FPGA registers access from Linux userspace

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Introduction

There are two methods to access FPGA registers on APF*.

  • For APF9328, APF27 and APF51 registers access are done with a memory bus (parallel).
  • For APF6_SP registers access are done with PCIe

Memory bus

fpgaregs

Installation

If the fpgaregs package is not already installed on your APF (/usr/bin/fpgaregs), you can select it in the Buildroot menuconfig: 

 $ make menuconfig

Package Selection for the target  ---> 
    [*] Hardware handling / blockdevices and filesystem maintenance  ---> 
         [*]   fpgaregs 

 $ make

Then reflash your rootfs or install it manually.

Usage

fpgaregs can be used to do read or write accesses (16 or 32 bits wide) to the FPGA, from Linux userspace/console.

16 bits read
# fpgaregs w <address>

Where <address> is an address relative to FPGA's mapping in hexadecimal value. Example: 

# fpgaregs w 0
16 bits write
# fpgaregs w <address> <value>

Where <value> is hexadecimal value to write.

32 bits read
# fpgaregs l <address>
32 bits write
# fpgaregs l <address> <value>

the mmap problem

Note Note: This problem is corrected with latest Toolchains compilation options


First of all, you need to get a file descriptor for /dev/mem using the open() function 

ffpga=open("/dev/mem",O_RDWR|O_SYNC);

Now you have a valid file descriptor to access your FPGA.

The O_SYNC option is recommended to avoid Linux to cache the content of /dev/mem and delay any modification done in this file.


To access fpga register, fpgaregs use the mmap() system call : 

void * ptr_fpga;
ptr_fpga = mmap (0, 8192, PROT_READ|PROT_WRITE, MAP_SHARED, ffpga, FPGA_ADDRESS);

Thanks to this function, fpga registers are accessible directly on memory with pointer ptr_fpga. To read and write in 16bits or in 32 bits we will cast the pointer value in unsigned short or unsigned int :

16bits write 

        *(unsigned short*)(ptr_fpga+(address)) = (unsigned short)value;

read 

          value = *(unsigned short*)(ptr_fpga+(address));

32 bits write 

        *(unsigned int*)(ptr_fpga+(address)) = (unsigned short)value;

read 

          value = *(unsigned int*)(ptr_fpga+(address));

The problem

By default, if the specific arm920t target is not specified, arm-linux-gcc will try to generate compatible read/write for all ARM9 model when it access register in 16bits. Indeed it seems that not all ARM9 have 16bits read/write capabilities (ldrh asm instruction).

As the interface between i.MXL and FPGA on APF9328 has no 8bits read/write capabilities, each 8 bits access is recognized by the FPGA as a 16bits access. So on each 16bits access of the i.MXL, FPGA will process two 16bits access instead of 1. That is a problem when accessing a FIFO for example.

To avoid this painful problem don't forget the -mcpu=arm920t option when compiling fpgaregs for APF9328 and -mcpu=arm926ej-s for APF27.

PCIe

On APF6_SP FPGA memory is accessed with PCIe, to read/write on FPGA space an example is available with pci_debug.

Pci_debug

See pci_debug page to know how to access PCIe address space with a command line tool.

Links

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