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Re: Can't read from mmaped PCI memory space

To: "tiejun.chen" <tiejun.chen@windriver.com>
Subject: Re: Can't read from mmaped PCI memory space
From: Wolfgang Grandegger <wg@grandegger.com>
Date: Mon, 29 Nov 2010 14:23:23 +0100
Cc: Linux-MIPS <linux-mips@linux-mips.org>
In-reply-to: <4CF37F5D.9070709@windriver.com>
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References: <4CF2C7B1.5030007@grandegger.com> <4CF37F5D.9070709@windriver.com>
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On 11/29/2010 11:24 AM, tiejun.chen wrote:
> Wolfgang Grandegger wrote:
>> Hello,
>>
>> I'm trying to read from mmapped PCI memory space on an alchemy board,
>> but I can't get it to work. Here's the lspci output of the PCI card:
>>
>>   bash-3.00# lspci -v
>>   00:00.0 Class 0200: 168c:001b (rev 01)
>>      Subsystem: 168c:2063
>>      Flags: bus master, medium devsel, latency 168, IRQ 9
>>      Memory at 0000000040000000 (32-bit, non-prefetchable) [size=64K]
>>      Capabilities: [44] Power Management version 2
>>
>> I used mmap on "/dev/mem" and "/sys/bus/pci/.../resource0", but I do not
>> read the expected values using "*(volatile u32 *)mmap_addr" from that
>> region. The value also changes from read to read. Reading from kernel
>> space just work fine. Am I doing something illegal? Any idea why it does
>> not work?
> 
> Form here I'm not sure how you did exactly.
> 
> Theoretically, you can mmap() directly that at least from the sys resource. 
> But
> I think you have to notice the aligning requirement for a page. I means you
> should firstly map one given base_address & ~(PAGE_SIZE - 1). Then access the
> last destination address with adding the corresponding offset as you want.
> 
> Hope its useful.
> 
> Tiejun
> 
>>
>> TIA,
>>
>> Wolfgang.
>>
>>
> 
> 
> 


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