target/linux/generic/patches-3.10/307-mips-Reimplement-get_cycles.patch

   1 From 9c9b415c50bc298ac61412dff856eae2f54889ee Mon Sep 17 00:00:00 2001
   2 From: Ralf Baechle <ralf@linux-mips.org>
   3 Date: Thu, 12 Sep 2013 13:47:32 +0200
   4 Subject: [PATCH] MIPS: Reimplement get_cycles().
   5
   6 This essentially reverts commit efb9ca08b5a2374b29938cdcab417ce4feb14b54
   7 (kernel.org) / 58020a106879a8b372068741c81f0015c9b0b96dbv [[MIPS] Change
   8 get_cycles to always return 0.]
   9
  10 Most users of get_cycles() invoke it as a timing interface.  That's why
  11 in modern kernels it was never very much missed for.  /dev/random however
  12 uses get_cycles() in the how the jitter in the interrupt timing contains
  13 some useful entropy.
  14
  15 Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
  16 ---
  17  arch/mips/include/asm/timex.h |   33 ++++++++++++++++++++++++++++++++-
  18  1 file changed, 32 insertions(+), 1 deletion(-)
  19
  20 --- a/arch/mips/include/asm/timex.h
  21 +++ b/arch/mips/include/asm/timex.h
  22 @@ -10,6 +10,7 @@
  23
  24  #ifdef __KERNEL__
  25
  26 +#include <asm/cpu-features.h>
  27  #include <asm/mipsregs.h>
  28
  29  /*
  30 @@ -33,9 +34,38 @@
  31
  32  typedef unsigned int cycles_t;
  33
  34 +/*
  35 + * On R4000/R4400 before version 5.0 an erratum exists such that if the
  36 + * cycle counter is read in the exact moment that it is matching the
  37 + * compare register, no interrupt will be generated.
  38 + *
  39 + * There is a suggested workaround and also the erratum can't strike if
  40 + * the compare interrupt isn't being used as the clock source device.
  41 + * However for now the implementaton of this function doesn't get these
  42 + * fine details right.
  43 + */
  44  static inline cycles_t get_cycles(void)
  45  {
  46 -       return 0;
  47 +       switch (cpu_data[0].cputype) {
  48 +       case CPU_R4400PC:
  49 +       case CPU_R4400SC:
  50 +       case CPU_R4400MC:
  51 +               if ((read_c0_prid() & 0xff) >= 0x0050)
  52 +                       return read_c0_count();
  53 +               break;
  54 +
  55 +        case CPU_R4000PC:
  56 +        case CPU_R4000SC:
  57 +        case CPU_R4000MC:
  58 +               break;
  59 +
  60 +       default:
  61 +               if (cpu_has_counter)
  62 +                       return read_c0_count();
  63 +               break;
  64 +       }
  65 +
  66 +       return 0;       /* no usable counter */
  67  }
  68
  69  #endif /* __KERNEL__ */