Re: TSC instead of ACPI: powerd doesn't work anymore (to be expected?)

M. Warner Losh schrieb:
> Actually, ioport reads can be faster than the 1us that's widely
> quoted.  ioport reads can be as fast as ~125ns (2 cycles at 16MHz).
> However, experience has shown that they are rarely this fast.  I've
> seen 8MB/s pio over the pci bus on some custom hardware we have, which
> 2Mreads/sec which is about 500ns per read.  I think that the pci
> hardware that I was reading had a few extra wait states...


According to "Software Optimization Guide for AMD64 Processors":
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF
the "in" machine instructions have a latency of ~180 clock cycles.
But this will be only the lower theoretical limit.


Daniel