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Can't read from mmaped PCI memory space

To: Linux-MIPS <>
Subject: Can't read from mmaped PCI memory space
From: Wolfgang Grandegger <>
Date: Sun, 28 Nov 2010 22:20:49 +0100
Original-recipient: rfc822;
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20100907 Fedora/3.0.7-1.fc12 Thunderbird/3.0.7

I'm trying to read from mmapped PCI memory space on an alchemy board,
but I can't get it to work. Here's the lspci output of the PCI card:

  bash-3.00# lspci -v
  00:00.0 Class 0200: 168c:001b (rev 01)
        Subsystem: 168c:2063
        Flags: bus master, medium devsel, latency 168, IRQ 9
        Memory at 0000000040000000 (32-bit, non-prefetchable) [size=64K]
        Capabilities: [44] Power Management version 2

I used mmap on "/dev/mem" and "/sys/bus/pci/.../resource0", but I do not
read the expected values using "*(volatile u32 *)mmap_addr" from that
region. The value also changes from read to read. Reading from kernel
space just work fine. Am I doing something illegal? Any idea why it does
not work?



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