Re: Linux 2.2.18 release notes

[Peter Samuelson <peter@cadcamlab.org>]

  [AC]
> >           ... added basic support for the Pentium IV.
[Android]
> How is the Pentium IV more advanced than the Pentium III, other than
> speed?  Why would LInux care about a 1500 MHz clock or 400 MHz bus
> speed?  Just treat the PIV as a faster PIII.

It all sounds so simple, right?  Several small things to worry about:

- CPU identification and categorization.  The P4 reports itself as CPU
  family 15 rather than family 6 like PPro, PII and PIII.  Thus, Linux
  code that tests for certain features by saying if(cpufamily==6)
  didn't notice that the P4 would work.  Also there was at least one
  format string like sprintf(buf, "i%d86", cpufamily) which is supposed
  to print "i686" but on the P4 would print "i1586"....

- metrics -- L1 cacheline size is the important one: you align array
  elements to this size when you want a per-cpu array, so that multiple
  CPUs do not share a cacheline for accessing their "own" structure.
  Proper alignment avoids "cacheline ping-pong", as it's called,
  whenever two CPUs need to access "their" element of the same array at
  the same time.

- as to the MHz -- there was a wraparound bug if your CPU is faster
  than 2 GHz (highest signed 32-bit int), which isn't a problem today
  but will be tomorrow.

- Tigran's microcode driver -- some small changes were made so that it
  could be used for P4's.

- maybe they'll need to patch lm_sensors to accommodate the increased
  temperature range since the P4 runs so hot. (: (:

Peter
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