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Re: use bootmem in platform code on MIPS

To: Manuel Lauss <manuel.lauss@googlemail.com>
Subject: Re: use bootmem in platform code on MIPS
From: Ralf Baechle <ralf@linux-mips.org>
Date: Wed, 28 Apr 2010 16:54:39 +0100
Cc: Linux-MIPS <linux-mips@linux-mips.org>
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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.

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.

> 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
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.

  Ralf

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