Can't read from mmaped PCI memory space

Can't read from mmaped PCI memory space

[Wolfgang Grandegger]

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?

TIA,

Wolfgang.

Re: Can't read from mmaped PCI memory space

[tiejun.chen]

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.
> 
> 

Re: Can't read from mmaped PCI memory space

[Wolfgang Grandegger]

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.
>>
>>
> 
> 
> 

Re: Can't read from mmaped PCI memory space

[Wolfgang Grandegger]

Hello,

I hit the send button too early, sorry...

On 11/29/2010 02:23 PM, Wolfgang Grandegger wrote:
> 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.

I'm aware of the alignment issue. Anyway, I'm mapping the above address,
which is already aligned. It must be something else. I'm using the
ath_info and devmem2 Program for testing.

Thanks,

Wolfgang.

Re: Can't read from mmaped PCI memory space

[Wolfgang Grandegger]

On 11/29/2010 02:41 PM, Wolfgang Grandegger wrote:
> Hello,
> 
> I hit the send button too early, sorry...
> 
> On 11/29/2010 02:23 PM, Wolfgang Grandegger wrote:
>> 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.
> 
> I'm aware of the alignment issue. Anyway, I'm mapping the above address,
> which is already aligned. It must be something else. I'm using the
> ath_info and devmem2 Program for testing.

It's an issue with 64-bit address mapping. Currently mmap tries to map
0x4000'0000 but the physical address of the PCI memory space on my CPU is
0x4'4000'0000. I wonder why this problem has not yet been discovered.
The attached patch below works for my board.

Wolfgang.