ARM counter registers and get_cyclecount()

[Mark Robert Vaughan Murray]

Hi folks

I asked a similar question to this a month or so ago, then got involved in other work, so apologies for the repetition!

The random(4) device benefits from having a decent hardware get_cyclecount() implementation. In i386 and arm, we have a stopgap version that uses binuptime(), which is slow and prone to quantisation error.

I've hacked up a minimalist hardware version for ARMv6/RPI (which is the only ARM I have access to, and I'm keen to use it for other things as well), and I'm looking for improvement advice and/or commit blessing.

Things it could conceivably do better:

1) The counter is 32 bits only. At clocks of hundreds-of-megahertz, this will overflow in some 10's of seconds to maybe a minute, so it would be nice (but to essential) to trap the overflow with an interrupt and increment an upper-half counter, making a 64-bit counter.

2) Set up the debug/profile/counter registers as some kind of device (suggestion from a couple of months back that I have no more information on)?

3) Support more ARM arches in a more general way? I have very little ARM hardware (only other is a BeagleBoard-xM) to test/develop this on.

Anyone interested in helping me or taking this over? :-)

M
-- 
Mark R V Murray
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