busmaster | Open Source Software tool to simulate , analyze and test | Business library

Ok, after a lot of trial and error I was able to find the problem. I will share my experience in hope for someone to find it useful (my pc has a HD620-H81 motherboard with IT8888F/G PCI-ISA bridge).

My PCI-ISA bridge (and probably many others) is configured by default in "Subtractive Decoding" pci mode, which means "if no other pci device claim an address, then the bridge claims it". You can check for subtractive decoding mode by running lspci -t and lscpi -vv, and you should see that every PCIe-PCI/PCI-PCI bridge in the tree leading to your PCI-ISA bridge is configured in subtractive decode mode (the ISA bridge itself won't appear as subtractive because from a PCI perspective the ISA bridge is just a PCI device and not a bridge).

That means that you don't have a BAR assigned to the bridge, nor you need to interact with the pci bridge device directly in any way. You can just access directly the absolute address you need and the bridge will take care of managing it correctly (a lot of information is already available on the internet for subtractive decoding anyway if you're interested in details).

To sum up: no action is needed regarding the bridge itself or pci devices, just make sure in the bios that no other device, such as serial or parallel ports, gets assigned the port/address range that you are interested into (if it does, either disable it or change its address).

I found that my ISA card was located at a port which is 0x0800 ports above the one where it should be (and that was the main problem for me). I'm not sure about why, maybe my bridge adds a fixed offset (if you know the reason maybe comment it below!).
What I did to find out the correct address was to run a function that iterates all port addresses from 0x0100 (which skips the first zone where is better not to write random data into) to 0xFFFF and runs a card check routine at each single port address until it finds the correct answer I expected from the card.