How does DMA work with PCI Express devices?

In the PCIe enumeration phase, the maximum allowed payload size is determined (it can be lower then the device's max payload size: e.g. a intermediate PCIe switch has a lower max. payload size).

Most PCIe devices are DMA masters, so the driver transfers the command to the device. The device will send several write packets to transmit 4 MiB in xx max sized TLP chunks.

Edit 1 in reply to comment 1:

A PCI based bus has no "DMA Controller" in form of a chip or a sub circuit in the chipset. Every device on the bus can become a bus master. The main memory is always a slave.

Let's assume you have build your own PCIe device card, which can act as an PCI master and your program (running on CPU) wants to send data from that card to main memory (4 MiB).

The device driver knows the memory mapping for that particular memory region from operating system (some keywords: memory mapped I/O, PCI bus enumeration, PCI BARs, ).

The driver transfers the command (write), source-address, destination-address and length to the device. This can be done by sending bytes to a special address inside an pre-defined BAR or by writing into the PCI config space. The DMA master on the cards checks these special regions for new tasks (scatter-gather lists). If so, theses tasks get enqueued.

Now the DMA master knows where to send, how many data. He will read the data from local memory and wrap it into 512 byte TLPs of max payload size (the max payload size on path device <---> main memory is known from enumeration) and send it to the destination address. The PCI address-based routing mechanisms direct these TLPs to the main memory.

@Paebbels has already explained most of it. In PCI/PCI-e, "DMA" is implemented in terms of bus mastering, and it's the bus-master-capable peripheral devices that hold the reins. The peripheral device has the memory read/write transactions at its disposal, and it's up to the peripheral device, what granularity and ordering of the writes (or reads) it will use. I.e. the precise implementation details are hardware-specific to the peripheral device, and the corresponding software driver running on the host CPU must know how to operate the particular peripheral device, to provoke the desired DMA traffic in it.

Regarding the "memory management aspect", let me refer my distinguished audience to two chapters of a neat book by Jon Corbet, on exactly this topic in Linux. Memory management bordering on DMA, under the hood of the OS kernel. Linux and its source code and documentation are generally a good place (open source) to start looking for "how things work under the hood". I'll try to summarize to topic a bit.

First of all, please note that DMA access to the host's RAM (from a peripheral PCI device) is a different matter than PCI MMIO = where the peripheral device possesses a private bank of RAM of its very own, wants to make that available to the host system via a MMIO BAR. This is different from DMA, a different mechanism (although not quite), or maybe "the opposite perspective" if you will... suppose that the difference between a host and a peripheral device on the PCI/PCI-e is not great, and the host bridge / root complex merely has a somewhat special role in the tree topology, bus initialization and whatnot :-) I hope I've confused you enough.

The computer system containing a PCI(-e) bus tree and a modern host CPU actually works with several "address spaces". You've probably heard about the CPU's physical address space (spoken at the "front side bus" among the CPU cores, the RAM controller and the PCI root bridge) vs. the "virtual address spaces", managed by the OS with the help of some HW support on part of the CPU for individual user-space processes (including one such virtual space for the kernel itself, not identical with the physical address space). Those two address spaces, the physical one and the manifold virtual, occur irrespective of the PCI(-e) bus. And, guess what: the PCI(-e) bus has its own address space, called the "bus space". Note that there's also the so called "PCI configuration space" = yet another parallel address space. Let's abstract from the PCI config space for now, as access to it is indirect and complicated anyway = does not "get in the way" of our topic here.

So we have three different address spaces (or categories): the physical address space, the virtual spaces, and the PCI(-e) bus space. These need to be "mapped" to each other. Addresses need to be translated. The virtual memory management subsystem in the kernel uses its page tables and some x86 hardware magic (keyword: MMU) to do its job: translate from virtual to physical addresses. When speaking to PCI(-e) devices, or rather their "memory mapped IO", or when using DMA, addresses need to be translated between the CPU physical address space and the PCI(-e) bus space. In the hardware, in bus transactions, it is the job of the PCI(-e) root complex to handle the payload traffic, including address translation. And on the software side, the kernel provides functions (as part of its internal API) to drivers to be able to translate addresses where needed. As much as the software is only concerned about its respective virtual address space, when talking to PCI(-e) peripheral devices, it needs to program their "base address registers" for DMA with addresses from the "bus space", as that's where the PCI(-e) peripherals live. The peripherals are not gonna play the "game of multiple address translations" actively with us... It's up to the software, or specifically the OS, to make the PCI(-e) bus space allocations a part of the host CPU's physical address space, and to make the host physical space accessible to the PCI devices. (Although not a typical scenario, a host computer can even have multiple PCI(-e) root complexes, hosting multiple trees of the PCI(-e) bus. Their address space allocations must not overlap in the host CPU physical address space.)

