[PATCH] WIP: HACK: LPAE, BOOTMEM and NO_BOOTMEM

[H. Peter Anvin]

3 makes sense to me.

Tejun Heo <tj at kernel.org> wrote:

>( Expanding cc list, original thread is at
>  http://thread.gmane.org/gmane.linux.kernel/1518046 )
>
>Hello,
>
>On Sat, Jun 29, 2013 at 06:21:24PM +0100, Russell King - ARM Linux
>wrote:
>> Unfortunately, that has not been true on ARM - it's very common for
>> there to be an offset on physical memory, sometimes of the order of
>> 3GB or more.  This is because on reset, ARMs start executing the code
>> at physical address zero, which therefore can't be RAM - and there's
>> a desire to avoid complex switching games in hardware to temporarily
>> map ROM there instead of RAM.
>> 
>> On these SoCs which Santosh is working on, the main physical memory
>> mapping is above 4GB, with just a small alias below 4GB to allow the
>> system to boot without the MMU being on, as they may have more than
>> 4GB of RAM.  As I understand it, the small alias below 4GB is not
>> suitable for use as a "lowmem" mapping.
>
>Ah, okay, so the @limit which is in physical address can be over 4GB
>even for lowmem mappings and alloc_bootmem takes them in ulongs,
>urghhh....
>
>Given that still about half of the archs aren't using memblock yet, I
>think there are three options.
>
>1. Converting all bootmem interface to use physaddr_t.  But that's
>   what memblock is.
>
>2. Introducing new interface.  Easier right now but the danger there
>   is that it might end up duplicating most of alloc_bootmem()
>   interface anyway and we'll have yet another variant of early mem
>   allocator to enjoy.
>
>3. Make all generic code use memblock interface instead of bootmem and
>   implement memblock wrapper on archs which don't use memblock yet.
>   We'll probably need to sort out different combinations of
>   HAVE_MEMBLOCK and NO_BOOTMEM.  If this is doable, it probably is
>   the most future proof way.  While it adds new memblock interface
>   built on top of bootmem, it would also allow removing the bootmem
>   interface built on top of memblock - ie. nobootmem.c, which
>   probably is what we should have done from the beginning.
>
>What do you guys think?
>
>Thanks.

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