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eea5c0b166
get_cycles() is used in some places as one part of the seed for the kernel PRNG. This backports the following commit from mainline linux kernel: 9c9b415c50bc298ac61412dff856eae2f54889ee Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de> SVN-Revision: 38834
70 lines
1.9 KiB
Diff
70 lines
1.9 KiB
Diff
From 9c9b415c50bc298ac61412dff856eae2f54889ee Mon Sep 17 00:00:00 2001
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From: Ralf Baechle <ralf@linux-mips.org>
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Date: Thu, 12 Sep 2013 13:47:32 +0200
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Subject: [PATCH] MIPS: Reimplement get_cycles().
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This essentially reverts commit efb9ca08b5a2374b29938cdcab417ce4feb14b54
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(kernel.org) / 58020a106879a8b372068741c81f0015c9b0b96dbv [[MIPS] Change
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get_cycles to always return 0.]
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Most users of get_cycles() invoke it as a timing interface. That's why
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in modern kernels it was never very much missed for. /dev/random however
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uses get_cycles() in the how the jitter in the interrupt timing contains
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some useful entropy.
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Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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---
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arch/mips/include/asm/timex.h | 33 ++++++++++++++++++++++++++++++++-
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1 file changed, 32 insertions(+), 1 deletion(-)
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--- a/arch/mips/include/asm/timex.h
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+++ b/arch/mips/include/asm/timex.h
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@@ -10,6 +10,7 @@
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#ifdef __KERNEL__
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+#include <asm/cpu-features.h>
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#include <asm/mipsregs.h>
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/*
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@@ -33,9 +34,38 @@
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typedef unsigned int cycles_t;
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+/*
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+ * On R4000/R4400 before version 5.0 an erratum exists such that if the
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+ * cycle counter is read in the exact moment that it is matching the
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+ * compare register, no interrupt will be generated.
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+ *
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+ * There is a suggested workaround and also the erratum can't strike if
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+ * the compare interrupt isn't being used as the clock source device.
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+ * However for now the implementaton of this function doesn't get these
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+ * fine details right.
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+ */
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static inline cycles_t get_cycles(void)
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{
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- return 0;
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+ switch (cpu_data[0].cputype) {
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+ case CPU_R4400PC:
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+ case CPU_R4400SC:
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+ case CPU_R4400MC:
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+ if ((read_c0_prid() & 0xff) >= 0x0050)
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+ return read_c0_count();
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+ break;
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+
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+ case CPU_R4000PC:
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+ case CPU_R4000SC:
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+ case CPU_R4000MC:
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+ break;
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+
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+ default:
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+ if (cpu_has_counter)
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+ return read_c0_count();
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+ break;
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+ }
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+
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+ return 0; /* no usable counter */
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}
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#endif /* __KERNEL__ */
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