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Date: Tue, 6 Oct 2015 20:37:57 +0100 From: Russell King - ARM Linux <linux@....linux.org.uk> To: Lee Jones <lee.jones@...aro.org> Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, peter@...sgaard.com, kernel@...inux.com, daniel.thompson@...aro.org, pankaj.dev@...com, festevam@...il.com, herbert@...dor.apana.org.au Subject: Re: [PATCH 3/3] hwrng: st: Use real-world device timings for timeout 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. :) -- FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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