Messages in this thread |  | | Date | Tue, 20 Aug 2002 09:03:46 -0500 | From | Oliver Xymoron <> | Subject | Re: [RFC] Improved add_timer_randomness for __CRIS__ (instead of rdtsc()) |
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On Tue, Aug 20, 2002 at 11:31:10AM +0200, johan.adolfsson@axis.com wrote: > The cris architecture don't have any tsc, but it has a couple of > timer registers that can be used to get better than jiffie resolution. > > I set the time to a 40 us resolution counter with a slight > "jump" since lower 8 bit only counts from 0 to 249, > the patch does not take wrapping of the register into account either > to save some cycles, is that a problem or a good thing?
That should be fine. More important is actually scaling the entropy count based on the timing granularity of the source. Keyboards and mice tend to have a granularity of about 1khz so timestamps better than milliseconds 'invent' entropy in the current code.
> The num is xor:d with the value from 2 timer registers, > which in turn contains different fields breifly described below. > > Does the patch below look sane?
Looks fine, but I think we want to come up with a cleaner scheme of having per-arch high-res timestamps. I'd hate to have that grow to several pages of ifdefs and not have it available anywhere else.
> +++ random.c 20 Aug 2002 09:10:04 -0000 > @@ -746,6 +746,15 @@ static void add_timer_randomness(struct > __u32 high; > rdtsc(time, high); > num ^= high; > +#elif defined (__CRIS__) > + /* R_TIMER0_DATA, 8 bit, 40 us resolution, counting down from 250 */ > + /* R_TIMER_DATA, 4*8 bit, timer1, timer0, 38.4kHz, 7.3728MHz */ > + /* R_PRESCALE_STATUS, upper 16 bit: 320ns resolution, > + lower 16 bit: 40 ns resolution, ~10 bits used, > + counting down from 1000 */ > + time = jiffies << 8; > + time |= (TIMER0_DIV - *R_TIMER0_DATA); > + num ^= *R_PRESCALE_STATUS ^ *R_TIMER_DATA; > #else > time = jiffies; > #endif
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