Userspace access to PCIe - SATA device memory BAR

Hi,

I have a PCIe - SATA bridge device, and I would like to access the I2C control registers in the PCI memory region (BAR5) - these use just two addresses in the BAR. I have tried, in userspace, to mmap() the /sys/bus/pci/devices/..../resource5 file and read from that, but all I get is 0xFF in every byte.

I used pcimem (https://github.com/billfarrow/pcimem.git) to try to read the BAR.

The device is already matched by the 'ahci' driver, and the attached disks are visible and mountable. If I unbind the ahci driver to use uio, I can of course no longer use the device as a SATA bridge.

In other PCI devices which are also matched by ahci, I can mmap and read published BARs fine without unbinding the ahci driver.

What is the correct way to fiddle with the memory in the BARs of a PCI device, while still letting the normal driver run the device? I am not doing anything related to AHCI or SATA, just reading and writing to the "side-channel" communication hardware.

Best regards,

John