lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Tue, 6 Oct 2015 21:51:22 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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, 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 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/

Powered by blists - more mailing lists