A simple guide on using RasPi DMA channels

Suppose you need to monitor the level changes of your Pi's GPIOs and report them as soon as possible. This could be as simple as a single interrupt if you are programming on bare metal. However, things are much more complicated if you are working on Pi's Linux system with tens of other processes running. To avoid your program from occupying all the CPU cycles, you need to use DMA.

I found that there actually exists very few resources on how to use Raspberry Pi's DMA channel properly. Pigpio uses DMA accesses to achieve microsecond-level sampling, but as the project is fairly large now it's not very suitable for beginners. However, pigpio, along with Wallacoloo's example (a little old) and hezller's demo are great references when you write code to control the DMA channels.

Background knowledge

As described in BCM2835-ARM-Peripherals (the "datasheet"), There are three types of addresses in Raspberry Pi:

• ARM Virtual Address: The address used in the virtual address space of a Linux process.
• ARM Physical Address: The address used when accessing RAM. Since peripherals on BCM2835 are memory-mapped, this address is used to access peripherals directly.
• Bus Address: This is the address used by the DMA engine.

Note that the physical addresses of the peripherals range from 0x3F000000 to 0x3FFFFFFF and are mapped onto bus address range 0x7F000000 to 0x7FFFFFFF.

The DMA control blocks on BCM2835 are organized as a linked list, with the pivot being DMA channel controller registers. Note that all "pointer"s in the following image are bus addresses.

Detailed information about the DMA controller can be found in the datasheet.

Mapping peripherals into virtual memory with mmap()

Allocating DMA Control Blocks and Result Buffer

The next step is to allocate our control blocks and a buffer to store DMA results. What's special about this is that since DMA accesses uses bus address, we at least need to know the physical address of our allocated memory region.

A natural way to get physical addresses out of virtual addresses would be using Linux's pagemap interface. However, this is not reliable because we can't guarantee the memory we have is cache coherent, which is crucial to proper DMA. Also, the physical address backing a certain virtual address may be subject to change. Thus, it's not recommended to use pagemap.

As pointed out here (although I can't open its referenced page) the RasPi memory can be partitioned into 4 sections regarding their cache usage:

Memory starting at	Cache
0x00000000	L1 & L2 cache
0x40000000	L2 cache coherent (meaning L1 write-through?)
0x80000000	L2 cache only
0xc0000000	direct uncached

The mailbox property interface documentation suggests that memory sections with 0x8 and 0xc alias seem to be coherent when using DMA.

Starting DMA transfer

Pacing DMA accesses with DREQ

Final example: Monitoring GPIO changes at the us-level accuracy