA operations if architecture specifies
> > > > +	 * _XXX_SUPPORT_CCA_ZERO. Otherwise, we do not support
> > > > +	 * DMA on this device and fallback to arch-specific default
> > > > +	 * handling.
> > > > +	 *
> > > > +	 * For the case when _CCA is missing (i.e. cca_seen=0) but
> > > > +	 * platform specifies ACPI_CCA_REQUIRED, we do not support DMA,
> > > > +	 * and fallback to arch-specific default handling.
> > > > +	 */
> > > > +	return adev && (adev->flags.is_coherent ||
> > > > +			(adev->flags.cca_seen &&
> > > > +			 IS_ENABLED(CONFIG_ARM64_SUPPORT_ACPI_CCA_ZERO)));
> > >
> > > So what exactly would be wrong with using IS_ENABLED(CONFIG_ARM64) here?
> >
> > I'm not sure I follow why we need to check for ARM64 here at all. Can we
> > not just have something like:
> >
> > 	return adev && (!IS_ENABLED(CONFIG_ACPI_CCA_REQUIRED) ||
> > 			adev->flags.cca_seen)
>
> If _CCA returns 0 on non-ARM64, DMA is not supported for this device, so
> in that case the function should return 'false' while the above check will
> make it return 'true'.

The main idea is basically to allow architecture to decide if it wants 
to specify if it wants to support _CCA=0. Currently, there are two 
approaches.
1. Do not support and disable DMA
2. Support and default to what architecture would normally do for 
non-coherent DMA.

Since, ARM64 being the only platform, which supports ACPI and would 
support _CCA=0. I can just put CONFIG_ARM64 then as Arnd and Rafael 
mentioned.

Thanks,
Suravee


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