On 06-05-08 05:15, Peter Teoh wrote:
Greg: added to the CC as one of the LDD3 authors. perhaps it's interesting
to note for a possible followup edition.
Please be aware that /proc/iomem lists physical addresses, while his
request was for availability of a virtual address.
I was aware of the difference, that is why before I posted, I referred
to LDD3, and found this mapping between /proc/*/maps (which is virtual
address) and iomem. And since Greg in the book speaked as if the two
address are the same (they are identical in the first place - could it
be due to identity mapping for the low mem? I suspect so.....) and
that's where I deduced that it is virtual memory.
Here (the whole paragraph under "The mmap Device Operation"):
http://people.freedesktop.org/~nagappan/LDD3/LDD3.html
Ok...logically it should physical address....being IOMEM...and being
process independent....
Ah, I see. That "definitive" example is actually a fairly poor chosen one
due this exact potential for confusion indeed. If I remember right the fact
that (in the book) you see that identify for the first megabyte is only due
to the X server setting up a VM86 environment ("a DOS box", if that
terminology still means anything to you) in which to call the VGA BIOS. The
VGA BIOS expects to find a real-mode environment; specifically, expects to
be called in an environment where it itself lives at 0xc0000 (expressed in
real-mode terms as 0xc000:0x0) and the VGA videomemory at 0xa0000 (0xa000:0x0).
These days it appears X is either no longer using VM86 or using segmentation
register tricks (google for "unreal mode" if you're interested) to get
around that issue:
root@7ixe4:~# cat /proc/$(pidof X)/maps | grep /dev/mem
b4a24000-b5224000 rw-s ef000000 08:05 905647 /dev/mem
b5224000-b5264000 rw-s 000c0000 08:05 905647 /dev/mem
b5b14000-b5b24000 rw-s 000a0000 08:05 905647 /dev/mem
b5b24000-b7b24000 rw-s e4000000 08:05 905647 /dev/mem
b7b24000-b7b28000 rw-s efefc000 08:05 905647 /dev/mem
Not much into X, so I don't know. In any case, in the book it's a very
non-generic example indeed. Badly picked.
Rene.
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