FPGA registers access from Linux userspace

fpgaregs

compile

To access FPGA registers a tool named fpgaregs is available under target/linux/module/fpga/dev_tools/ directory. To compile it for apf9328, use the command above :

arm-linux-gcc -mcpu=arm920t fpgaregs.c -o fpgaregs

use

fpgaregs can be used to read and write 16 or 32 bits registers.

read 16 bits

fpgaregs w <address>

Where <address> is an address relative to fpga mapping in hexadecimal value.

write 16 bits

fpgaregs w <address> <value>

Where <value> is hexadecimal value to write.

read 32 bits

fpgaregs l <address>

write 32 bits

fpgaregs l <address> <value>

the mmap problem

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 :

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.

Links

• http://sources.redhat.com/ml/crossgcc/2005-08/msg00120.html : Explanation of the problem.