CODEWITHSUNDEEP
cpcimemory-mapping
flashburn
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Reputation: 4508

communicating with a PCI device through /dev/mem

I'm need to communicate with a memory PCI device through /dev/mem. Why? There are many reasons, one of which is because my boss told me so.

I think I have my answer here. What I can't figure out however is the MMIO_ADDR in the answer. If I want to communicate with a specific memory region, would I use a value stored in a BAR register as my MMIO_ADDR? If no then how do I communicate with my PCI device?

Upvotes: 1

Views: 4076

Answers (1)

Gil Hamilton
Gil Hamilton

Reputation: 12337

Your BAR will have been assigned an address by the BIOS/kernel. That address should have been written into the BAR address registers in the PCI configuration header by the time the system is up.

For example, on a VM I have here, the e1000 device is as follows (from lspci -v):

02:03.0 Ethernet controller: Intel Corporation 82545EM Gigabit Ethernet Controller (Copper) (rev 01)
        Subsystem: VMware PRO/1000 MT Single Port Adapter
        Physical Slot: 35
        Flags: bus master, 66MHz, medium devsel, latency 0, IRQ 17
   =>   Memory at fd560000 (64-bit, non-prefetchable) [size=128K]
        Memory at fdfd0000 (64-bit, non-prefetchable) [size=64K]
        I/O ports at 2080 [size=64]
        [virtual] Expansion ROM at fd520000 [disabled] [size=64K]
        Capabilities: <access denied>
        Kernel driver in use: e1000

You'll need to read that info (you can reach it in binary form via /proc/bus/pci/<Bus>/<DevFn> or via /sys/bus/pci/devices/) to find the MMIO address.

For example, dumping the binary of the PCI configuration space for the device above shows:

od -tx1z -Ax /proc/bus/pci/02/03.0
000000 86 80 0f 10 17 01 30 02 01 00 00 02 10 00 00 00  >......0.........<
000010 04 00 56 fd 00 00 00 00 04 00 fd fd 00 00 00 00  >..V.............<
000020 81 20 00 00 00 00 00 00 00 00 00 00 ad 15 50 07  >. ............P.<
000030 00 00 00 00 dc 00 00 00 00 00 00 00 0b 01 ff 00  >................<
000040

The first BAR (bar region #0) is at offset 0x10, the second (bar region #2) is at offset 0x18.

See https://en.wikipedia.org/wiki/PCI_configuration_space for description of the layout and interpretation.

Upvotes: 2

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