There's a shortcut, although not quite: in an x86 PC, the PCI(-e) address space and the host CPU physical address space, are one. Not sure if this is hardwired in the HW (the root complex just doesn't have any specific mapping/translation capability) or if this is how "things happen to be done", in the BIOS/UEFI and in Linux. Suffice to say that this happens to be the case. But, at the same time, this doesn't make the life of a Linux driver writer any easier. Linux is made to work on various HW platforms, it does have an API for translating addresses, and the use of that API is mandatory, when crossing between address spaces.

Maybe interestingly, the API shorthands complicit in the context of PCI(-e) drivers and DMA, are "bus_to_virt()" and "virt_to_bus()". Because, to software, what matters is its respective virtual address - so why complicate things to the driver author by forcing him to translate (and keep track of) the virtual, the physical and the bus address space, right? There are also shorthands for allocating memory for DMA use: pci_alloc_consistent() and pci_map_single() - and their deallocation counterparts, and several companions - if interested, you really should refer to Jon Corbet's book and further docs (and kernel source code).

So as a driver author, you allocate a piece of RAM for DMA use, you get a pointer of your respective "virtual" flavour (some kernel space), and then you translate that pointer into the PCI "bus" space, which you can then quote to your PCI(-e) peripheral device = "this is where you can upload the input data".

You can then instruct your peripheral to do a DMA transaction into your allocated memory window. The DMA window in RAM can be bigger (and typically is) than the "maximum PCI-e transaction size" - which means, that the peripheral device needs to issue several consecutive transactions to accomplish a transfer of the whole allocated window (which may or may not be required, depending on your application). Exactly how that fragmented transfer is organized, that's specific to your PCI peripheral hardware and your software driver. The peripheral can just use a known integer count of consecutive offsets back to back. Or it can use a linked list. The list can grow dynamically. You can supply the list via some BAR to the peripheral device, or you can use a second DMA window (or subsection of your single window) to construct the linked list in your RAM, and the peripheral PCI device will just run along that chain. This is how scatter-gather DMA works in practical contemporary PCI-e devices.

The peripheral device can signal back completion or some other events using IRQ. In general, the operation of a peripheral device involving DMA will be a mixture of direct polling access to BAR's, DMA transfers and IRQ signaling.

As you may have inferred, when doing DMA, the peripheral device need NOT necessarily possess a private buffer on board, that would be as big as your DMA window allocation in the host RAM. Quite the contrary - the peripheral can easily "stream" the data from (or to) an internal register that's one word long (32b/64b), or a buffer worth a single "PCI-e payload size", if the application is suited for that arrangement. Or a minuscule double buffer or some such. Or the peripheral can indeed have a humongous private RAM to launch DMA against - and such a private RAM need not be mapped to a BAR (!) if direct MMIO access from the bus is not required/desired.

Note that a peripheral can launch DMA to another peripheral's MMIO BAR just as easily, as it can DMA-transfer data to/from the host RAM. I.e., given a PCI bus, two peripheral devices can actually send data directly to each other, without using bandwidth on the host's "front side bus" (or whatever it is nowadays, north of the PCI root complex: quickpath, torus, you name it).

During PCI bus initialization, the BIOS/UEFI or the OS allocates windows of bus address space (and physical address space) to PCI bus segments and peripherals - to satisfy the BARs' hunger for address space, while keeping the allocations non-overlapping systemwide. Individual PCI bridges (including the host bridge / root complex) get configured to "decode" their respective allocated spaces, but "remain in high impedance" (silent) for addresses that are not their own. Feel free to google on your own on "positive decode" vs. "subtractive decode", where one particular path down the PCI(-e) bus can be turned into an "address sink of last resort", maybe just for the range of the legacy ISA etc.

Another tangential note maybe: if you've never programmed simple MMIO in a driver, i.e. used BAR's offered by PCI devices, know ye that the relevant keyword (API call) is ioremap() (and its counterpart iounmap, upon driver unload). This is how you make your BAR accessible to memory-style access in your living driver.

And: you can make your mapped MMIO bar, or your DMA window, available directly to a user-space process, using a call to mmap(). Thus, your user-space process can then access that memory window directly, without having to go through the expensive and indirect rabbit hole of the ioctl().

Umm. Modulo PCI bus latencies and bandwidth, the cacheable attribute etc.

I feel that this is where I'm getting too deep down under the hood, and running out of steam... corrections welcome.

I am missing the memory management aspect. Bus masters send data in chunks to memory, using a physical address, which is formerly resolved by software using APIs to the OS. But a size of 4 MBytes spans a large number of MMU pages and the MMU is part of the CPU. Not in the chipset driving memory and PCIe. So, I don't beleive, this is the full story. I am afraid, each block must be handled separately.