Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I am trying to access a memory-mapped register outside of system ram. I am opening /dev/mem to do so. When using seek, I only get access to the address space of the system ram. Using an address above that limit gives me a "Bad address" error.

Doing a similar operation in 'C' allows access to addresses above that limitation. Why does perl limit my access? Is there anyway around the limitation?

Replies are listed 'Best First'.
Re: Accessing Memory-mapped I/O
by stevieb (Canon) on Oct 31, 2019 at 15:56 UTC

    Would you please provide the smallest snippet of code that doesn't work, so that we can reproduce the problem to better understand it?

    Adding the C example showing it working would be of benefit as well.

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      Here is a snippet of perl code:

      open(FH, "<", "/dev/mem");
binmode(FH);
seek(FH,0xff230180,0);  #Fails with bad address

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      Here is a snippet of C code:

      devmem_fd = open("/dev/mem", O_RDWR | O_SYNC);
bridge_map = (uint32_t*)mmap(NULL, 0x100, PROT_READ|PROT_WRITE, 
    MAP_SHARED, devmem_fd, 0xff230180);
reg = (uint32_t*) bridge_map;
printf("Register contents: 0x08x\n", *reg);
munmap (bridge_map, 0x100);
close(devmem_fd);

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        You're doing two completely different thing between the Perl code and the C code. Does the C code still work when using seek? Does the Perl code work when using (for example) File::Map?

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Re: Accessing Memory-mapped I/O
by jdmonk (Initiate) on Nov 04, 2019 at 19:29 UTC
    So, I found a way to install the Sys::Mmap module. With it, I am able to access the memory-mapped registers.
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Re: Accessing Memory-mapped I/O
by jcb (Parson) on Nov 04, 2019 at 01:32 UTC

    Perl is not limiting your access, but you are using buffered I/O to access MMIO space, so perl is trying to read much more than you actually have. Try sysopen/sysseek/sysread/syswrite instead.

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      I've tried using sysseek and have the same problem: bad address.
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Re: Accessing Memory-mapped I/O
by Anonymous Monk on Nov 01, 2019 at 13:23 UTC
    You need to publish your C-code to let us demonstrate that it works. I do not think that it does. In any case what you really want to do is to use system-calls to directly map the requested address space ... not rely on /dev/mem.
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