linux-kernel - Re: [PATCH 3/3] hwrng: st: Use real-world device timings for timeout

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Message-ID: <20151006193757.GY21513@n2100.arm.linux.org.uk>
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. :)

-- 
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according to speedtest.net.
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