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 12:56:54 -0500
From:	Cyril Chemparathy <cyril@...com>
To:	Subash Patel <subashrp@...il.com>
CC:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Hui Wang <jason77.wang@...il.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	<marc.zyngier@....com>, Will Deacon <will.deacon@....com>,
	<linus.walleij@...aro.org>, <sboyd@...eaurora.org>,
	<linux-kernel@...r.kernel.org>, Vitaly Andrianov <vitalya@...com>,
	<paulmck@...ux.vnet.ibm.com>,
	Catalin Marinas <catalin.marinas@....com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 02/13] ARM: LPAE: use phys_addr_t in alloc_init_pud()

On 02/01/2013 12:33 PM, 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.
>

On the KeyStone platform, memory is located at 08:0000:0000, i.e., 
outside the 32-bit addressable range.  The hardware provides a limited 
aliased map of the very same memory at 8000:0000, but this alias is 
limited and intended only for boot time usage.

We boot the system while running out of this 32-bit physical address 
range.  We then switch over to the high physical address range fairly 
early in the kernel boot, by rewriting boot-time page tables and the 
TTBRs.  Once this switch over has happened, lowmem is indeed outside the 
32-bit physical address space.

Thanks
-- Cyril.
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