PCI Express BAR memory mapping basic understanding

Question

I am trying to understand how PCI Express works so i can write a windows driver that can read and write to a custom PCI Express device with no on-board memory.

I understand that the Base Address Registers (BAR) in the PCIE configuration space hold the memory address that the PCI Express should respond to / is allowed to write to. (Is that correct understood?)

My questions are the following:

• What is a "bus-specific address" compared to physical address when talking about PCIE?
• When and how is the BAR populated with addresses? Is the driver responsible for allocating memory and writing the address to the peripheral BAR?
• Is DMA used when transferring data from peripheral to host memory?

I appreciate your time.

Best regards,

Answer 1

i'm also working on device driver (albeit on linux) with a custom board. Here is my attempt on answering your questions:

The BARs represent memory windows as seen by the host system (CPUs) to talk to the device. The device doesn't write into that window but merely answers TLPs (Transaction Layer Packets) requests (MRd*, MWr*).

I would say "bus-specific" = "physical" addresses if your architecture doesn't have a bus layer traslation mechanism. Check this thread for more info.

In all the x86 consumer PCs i've used so far, the BAR address seemed to be allocated either by the BIOS or at OS boot. The driver has to work with whatever address has been allocated.

The term DMA seems to abused instead of bus mastering which I believe is the correct term in PCIe. In PCIe every device may be a bus master (if allowed in its command register bit 2). It does so by sending MRd, MWr TLPs to other devices in the bus (but generally to the system memory) and signalling interrupts to the CPU.

Answer 2

From your query its clear that you want to write a driver for a PCIe slave device. To understand the scheme of things happening behind PCIe transfer, a lot of stuff is available on internet (Like PCIe Bus Enumeration, Peripheral Address Mapping to the memory etc).

Yes your understanding is correct regarding the mapping of PCIe registers to the memory and you can read/write them.( For e.g in case of linux PCIe device driver you can do this using "ioremap" ).

An address bus is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus. Nothing more to add to that. "PCIe bus enumeration" topic will answer your 2nd question.

Your third question is vague. You mean slave PCIe device. Assuming it is, yes you can transfer data between a slave PCIe device and host using a DMA controller. I am working on a project which involves "PCIe-DMA" connected with the host over the PCIe bus. Really depends on your design and implementation. So in my case PCIe-DMA is itself a slave PCIe device on the target board connected to the host over PCIe.