Commit 9c9b415c authored by Ralf Baechle's avatar Ralf Baechle

MIPS: Reimplement get_cycles().

This essentially reverts commit efb9ca08
(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: default avatarRalf Baechle <ralf@linux-mips.org>
parent 69f24d17
......@@ -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__ */
......
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