Re: [PATCH 5/6] ARM: ux500: Enable HIGHMEM on all mop500 platforms

From: Lee Jones
Date: Wed Aug 01 2012 - 03:56:27 EST


On 31/07/12 23:01, Russell King - ARM Linux wrote:
On Tue, Jul 31, 2012 at 08:50:02PM +0000, Arnd Bergmann wrote:
On Tuesday 31 July 2012, Russell King - ARM Linux wrote:
I still fail to see how not having highmem enabled would ever cause memory
corruption errors (unless something dealing with memory in a very very
wrong way - iow, not using one of the reservation or memory allocation
methods provided by the kernel.)

The problem is that all users of ux500 systems pass a command line like

vmalloc=256M mem=128M@0 mali.mali_mem=32M@128M hwmem=168M@160M mem=48M@328M mem_issw=1M@383M mem=640M@384M

This is of course totally bogus and should not be done. If I understand
Lee correctly, one of the issues resulting from passing a command
line like this without enabling highmem is memory corruption.

But the question is _why_ does that corruption happen.

From the above, we will end up with the kernel getting:

0x00000000 - 0x07ffffff (128M @ 0)
0x14800000 - 0x177fffff (48M @ 328M)
0x18000000 - 0x3fffffff (640M @ 384M)

with:

0x08000000 - 0x081fffff used for mali
0x0a000000 - 0x147fffff used for hwmem
0x17f00000 - 0x17ffffff used for mem_issw

Now, with highmem disabled, the kernel should still map exactly the
regions: 0x00000000 - 0x07ffffff, 0x14800000 - 0x177fffff, into the
direct mapped region, and truncate the 0x18000000 - 0x3fffffff
region appropriately, reducing the amount of memory available such
that it won't overlap the vmalloc area (which you've specified to be
a minimum of 256M.)

This should _NOT_ cause any memory corruption.

So, come on guys. Debugging is *mandatory* for this kind of problem.
Papering over it is obscene.

Actually I didn't go any further with it, as I changed to another identical piece of hardware and couldn't reproduce the issue.

FYI, here's the boot log from the broken board:

http://paste.ubuntu.com/1102017/

--
Lee Jones
Linaro ST-Ericsson Landing Team Lead
Linaro.org â Open source software for ARM SoCs
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