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

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Message-ID: <510C2647.3080305@gmail.com>
Date:	Fri, 01 Feb 2013 12:32:07 -0800
From:	Subash Patel <subashrp@...il.com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
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 Friday 01 February 2013 10:14 AM, Nicolas Pitre wrote:
> 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.

Yes, I mean LPAE capable MMU's introduced with Cortex-A15. Infact I have 
programmed that MMU with 1G super-section for non-linux usage :) Cyril 
explained his platform and aliasing. My platform has a real memory in 
32-bit area, and we use it for 32-bit DMA's as well. So it looks we are 
speaking of various configurations of LPAE systems here.

Regards,
Subash

>
>
> Nicolas
>
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