Messages in this thread Patch in this message |  | | | Subject | Re: [PATCH 08/57] microblaze_v7: Interrupt handling, timer support, selfmod code | | From | John Stultz <> | | Date | Thu, 19 Mar 2009 19:24:21 -0700 |
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On Thu, 2009-03-19 at 22:47 +0100, Thomas Gleixner wrote: > On Thu, 19 Mar 2009, Michal Simek wrote: > > And the second question is about shift and rating values. > > I wrote one message in past http://lkml.org/lkml/2009/1/11/291 > > Here is the important of part of that message. > > > > ... > > > > And the second part is about shift and rating values. Rating is > > describe(linux/clocksource.h) and seems to me that should be > > corresponded with CONFIG_HZ value,right?
Not sure where the idea of correspondence w/ CONFIG_HZ came from. The rating value just provides a relative ordering of preferences between possible clocksources. Since different hardware may have a number of different clocksources available, we just need to have a method of selecting a preferred clocksource, and the rating value is used for that. The guide in linux/clocksource.h is just a guide. Most arches, which only have one or two clocksource options probably won't need much care, and a rating of 200 or 300 will probably suffice. Or if there really isn't any option about it and there is only one which is a must-use clocksource, 400.
> > And I found any explanation of shift value -> max value for equation > > (2-5) * freq << shift / NSEC_PER_SEC should be for my case still 32bit > > number, where (2-5s) are because of NTP > > @John, can you explain the shift vlaue please ?
The shift value is a bit more difficult to explain. The algorithm you describe above is used by sparc to generate shift, and I think it will work, but may not be optimal.
This question comes up over and over, so I figured I should sit down and really solve it.
Basically the constraint is you want to calculate a mult value using the highest shift possible. However we have to be careful not to overflow 64bits when we multiply ~5second worth of cycles times the mult value.
So I finally put this down into code and here it is. No promises that it is 100% right, but from my simple test examples it worked ok.
Let me know if it helps.
thanks -john
Add helper functions to calculate ideal clocksource shift values given its frequency (either hz or khz). Lightly tested, use with care. Also fixes a few old-old-old timesource (well, timsource) references my find and replace attempts failed on.
Signed-off-by: John Stultz <johnstul@us.ibm.com>
diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h index f88d32f..9ef062d 100644 --- a/include/linux/clocksource.h +++ b/include/linux/clocksource.h @@ -116,7 +116,7 @@ extern struct clocksource *clock; /* current clocksource */ * @khz: Clocksource frequency in KHz * @shift_constant: Clocksource shift factor * - * Helper functions that converts a khz counter frequency to a timsource + * Helper functions that converts a khz counter frequency to a clocksource * multiplier, given the clocksource shift value */ static inline u32 clocksource_khz2mult(u32 khz, u32 shift_constant) @@ -142,7 +142,7 @@ static inline u32 clocksource_khz2mult(u32 khz, u32 shift_constant) * @shift_constant: Clocksource shift factor * * Helper functions that converts a hz counter - * frequency to a timsource multiplier, given the + * frequency to a clocksource multiplier, given the * clocksource shift value */ static inline u32 clocksource_hz2mult(u32 hz, u32 shift_constant) @@ -162,6 +162,10 @@ static inline u32 clocksource_hz2mult(u32 hz, u32 shift_constant) return (u32)tmp; } +extern u32 clocksource_khz2shift(u32 freq_khz); +extern u32 clocksource_hz2shift(u32 freq_hz); + + /** * clocksource_read: - Access the clocksource's current cycle value * @cs: pointer to clocksource being read diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index ca89e15..0a2538d 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -212,6 +212,83 @@ static void clocksource_check_watchdog(struct clocksource *cs) static inline void clocksource_resume_watchdog(void) { } #endif + +/* Maximum seconds worth of cycles we consider valid */ +#define MAX_SEC_OF_CYCLES 5ULL + +/** + * clocksource_khz2shift - calculates shift from khz + * @freq_khz: Clocksource frequency in KHz + * + * Helper functions that converts a khz counter + * frequency to a clocksource shift. + */ +u32 clocksource_khz2shift(u32 freq_khz) +{ + s32 shift = 32; + u32 mult; + + while (1) { + s64 tmp; + u64 cycles; + mult = clocksource_khz2mult(freq_khz, shift); + + cycles = (u64)freq_khz*1000*MAX_SEC_OF_CYCLES; + tmp = (cycles * mult) >> shift; + + tmp = (MAX_SEC_OF_CYCLES*NSEC_PER_SEC) - tmp; + if (tmp < 0) + tmp = -tmp; + + /* if we are witin one mult unit, we're good */ + if (tmp < cycles>>shift) + break; + + shift--; + if (shift < 1) + BUG(); + } + return shift; +} + +/** + * clocksource_hz2shift - calculates shift from hz + * @freq_hz: Clocksource frequency in Hz + * + * Helper functions that converts a hz counter + * frequency to a clocksource shift. + */ + +u32 clocksource_hz2shift(u32 freq_hz) +{ + s32 shift = 32; + u32 mult; + + while (1) { + s64 tmp; + u64 cycles; + mult = clocksource_hz2mult(freq_hz, shift); + + cycles = (u64)freq_hz*MAX_SEC_OF_CYCLES; + tmp = (cycles * mult) >> shift; + + tmp = MAX_SEC_OF_CYCLES*NSEC_PER_SEC - tmp; + if (tmp < 0) + tmp = -tmp; + + /* if we are witin one mult unit, we're good */ + if (tmp < cycles>>shift) + break; + + shift--; + if (shift < 1) + BUG(); + } + return shift; +} + + + /** * clocksource_resume - resume the clocksource(s) */
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