This change “MIPS: Make (UN)CAC_ADDR() PHYS_OFFSET-agnostic” in Linux kernel is authored by Paul Burton <paul.burton [at] mips.com> on Fri Jul 27 18:23:18 2018 -0700.


Converting an address between cached & uncached (typically addresses in
(c)kseg0 & (c)kseg1 or 2 xkphys regions) should not depend upon
PHYS_OFFSET in any way - we're converting from a virtual address in one
unmapped region to a virtual address in another unmapped region.

For some reason our CAC_ADDR() & UNCAC_ADDR() macros make use of
PAGE_OFFSET, which typically includes PHYS_OFFSET. This means that
platforms with a non-zero PHYS_OFFSET typically have to workaround
miscalculation by these 2 macros by also defining UNCAC_BASE to a value
that isn't really correct.

It appears that an attempt has previously been made to address this with
commit 3f4579252aa1 ("MIPS: make CAC_ADDR and UNCAC_ADDR account for
PHYS_OFFSET") which was later undone by commit ed3ce16c3d2b ("Revert
"MIPS: make CAC_ADDR and UNCAC_ADDR account for PHYS_OFFSET"") which
also introduced the ar7 workaround. That attempt at a fix was roughly
equivalent, but essentially caused the CAC_ADDR() & UNCAC_ADDR() macros
to cancel out PHYS_OFFSET by adding & then subtracting it again. In his
revert Leonid is correct that using PHYS_OFFSET makes no sense in the
context of these macros, but appears to have missed its inclusion via
PAGE_OFFSET which means PHYS_OFFSET actually had an effect after the
revert rather than before it.

Here we fix this by modifying CAC_ADDR() & UNCAC_ADDR() to stop using
PAGE_OFFSET (& thus PHYS_OFFSET), instead using __pa() & __va() along

For UNCAC_ADDR(), __pa() will convert a cached address to a physical
address which we can simply use as an offset from UNCAC_BASE to obtain
an address in the uncached region.

For CAC_ADDR() we can undo the effect of UNCAC_ADDR() by subtracting
UNCAC_BASE and using __va() on the result.

With this change made, remove definitions of UNCAC_BASE from the ar7 &
pic32 platforms which appear to have defined them only to workaround
this problem.

Signed-off-by: Paul Burton <paul.burton@mips.com>
References: 3f4579252aa1 ("MIPS: make CAC_ADDR and UNCAC_ADDR account for PHYS_OFFSET")
References: ed3ce16c3d2b ("Revert "MIPS: make CAC_ADDR and UNCAC_ADDR account for PHYS_OFFSET"")
Patchwork: https://patchwork.linux-mips.org/patch/20046/
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: Vladimir Kondratiev <vladimir.kondratiev@intel.com>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit 0d0e147.

There are 12 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 arch/mips/include/asm/mach-ar7/spaces.h   | 3 ---
 arch/mips/include/asm/mach-pic32/spaces.h | 1 -
 arch/mips/include/asm/page.h              | 4 ++--
 arch/mips/jazz/jazzdma.c                  | 2 +-
 arch/mips/mm/dma-noncoherent.c            | 2 +-
 5 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/arch/mips/include/asm/mach-ar7/spaces.h b/arch/mips/include/asm/mach-ar7/spaces.h
index 660ab64..a004d94d 100644
--- a/arch/mips/include/asm/mach-ar7/spaces.h
+++ b/arch/mips/include/asm/mach-ar7/spaces.h
@@ -17,9 +17,6 @@
 #define PAGE_OFFSET	_AC(0x94000000, UL)
 #define PHYS_OFFSET	_AC(0x14000000, UL)
-#define UNCAC_BASE	_AC(0xb4000000, UL)	/* 0xa0000000 + PHYS_OFFSET */
 #include <asm/mach-generic/spaces.h>
 #endif /* __ASM_AR7_SPACES_H */
diff --git a/arch/mips/include/asm/mach-pic32/spaces.h b/arch/mips/include/asm/mach-pic32/spaces.h
index 046a0a9..a1b9783 100644
--- a/arch/mips/include/asm/mach-pic32/spaces.h
+++ b/arch/mips/include/asm/mach-pic32/spaces.h
@@ -16,7 +16,6 @@
 #define PHYS_OFFSET	_AC(0x08000000, UL)
-#define UNCAC_BASE	_AC(0xa8000000, UL)
 #include <asm/mach-generic/spaces.h>
diff --git a/arch/mips/include/asm/page.h b/arch/mips/include/asm/page.h
index ad46121..a051b82 100644
--- a/arch/mips/include/asm/page.h
+++ b/arch/mips/include/asm/page.h
@@ -252,8 +252,8 @@ static inline int pfn_valid(unsigned long pfn)
 	 ((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0) | 
-#define UNCAC_ADDR(addr)	((addr) - PAGE_OFFSET + UNCAC_BASE)
-#define CAC_ADDR(addr)		((addr) - UNCAC_BASE + PAGE_OFFSET)
+#define UNCAC_ADDR(addr)	(UNCAC_BASE + __pa(addr))
+#define CAC_ADDR(addr)		((unsigned long)__va((addr) - UNCAC_BASE))
 #include <asm-generic/memory_model.h>
 #include <asm-generic/getorder.h>
diff --git a/arch/mips/jazz/jazzdma.c b/arch/mips/jazz/jazzdma.c
index 446fc8c..d31bc2f 100644
--- a/arch/mips/jazz/jazzdma.c
+++ b/arch/mips/jazz/jazzdma.c
@@ -576,7 +576,7 @@ static void *jazz_dma_alloc(struct device *dev, size_t size,
 	if (!(attrs & DMA_ATTR_NON_CONSISTENT)) {
 		dma_cache_wback_inv((unsigned long)ret, size);
-		ret = UNCAC_ADDR(ret);
+		ret = (void *)UNCAC_ADDR(ret);
 	return ret;
diff --git a/arch/mips/mm/dma-noncoherent.c b/arch/mips/mm/dma-noncoherent.c
index 25edf6d..2aca123 100644
--- a/arch/mips/mm/dma-noncoherent.c
+++ b/arch/mips/mm/dma-noncoherent.c
@@ -78,7 +78,7 @@ void *arch_dma_alloc(struct device *dev, size_t size,
 	if (!dev_is_coherent(dev) && !(attrs & DMA_ATTR_NON_CONSISTENT)) {
 		dma_cache_wback_inv((unsigned long) ret, size);
-		ret = UNCAC_ADDR(ret);
+		ret = (void *)UNCAC_ADDR(ret);
 	return ret;

The commit for this change in Linux stable tree is 0d0e147 (patch).

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