Accessing APB and AXI Peripherals Through Linux#

AXI#

AXI is part of the ARM AMBA (Advanced Microcontroller Bus Architecture) protocol family.
It is designed for high-performance, high-frequency system-on-chip (SoC) designs.
AXI provides high-speed data transfer with minimal latency and is widely used in various applications, including high-end embedded systems and complex digital circuits.

APB#

APB is also part of the ARM AMBA protocol family, designed for low-power and low-latency communication with peripheral devices.

It is simpler and lower performance compared to AXI, making it suitable for slower peripheral devices. An APB peripheral also consumes less resources on the FPGA fabric compared to an AXI peripheral.

Accessing AXI and APB Peripherals from Linux#

To access AXI and APB peripherals from Linux, memory-mapped I/O (MMIO) is commonly used.
This involves mapping the physical addresses of the peripherals into the virtual address space of a user-space application.
The following sections demonstrate how to access APB peripherals using the Linux /dev/mem interface and AXI peripherals using the UIO (Userspace I/O) framework.

Note

The codes for accessing the interfaces are available in the snippets here: APB Interfaces and AXI Interfaces

APB Interfaces#

The MSS includes fabric interfaces for interfacing FPGA fabric with the CPU Core Complex.

It provides one 32-bit APB master interface, FIC3, and can be connected to a slave in the fabric.

Design Details#

For this example, you can try to write to the APB slave present in the Verilog Tutorial Cape gateware. Select the gateware by changing custom-fpga-design/my_custom_fpga_design.yaml to include VERILOG_TUTORIAL as the cape option.

The APB Slave has two registers, one read-only register at 0x00, one read-write register at 0x10, and a status register containing the last read value at 0x20.

Having a look at the design, we can see that the APB slave is connected with a CoreAPB3 interconnect, which assigns it the 0xXX10_0000 address, the top two bits being ignored.
Tracing to the master connected with the CoreAPB3 device, we can see that another interconnect is present, which gives our slave the 0xX100_0000 address.
The polarfire technical manual shows that FIC3 peripherals can start from the 0x4000_0000 address.
Therefore, the final address of our APB slave becomes 0x4110_0000.

Now, we shall access this address through a memory-mapped interface in Linux.

Important

The following paragraphs will present to you several ways to test APB/AXI traffic.
Normally, this isn’t harmful, but reading/writing to addresses
with no gateware behind it will lead to you stalling a CPU.
In rare cases, this stalling of a CPU can lead to loss of content on your eMMC,
so please make sure you have a known good backup!
For more information about the stalls, please read the section on issues faced with the interfaces.

Accessing the Interface#

There are two ways to access such registers. One can use the devmem2 utility or write a C program for accessing the memory region. The first method is quite simple.

To read from a register:
```
sudo devmem2 0x41100000 w
```
To write to a register:
```
sudo devmem2 0x41100010 w 0x1
```

In the second method, we can use the /dev/mem interface to access the registers inside the APB Slave. Here is an example C program which demonstrates this:

AXI Interfaces#

The MSS includes three 64-bit AXI FICs out of which FIC0 is used for data transfers to/from the fabric. FIC0 is connected as both master and slave. For usage of AXI peripherals, an example is also provided by microchip in their Polarfire SoC Linux examples. The example here takes reference from the AXI LSRAM example.

Design Details#

A simple design can be created by first connecting the FIC0 Initiator from the MSS to a CoreAXI4Interconnect.
Now, you can connect an AXI slave to this interconnect. We will be using the Polarfire AXI LSRAM.

Both the CoreAXI4Interconnect and the PF AXI LSRAM will have to be configured.
The AXI ID Width of both the modules will have to be matched, as well as the address space of the only slave will have to be configured.
In this example, LSRAM gets an address of 0x6000_0000 to 0x6000_ffff, and the AWID is kept at 9 bits.

Fig. 509 AXI LSRAM slave (example design)#

Finally, an entry will be added to the device tree to make a UIO device point to our LSRAM’s memory region.

&{/} {
    fabric-bus@40000000 {
        fpgalsram: uio@60000000 {
            compatible = "generic-uio";
            linux,uio-name = "fpga_lsram"; // mandatory for program. If changed, please update program as well.
            reg = <0x0 0x60000000 0x0 0x1000>;
            status = "enabled";
        };
    };
};

Once the gateware is compiled, we can access the memory-mapped interface by the same methods, and by the UIO device as well.

Using devmem2:

sudo devmem2 0x60000000 w # for read
sudo devmem2 0x60000000 w 0x1 # for write

Using the UIO device:

Issues that can be faced when using an improperly configured AXI/APB interface#

A CPU stall can be faced when accessing the FIC interfaces without any slaves connected to the memory region being accessed. Your BVF will stop responding if connected to SSH, and on serial you will see the following kernel messages:

[   24.110099] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
[   24.116041] rcu:     0-...0: (1 GPs behind) idle=e00c/0/0x1 softirq=40/41 fqs=2626
[   24.123377]     (detected by 3, t=5255 jiffies, g=-1131, q=9 ncpus=4)
[   24.129573] Task dump for CPU 0:
[   24.132810] task:swapper/0       state:R  running task     stack:0     pid:0     ppid:0      flags:0x00000008
[   24.142757] Call Trace:
[   24.145213] [<ffffffff80a67ba0>] __schedule+0x27c/0x834

If this happens, please double check your design. Specifically, check the address configured for the slaves, the AXI ID wire width and other AXI parameters.

In any case, this state is virtually impossible to recover from gracefully, so the reset button may be your last resort.

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