On Tue, 06 Oct 2015, Russell King - ARM Linux wrote: > 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?
I measured it using ktime. Hopefully that was adequate. > > +/* > > + * 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. :) Thanks for the info. I did do testing, again using ktime, to make sure and on our platform (is it platform specific?) I measured udelay(1) to be ~1100ns. After moving to a 12us timeout and reading many MBs of randomness I am yet to receive any more timeouts. -- Lee Jones Linaro STMicroelectronics Landing Team Lead Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
