Re: Can't read from mmaped PCI memory space

[Wolfgang Grandegger <wg@grandegger.com>]

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.
>>
>>
> 
> 
>