ARM bare-metal with MMU: write to non-cachable,non-bufferable mapped area fail

Question

I am ARM Cortex A9 CPU with 2 cores. But I just use 1 core and the other is just in a busy loop. I setup the MMU table using section (1MB per entry) like this:

0x00000000-0x14ffffff => 0x00000000-0x14ffffff (non-cachable, non-bufferable)
0x15000000-0x24ffffff => 0x15000000-0x24ffffff (cachable, bufferable)
0x25000000-0x94ffffff => 0x25000000-0x94ffffff (non-cachable, non-bufferable)
0x15000000-0x24ffffff => 0x95000000-0xa4ffffff (non-cachable, non-bufferable)
0xa5000000-0xffffffff => 0xa5000000-0xffffffff (non-cachable, non-bufferable)

It is rather simple. I just want to have a mirror of 256MB memory for non-cachable access. However, when I do several write to the the non-cachable memory section at 0x95000000-0xa4ffffff. I find the write is not actually written until I explicitly give a cache flush.

Am I doing something wrong or this kind of mapping is not valid? If that is the case, I don't understand how Linux's ioremap will be working on ARM. It will be good if anyone can give some explanation to me. Thanks very much.

Answer 1

First of all: the Cortex-A9 is an ARMv7-A processor. The terms non-cacheable/non-bufferable/cacheable/bufferable are no longer correct descriptions of the mappings. The actual mapping type is determined by TEX[2:0], C and B bits.

So I am actually having to guess a bit here as to what your mappings actually are. And my guess is that you have the majority of your mappings set as Strongly-ordered, and the mirrored region as Normal Write-Back cacheable.

Having multiple virtual mappings with different memory types pointing to the same physical location is generally not a good idea in the ARM architecture. It used to be explicitly banned, but the latest version of the ARMv7-AR Architecture reference manual (DDI 0406C.b) has a (fairly long) section dedicated to the implications of "Mismatched memory attributes".

I would recommend finding a different way of achieving your goal.

Simply changing the mapping of the uncached regions to Normal Non-cacheable would be a good start. There is no valid reason for using Strongly-ordered mappings for RAM.