linux-kernel - Re: [PATCH] /dev/random: Insufficient of entropy on many architectures

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Date:	Tue, 10 Sep 2013 12:44:56 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Stephan Mueller <smueller@...onox.de>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	LKML <linux-kernel@...r.kernel.org>, dave.taht@...ferbloat.net
Subject: Re: [PATCH] /dev/random: Insufficient of entropy on many architectures

On Tue, Sep 10, 2013 at 12:38 PM, John Stultz <john.stultz@...aro.org> wrote:
> On Tue, Sep 10, 2013 at 4:31 AM, Stephan Mueller <smueller@...onox.de> wrote:
>> Hi,
>>
>> /dev/random uses the get_cycles() function to obtain entropy in addition to jiffies and the event value of hardware events.
>>
>> Typically the high-resolution timer of get_cycles delivers the majority of entropy, because the event value is quite deterministic and jiffies are very coarse.
>>
> [snip]
>> The following patch uses the clocksource clock for a time value in case get_cycles returns 0. As clocksource may not be available during boot time, a flag is introduced which allows random.c to check the availability of clocksource.

Also, note that at the end of timekeeping_init(), we may not have been
able to access the RTC (as some RTCs require interrupts to access), so
on those systems getnstimeofday() may return something close to 0
until the RTC subsystem initializes and sets the current wall time.

thanks
-john
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