These macros are used to convert between physical addresses (as
seen by the CPU) and bus addresses (as seen by various peripherals
such as the display controller, cedar, g2d, mali, ...).
The difference between physical and bus addresses is 0x40000000,
because that's the physical address of SDRAM (and PHYS_OFFSET
define in the kernel).
Huang Benn has provided an explanation earlier:
http://thread.gmane.org/gmane.comp.hardware.netbook.arm.sunxi/2827
But some additional demystification might be useful.
Basically, when we write an address to some hardware register of
the display controller, it is expected to be a bus address.
However as long as the size of RAM is just 1 GiB or less, adding
or subtracting a multiple of PHYS_OFFSET (which happens to be
1 GiB) to this address does not have any adverse effects. We just
get some garbage in the high address lines on the memory interface,
but these high address lines are not connected anywhere and have
no effect. So using either bus addresses or just physical
addresses was fine for the systems with 1 GiB of RAM or less.
But when having 2 GiB of RAM, using proper bus addresses becomes
really important. A failure to do so and an incorrect value in
the high address bit results in memory accesses going to a low
or high 1 GiB part of DRAM based on the value of this bit.
This behaviour is expected to be the same for all sunxi hardware
with 2 GiB of RAM. There is nothing sun7i specific in it and it
makes sense to treat sun4i/sun5i/sun7i in the same way. Also the
conversion between bus and physical addresses is a simple addition
or subtraction of PHYS_OFFSET without any need for convoluted logic.
Signed-off-by: Siarhei Siamashka <[email protected]>
---
arch/arm/plat-sunxi/include/plat/memory.h | 10 ++--------
1 file changed, 2 insertions(+), 8 deletions(-)
diff --git a/arch/arm/plat-sunxi/include/plat/memory.h
b/arch/arm/plat-sunxi/include/plat/memory.h
index 20c27bd..42a4afa 100644
--- a/arch/arm/plat-sunxi/include/plat/memory.h
+++ b/arch/arm/plat-sunxi/include/plat/memory.h
@@ -30,13 +30,7 @@
/* default ATAG_MEM size */
#define MEM_SIZE (512*1024*1024)
-#ifdef CONFIG_ARCH_SUN7I
-#define __phys_to_bus(x) (((x)>=PLAT_PHYS_OFFSET)?(x)-PLAT_PHYS_OFFSET:(x))
-#define __bus_to_phys(x) (((x)<PLAT_PHYS_OFFSET)?(x)+PLAT_PHYS_OFFSET:(x))
-#else
-#define __phys_to_bus(x) (x)
-#define __bus_to_phys(x) (x)
-#endif
-
+#define __phys_to_bus(x) ((x) - PLAT_PHYS_OFFSET)
+#define __bus_to_phys(x) ((x) + PLAT_PHYS_OFFSET)
#endif
--
1.8.3.2
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