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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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.