freeze when reading a PCI bridge register

Nicolas Lavocat nicolas.lavocat at
Wed Apr 22 18:04:29 EST 2009

Hi everybody!

I' am trying to configure a PCI bridge on a private board, with a 
powerpc . In a first time, I tried to get informations about PCI 
devices, in order to be sure  that  my read  and write methods work ( 
using 2 configuration registers, like on an x86 architecture.) . 2 
configuration registers  are used, for example we  write  an encoded 
address (it is a request to a PCI device) in the first and the answer of 
the PCI device can be read in the second register (it is a configuration 
Firstly, I did it by JTAG: it works. Then, under uboot, it is ok.

For example, the code used under u-boot:

volatile u32* addr;
u32 vendor_device_ID;

puts("PCI1 reading PCI VENDOR and DEVICE ID\n");

addr= CFG_DATA_PCI1;
vendor_device_ID= *addr;
printf("PCI1: PCI1_VENDOR_DEVICE_ID= %08x  \n" ,vendor_device_ID);

Therefore, when I do the same thing under Linux, the system crash when I 
try to read the second register...
Linux is frozen, and there is no error message.
Under Linux, I made an ioremap before use the registers and access to 
these registers thanks to  functions  "in_be32" and "out_be32".
I tried with different endianness to avoid an error of this type.

If I understand, the main difference between u-boot and Linux (about 
registers access) is the activation of the MMU.
So I thought that  problem could come from it. 
I think the problem could came from the configuration of DBAT and IBATS 
registers of the MMU, but I didn't found any information about the MMU 
configuration under Linux.
So after this novel, I have some questions:

-Is the MMU configuration generic under Linux?
-Does somebody think that the problem doesn't come from MMU?
-How does work ioremap? Is it a fully software function, or does it 
speaks to MMU to get the effective address from physical address?
-Does somebody have an idea, or a documentation about MMU configuration 
under linux?
-Do you think that my MMU is under the control of an evil spirit? ^^

thank you for your help

Nicolas Lavocat

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