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Re: use bootmem in platform code on MIPS

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: use bootmem in platform code on MIPS
From: Manuel Lauss <manuel.lauss@googlemail.com>
Date: Wed, 28 Apr 2010 18:28:45 +0200
Cc: Linux-MIPS <linux-mips@linux-mips.org>
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On Wed, Apr 28, 2010 at 5:54 PM, Ralf Baechle <ralf@linux-mips.org> wrote:
> On Tue, Apr 27, 2010 at 02:14:32PM +0200, Manuel Lauss wrote:
>
>> I'd like to use bootmem to reserve large chunks of RAM (at a particular 
>> physical
>> address; for Au1200 MAE, CIM and framebuffer, and later Au1300 OpenGL block)
>> but it seems that it can't be done:  Doing __alloc_bootmem() in
>> plat_mem_setup() is
>> too early, while an arch_initcall() is too late because by then the
>> slab allocator is
>> already up and handing out random addresses and/or refusing allocations 
>> larger
>> than a few MBytes.
>
> The maximum is actually configurable.  CONFIG_FORCE_MAX_ZONEORDER defaults
> to 11 which means with 4kB pages you get 8MB maximum allocation - more for
> larger pages.

I already had to modify it for large display resolutions.


> CONFIG_FORCE_MAX_ZONEORDER is a tradeoff though.  A smaller value will give
> slightly better performance and safe a bit of memory but I can't really
> quantify these numbers - I assume it's a small difference.
>
> It may actually be preferable to never tell the bootmem allocator about the
> memory you need for these devices that is bypass the mm code entirely.

Do you mean by not adding the whole RAM area with add_memory_region()?
Can I give the memory back later (if it's not required)?  Right now I think with
bootmem that is actually possible.


>> Is there another callback I could use which would allow me to use bootmem 
>> (short
>> of abusing plat_smp_setup)?
>>
>> Would a separate callback like this be an acceptable solution?
>
> Certainly better than using plat_smp_setup which would require enabling
> SMP support for no good reason at all.
>
> I know we will eventually have to add another platform hooks to run after
> bootmem_init.  The name of plat_mem_setup() already shows what this hook
> originally was meant for but it ended up as the everything-and-the-kitchen-
> sink hook for platform-specific early initialization.  I just dislike

The comment above arch_mem_init() too mentions a separate function.


> conditional hooks.  Let's add a call to a new hook function and fix whatever
> breaks or think about what other hooks needs there should be.

Okay, I'll cook something up.

Thank you,
        Manuel Lauss

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