[PATCH 6/6] MIPS: document mixing "slightly different CCAs"

[Christoph Hellwig]

Based on an email from Paul Burton, quoting section 4.8 "Cacheability and
Coherency Attributes and Access Types" of "MIPS Architecture Volume 1:
Introduction to the MIPS32 Architecture" (MD00080, revision 6.01).

Signed-off-by: Christoph Hellwig <hch at lst.de>
---
 arch/mips/Kconfig | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index fc88f68ea1ee..aff1cadeea43 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -1119,6 +1119,13 @@ config DMA_PERDEV_COHERENT
 
 config DMA_NONCOHERENT
 	bool
+	#
+	# MIPS allows mixing "slightly different" Cacheability and Coherency
+	# Attribute bits.  It is believed that the uncached access through
+	# KSEG1 and the implementation specific "uncached accelerated" used
+	# by pgprot_writcombine can be mixed, and the latter sometimes provides
+	# significant advantages.
+	#
 	select ARCH_HAS_DMA_WRITE_COMBINE
 	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
 	select ARCH_HAS_UNCACHED_SEGMENT
-- 
2.20.1