How do I obtain PCI Region size in Windows?

Question

I needed to scan my PCI bus and obtain information for specific devices from specific vendors. My goal is to find the PCI Region size for the AMD Graphics card, in order to map the PCI memory of that card to userspace in order to do i2c transfers and view information from various sensors.

For scanning the PCI bus I downloaded and compiled pciutils 3.1.7 for Windows x64 around a year ago. It supposedly uses DirectIO.

This is my code.

int scan_pci_bus()
{
    struct pci_access *pci;
    struct pci_dev *dev;
    int i;

    pci = pci_alloc();
    pci_init(pci);

    pci_scan_bus(pci);

    for(dev = pci->devices; dev; dev = dev->next) 
    {
        pci_fill_info(dev, PCI_FILL_IDENT | PCI_FILL_CLASS | PCI_FILL_IRQ | PCI_FILL_BASES | PCI_FILL_ROM_BASE | PCI_FILL_SIZES | PCI_FILL_PHYS_SLOT);
        if(dev->vendor_id == 0x1002 && dev->device_id == 0x6899)
        {
            //Vendor is AMD, Device ID is a AMD HD5850 GPU
            for(i = 0; i < 6; i++) 
            {
                printf("Region Size %d %x ID %x\n", dev->size[i], dev->base_addr[i], dev->device_id);
            }
        }
    }


    pci_cleanup(pci);

    return 0;
}

As you see in my printf line, I try to print some data, I am successfully printing device_id and base_addr however size which should contain the PCI region size for this device is always 0. I expected, at least one of the cycles from the loop to display a size > 0.

My code is based on a Linux application which uses the same code, though it uses the pci.h headers that come with Linux(pciutils apparenltly has the same APIs). Apparently, Windows(that is Windows 7 x64 in my case) does not show this information or the at the very least is not exposed to PCIUtils.

How do you propose I obtain this information? If there are alternatives to pciutils for Windows and provide this information, I'd be glad to obtain a link to them.

EDIT:I have still found no solution. If there are any solutions to my problem and also work for 32-bit Windows, It would be deeply appreciated.

Answer 1

How this works is pretty complicated. PCI devices use Base Address Registers to let the BIOS and Operating System decide where to locate their memory regions. Each PCI device is allowed to specify several memory or IO regions it wants, and lets the BIOS/OS decide where to put it. Complicating matters, there's only one register that is used both to specify the size AND the address. How does this work?

When the card is first powered up, it's 32-bit address register will have something like 0xFFFF0000 in it. Any binary 1 means "the OS can change this", any binary 0 means "must stay zero". So this is telling the OS that any of the top 16 bits can be set to whatever the OS wants, but the bottom 16 bits have to stay zero. Which also means that this memory region takes up 16 bits of address space, or 64k. Because of this, memory regions have to be aligned to their size. If a card wants 64K of address space, the OS can only put it on memory addresses that are a multiple of 64K. When the OS has decided where it wants to locate this card's 64K memory space, it writes it back into this register, overwriting the initial 0xFFFF0000 that was in there.

In other words, the card tells the OS what size/alignment it needs for the memory, then the OS overwrites that same register/variable with the address for the memory. Once it's done this, you can't get the size back out of the register without resetting the address.

This means there's no portable way of asking a card how big its region is, all you can ask it is WHERE the region is.

So why does this work in Linux? Because it's asking the kernel for this information. The kernel has an API to provide this stuff, the same way that lspci works. I am not a Windows expert, but I'm unaware of any way for an application to ask the Windows kernel this information. There may be an API to do this somehow, or you may need to write something that runs on the kernel side to pass this information back to you. If you look in the libpci source, for windows it calls the "generic" version of pci_fill_info(), which returns:

return flags & ~PCI_FILL_SIZES;

which basically means "I'm returning everything you asked for, but the sizes."

BUT, this may not matter anyway. If all you're doing is wanting to read/write to the I2C registers, they're usually (always?) in the first 4K of the control/configuration region. You can probably just map 4K (one page) and ignore the fact that there might be more. Also be warned that you may need to take additional steps to stop the real driver for this card from reading/writing while you are. If you're bit-banging the I2C bus manually, and the driver tries to at the same time, it's likely to cause a mess on the bus.

There also may be an existing way to ask the radeon driver to do I2C requests for you, which might avoid all of this.

(also note I'm simplifying and glossing over a lot of details with how the BARs work, including 64 bit addresses, I/O space, etc, read PCI documentation if you want to learn more)