linux-kernel - Re: [PATCH v4 02/13] ARM: LPAE: use phys_addr_t in alloc_init

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Date:	Fri, 1 Feb 2013 13:14:51 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Subash Patel <subashrp@...il.com>
cc:	Hui Wang <jason77.wang@...il.com>,
	Cyril Chemparathy <cyril@...com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	marc.zyngier@....com, sboyd@...eaurora.org,
	linus.walleij@...aro.org, Will Deacon <will.deacon@....com>,
	Vitaly Andrianov <vitalya@...com>,
	Catalin Marinas <catalin.marinas@....com>,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH v4 02/13] ARM: LPAE: use phys_addr_t in
 alloc_init_pud()

On Fri, 1 Feb 2013, Subash Patel wrote:

> Hi Nicolas,
> 
> On Thursday 31 January 2013 07:35 PM, Nicolas Pitre wrote:
> > On Fri, 1 Feb 2013, Hui Wang wrote:
> > 
> > > Cyril Chemparathy wrote:
> > > > From: Vitaly Andrianov <vitalya@...com>
> > > > 
> > > > This patch fixes the alloc_init_pud() function to use phys_addr_t
> > > > instead of
> > > > unsigned long when passing in the phys argument.
> > > > 
> > > > This is an extension to commit 97092e0c56830457af0639f6bd904537a150ea4a
> > > > (ARM:
> > > > pgtable: use phys_addr_t for physical addresses), which applied similar
> > > > changes
> > > > elsewhere in the ARM memory management code.
> > > > 
> > > > Signed-off-by: Vitaly Andrianov <vitalya@...com>
> > > > Signed-off-by: Cyril Chemparathy <cyril@...com>
> > > > Acked-by: Nicolas Pitre <nico@...aro.org>
> > > > Acked-by: Catalin Marinas <catalin.marinas@....com>
> > > > ---
> > > >   arch/arm/mm/mmu.c |    3 ++-
> > > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
> > > > index 9f06102..ef43689 100644
> > > > --- a/arch/arm/mm/mmu.c
> > > > +++ b/arch/arm/mm/mmu.c
> > > > @@ -612,7 +612,8 @@ static void __init alloc_init_section(pud_t *pud,
> > > > unsigned long addr,
> > > >   }
> > > >    static void __init alloc_init_pud(pgd_t *pgd, unsigned long addr,
> > > > -	unsigned long end, unsigned long phys, const struct mem_type *type)
> > > > +				  unsigned long end, phys_addr_t phys,
> > > > +				  const struct mem_type *type)
> > > > 
> > > The change is correct but seems useless so far. This function only be
> > > called
> > > from map_lowmem and devicemaps_init, from i know neither lowmem nor device
> > > io
> > > registers of existing platforms exceed 32bit address.
> > 
> > It is not because you are not aware of any existing platforms with RAM
> > or device IO above the 4GB mark that they don't exist.
> > 
> > For example, some LPAE systems have all their RAM located above the 4G
> > physical address mark. A simple (potentially non DMA capable) alias
> > exists in the low 32-bit address space to allow the system to boot and
> > switch to the real physical RAM addresses once the MMU is turned on.
> > Some of that RAM is still qualified as "low mem" i.e. the portion of RAM
> > that the kernel keeps permanently mapped in the 32-bit virtual space
> > even if all of it is above the 4G mark in physical space.
> 
> I think he is right. You cannot have low_mem and devices in 36-bit areas.
> Atleast this is what I saw in one of the platforms on which I tested these
> patches. I am not sure what you mean by hardware address aliasing(as I have
> real RAM), but we need 32-bit areas to boot the CPU and I have mapped them for
> the LOW_MEM. But, I have used 36-bit areas for the HIGH_MEM. Since you said
> about aliasing DDR area in 32-bits, and then switching to 36-bit RAM, does the
> dma of the devices still use 32-bit aliased addresses?
> 
> I haven't tested a configuration where LOW_MEM can have both 32-bit and 36-bit
> DDR PA though. I think its not possible too.

Don't get confused by the 36-bit supersections introduced with ARMv6.  
This patch series is about LPAE capable systems using a completely 
different page table format providing physical addressing beyond 36 
bits.


Nicolas
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