Date: Sun, 20 Oct 2013 17:13:31 +0100 From: Mark Robert Vaughan Murray <markm@FreeBSD.org> To: freebsd-arm <freebsd-arm@freebsd.org> Subject: ARM counter registers and get_cyclecount() Message-ID: <0D53AF4E-9EC4-42E1-8D9E-1ECB87A9CCE6@FreeBSD.org>
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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
--=20
Mark R V Murray
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Index: sys/arm/arm/locore.S
===================================================================
--- sys/arm/arm/locore.S (revision 256775)
+++ sys/arm/arm/locore.S (working copy)
@@ -175,6 +175,18 @@
mcr p15, 0, r0, c13, c0, 1 /* Set ASID to 0 */
#endif
+#if defined(CPU_CORTEXA)
+ /* TODO: Better to do access to these counters as some kind of device?
+ * This is 32 bits. Use interrupt to make 64 bits? Random number generator
+ * doesn't care, but others might. See get_cyclecount().
+ */
+
+ mov r7, #0x8000000F /* Set INTENS to 0 to block interrupts */
+ mcr p15, 0, r7, c9, c14, 2
+ mov r7, #1 /* Set PMNC[0] to 1 to enable CCNT */
+ mcr p15, 0, r7, c9, c12, 0
+#endif
+
/* Set the Domain Access register. Very important! */
mov r0, #((DOMAIN_CLIENT << (PMAP_DOMAIN_KERNEL*2)) | DOMAIN_CLIENT)
mcr p15, 0, r0, c3, c0, 0
Index: sys/arm/include/cpu.h
===================================================================
--- sys/arm/include/cpu.h (revision 256775)
+++ sys/arm/include/cpu.h (working copy)
@@ -13,11 +13,21 @@
static __inline uint64_t
get_cyclecount(void)
{
+#if defined(CPU_CORTEXA)
+ uint32_t ccnt;
+
+ /*
+ * Read CCNT. Curses! Its only 32 bits.
+ * Fix this catching overflow with interrupt in locore.S?
+ */
+ __asm __volatile("mrc p15, 0, %0, c9, c13, 0": "=r" (ccnt));
+ return ((uint64_t)ccnt);
+#else
struct bintime bt;
binuptime(&bt);
return ((uint64_t)bt.sec << 56 | bt.frac >> 8);
-
+#endif
}
#endif
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