[U-Boot-Users] Please explain this code in drivers/pci.c

[Donald White <dbwhite@asu.edu>]

Group,

Please help me to understand the following code.  It occurs in drivers/pci.c in the 
function pci_hose_config_device().  I have ported U-Boot 0.3.0 and HHL 2.0 to our 
custom MPC8265A board.  It has one device on the PCI bus.  The driver develop was 
wanting a specific memory space allocation for the device.  So I created a PCI 
configuration table with the memory base of 0x90000000.  The card wants 4 regions 0f 
0x8000 bytes in size.  The driver writer thinks this should be 0x90000000, 
0x90008000, 0x90010000 and 0x90018000.  This brings us to the code snippet:

	bar_size = ~(bar_response & PCI_BASE_ADDRESS_MEM_MASK) + 1;
	bar_value = mem;

	mem = ((mem - 1) | (bar_size - 1)) + 1;

The value of bar_response is 0xffff8008 which looks good.  This gives bar size of 
0x8000 which also looks good.  The value of mem is initially 0x90000000.  So,
(mem -  1) = 0x8fffffff and (bar_size - 1) = 0x7fff).  When these are or'd one gets 
0x8fffffff and adding 1 gives 0x90000000.  This result is that the card is given 4 
address bases all at 0x90000000.

Thanks for any help,

Don