how does macro to get base address register in configuration space works

jamesbon · 11-21-2010, 09:06 PM

I am trying to understand working of pci_resource_start function
So I browsed code via cscope and searched for string pci_resource_start
and got following in pci.h

Code:

 #define pci_resource_start(dev, bar)    ((dev)->resource[(bar)].start)

I am not able to understand how does this above macro works.
How does it above macro gets appropriate base address register in
configuration space?

paulsm4 · 11-22-2010, 07:19 PM

Hi -

1. I'm sure you're probably familiar with this document:
http://tldp.org/LDP/tlk/dd/pci.html

Note in particular the "PCI Base Address" diagrams

2. Next, look at resource.c in your kernel source:

/usr/src/linux/kernel/resource.c =>

Code:

struct resource ioport_resource = {
        .name   = "PCI IO",
        .start  = 0x0000,
        .end    = IO_SPACE_LIMIT,
        .flags  = IORESOURCE_IO,
};

EXPORT_SYMBOL(ioport_resource);
...

3. This is a "Linux kernel thing", and not a "PCI thing".
Specifically,
a) The kernel has a "dev" pointer
b) "dev" has an array of "ioport_resources"
c) The macro you cited "knows" the values of start, end, etc based on other (source-configurable) definitions in the kernel.

'Hope that helps ... at least a little ...