Messages in this thread | | | Date | Tue, 6 Oct 2015 20:37:57 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH 3/3] hwrng: st: Use real-world device timings for timeout |
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On Tue, Oct 06, 2015 at 03:44:00PM +0100, Lee Jones wrote: > Samples are documented to be available every 0.667us, so in theory > the 8 sample deep FIFO should take 5.336us to fill. However, during > thorough testing, it became apparent that filling the FIFO actually > takes closer to 12us.
Is that measured?
> +/* > + * Samples are documented to be available every 0.667us, so in theory > + * the 8 sample deep FIFO should take 5.336us to fill. However, during > + * thorough testing, it became apparent that filling the FIFO actually > + * takes closer to 12us. > + */ > +#define ST_RNG_FILL_FIFO_TIMEOUT 12
I hope you're not using such a precise figure with udelay(). udelay() is not guaranteed to give exactly (or even at least) the delay you request. It's defined to give an approximate delay.
Many people have a problem understanding that, so I won't explain why it is that way, just accept that it is and move on... it's not going to magically get "fixed" because someone has just learnt about this. :)
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