linux-kernel - Re: Reserving a fixed physical address page of RAM.

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Date:	Thu, 30 Nov 2006 23:32:14 +0500
From:	"Fawad Lateef" <fawadlateef@...il.com>
To:	"Jon Ringle" <JRingle@...tical.com>
Cc:	"Dave Airlie" <airlied@...il.com>,
	"Robert Hancock" <hancockr@...w.ca>, linux-kernel@...r.kernel.org
Subject: Re: Reserving a fixed physical address page of RAM.

On 11/30/06, Jon Ringle <JRingle@...tical.com> wrote:
> Fawad Lateef wrote:
> > On 11/30/06, Jon Ringle <JRingle@...tical.com> wrote:
> > > Fawad Lateef wrote:
> > > > Yes, this can be used if required physical-memory exists
> > in the last
> > > > part of RAM as if you use mem=<xxxM> then kernel will only use
> > > > memory less than or equal-to <xxxM> and above can be used
> > by drivers
> > > > (or any kernel module) might be through ioremap which takes
> > > > physical-address.
> > >
> > > Seems that using mem= has to be in 1MB increments, where I
> > only need 4K.
> > >
> >
> > No AFAIK you can specify it in KBs (see
> > http://sosdg.org/~coywolf/lxr/source/Documentation/kernel-para
> > meters.txt#L869)
>
> Yes, you can specify the mem= using K notation, but there is a test in
> arch/arm/mm/mm-armv.c:create_mapping() that prevents the mapping from
> being created if the boundaries are not MB aligned:
>
>         if (mem_types[md->type].prot_l1 == 0 &&
>             (virt & 0xfffff || (virt + off) & 0xfffff || (virt + length)
> & 0xfffff)) {
>                 printk(KERN_WARNING "BUG: map for 0x%08lx at 0x%08lx can
> not "
>                        "be mapped using pages, ignoring.\n",
>                        __pfn_to_phys(md->pfn), md->virtual);
>                 return;
>         }
>
> This is in linux-2.6.16.29.
>

Ohh ok, I don't know about this architecture related information.

> >
> > > >
> > > > But if lets say we need only 1MB portion of specific
> > physical-memory
> > > > region then AFAIK it must be done by hacking in kernel
> > code during
> > > > memory-initialization (mem_init
> > > > function) where it is marking/checking pages as/are reserved; you
> > > > can simply mark you required pages as reserved too and set their
> > > > count to some-value if you want to know later which pages are
> > > > reserved by you. (can do this reservation work
> > > > here:
> > http://lxr.free-electrons.com/source/arch/i386/mm/init.c#605).
> > >
> > > Do you think that the following would work to properly reserve the
> > > memory. If it does, then I think I can just do a ioremap(0x0ffff000,
> > > 0x1000) to obtain a virtual address. (Ofcourse I would actually use
> > > symbolic names rather than the hardcoded addesses shown here).
> > >
> > > Index: linux/arch/arm/mm/init.c
> > > ===================================================================
> > > --- linux.orig/arch/arm/mm/init.c       2006-11-30
> > 11:03:00.000000000
> > > -0500
> > > +++ linux/arch/arm/mm/init.c    2006-11-30 11:09:09.000000000 -0500
> > > @@ -429,6 +429,10 @@
> > >         unsigned long addr;
> > >         void *vectors;
> > >
> > > +#ifdef CONFIG_MACH_VERTICAL_RSC4
> > > +       reserve_bootmem (0x0ffff000, 0x1000); #endif
> > > +
> > >         /*
> > >          * Allocate the vector page early.
> > >          */
> > >
> > >
> >
> > I think you can do like this but can't say accurately because
> > I havn't worked on arm architecture and also you havn't
> > mentioned your kernel-version or function (in file
> > arch/arm/mm/init.c) which you are going to do call reserve_bootmem !
>
> Kernel version is 2.6.16.29 and the reserve_bootmem() call above is at
> the top of the function devicemaps_init().
>

reserving_bootmem in devicemaps_init will work but I think it will be
better if you do this in mem_init function (here:
http://sosdg.org/~coywolf/lxr/source/arch/arm/mm/init.c?v=2.6.16;a=arm#L620),
so that all the paging and other map related stuff completes. (CMIIW)

-- 
Fawad Lateef
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