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

From: Suravee Suthikulanit
Date: Tue May 12 2015 - 11:06:32 EST

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

+ bool

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 &&

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:


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.


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