From: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
This essentially reverts commit efb9ca08b5a2374b29938cdcab417ce4feb14b54
(kernel.org) / 58020a106879a8b372068741c81f0015c9b0b96dbv [[MIPS] Change
get_cycles to always return 0.]
Most users of get_cycles() invoke it as a timing interface. That's why
in modern kernels it was never very much missed for. /dev/random however
uses get_cycles() in the how the jitter in the interrupt timing contains
some useful entropy.
Signed-off-by: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
---
arch/mips/include/asm/timex.h | 33 ++++++++++++++++++++++++++++++++-
1 file changed, 32 insertions(+), 1 deletion(-)
diff --git a/arch/mips/include/asm/timex.h b/arch/mips/include/asm/timex.h
index 6529704..c542475 100644
--- a/arch/mips/include/asm/timex.h
+++ b/arch/mips/include/asm/timex.h
@@ -10,7 +10,9 @@
#ifdef __KERNEL__
+#include <asm/cpu-features.h>
#include <asm/mipsregs.h>
+#include <asm/cpu-type.h>
/*
* This is the clock rate of the i8253 PIT. A MIPS system may not have
@@ -33,9 +35,38 @@
typedef unsigned int cycles_t;
+/*
+ * On R4000/R4400 before version 5.0 an erratum exists such that if the
+ * cycle counter is read in the exact moment that it is matching the
+ * compare register, no interrupt will be generated.
+ *
+ * There is a suggested workaround and also the erratum can't strike if
+ * the compare interrupt isn't being used as the clock source device.
+ * However for now the implementaton of this function doesn't get these
+ * fine details right.
+ */
static inline cycles_t get_cycles(void)
{
- return 0;
+ switch (boot_cpu_type()) {
+ case CPU_R4400PC:
+ case CPU_R4400SC:
+ case CPU_R4400MC:
+ if ((read_c0_prid() & 0xff) >= 0x0050)
+ return read_c0_count();
+ break;
+
+ case CPU_R4000PC:
+ case CPU_R4000SC:
+ case CPU_R4000MC:
+ break;
+
+ default:
+ if (cpu_has_counter)
+ return read_c0_count();
+ break;
+ }
+
+ return 0; /* no usable counter */
}
#endif /* __KERNEL__ */
--
1.7.10.4
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