Accessing HDMI hardware register in C

Asked Aug 10 '17 at 13:07

Active Aug 10 '17 at 13:21

Viewed 131 times

I am trying to access the HDMI hardware registers on a Phytec MIRA board using C. This is my first real step into embedded programming.

The memory address for the Design ID register, according to the i.MX 6 Reference Manual, is 0x120000 (See page 1568).

I wrote some code to try to access this register, and it just gives a segmentation fault when I try to dereference the pointer.

Can anyone tell me what I am doing wrong and point me in the right direction please?

Minimal reproducing code below:

#include <stdio.h>
#include <inttypes.h>

int main()
{
  uint8_t const volatile * const HDMI_DESIGN_ID = (uint8_t const volatile * const)(0x120000);
  printf("Design ID: %u\n", *HDMI_DESIGN_ID);
  return 0;
}

edited Aug 10 '17 at 13:21

Tony Tannous

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asked Aug 10 '17 at 13:07

Phillip Hamnett

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Are you trying to access from user space ? – Tony Tannous Aug 10 '17 at 13:09
That would only have a chance of working if memory mapping is turned off. What processor and what OS? – stark Aug 10 '17 at 13:11
@TonyTannous Yes I am running the program in user space. – Phillip Hamnett Aug 10 '17 at 13:14
@Neztor I'd take a shot and say that the registers are in memory region where only the `OS` has access. Trying to access this will cause segmentation fault. – Tony Tannous Aug 10 '17 at 13:15
@stark i.MX 6 Processor, running linux – Phillip Hamnett Aug 10 '17 at 13:15
@TonyTannous If the memory is in a region where only the OS has access, how can I read or write to it? – Phillip Hamnett Aug 10 '17 at 13:16
You could possibly implement a new system call which returns values of addresses in kernel space. – Tony Tannous Aug 10 '17 at 13:18
@TonyTannous I just did some googling, is this what you mean? `int hdmi_fd = open("/dev/mem", O_RDWR); HDMI_DESIGN_ID = mmap((void*)(0x120000), sizeof(uint8_t), PROT_READ, MAP_SHARED, hdmi_fd, 0); ` – Phillip Hamnett Aug 10 '17 at 13:28
I don't understand what you wrote and I think there should be easier ways than implementing a new syscall. What you wrote here is not what I meant. – Tony Tannous Aug 10 '17 at 13:29
@TonyTannous Then what did you mean? – Phillip Hamnett Aug 10 '17 at 13:34
2

Follow this https://stackoverflow.com/questions/19494812/convert-physical-address-to-virtual-in-linux – Tony Tannous Aug 10 '17 at 13:45
@TonyTannous Thanks I'll keep an eye on it – Phillip Hamnett Aug 10 '17 at 14:02

Accessing HDMI hardware register in C

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