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random: allow architectures to optionally define random_get_entropy()

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Allow architectures which have a disabled get_cycles() function to
provide a random_get_entropy() function which provides a fine-grained,
rapidly changing counter that can be used by the /dev/random driver.

For example, an architecture might have a rapidly changing register
used to control random TLB cache eviction, or DRAM refresh that
doesn't meet the requirements of get_cycles(), but which is good
enough for the needs of the random driver.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org
This commit is contained in:
Theodore Ts'o 2013-09-21 13:58:22 -04:00
commit 61875f30da
2 changed files with 18 additions and 4 deletions
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14
include/linux/timex.h
View file

@ -64,6 +64,20 @@
#include <asm/timex.h>
#ifndef random_get_entropy
/*
* The random_get_entropy() function is used by the /dev/random driver
* in order to extract entropy via the relative unpredictability of
* when an interrupt takes places versus a high speed, fine-grained
* timing source or cycle counter. Since it will be occurred on every
* single interrupt, it must have a very low cost/overhead.
*
* By default we use get_cycles() for this purpose, but individual
* architectures may override this in their asm/timex.h header file.
*/
#define random_get_entropy() get_cycles()
#endif
/*
* SHIFT_PLL is used as a dampening factor to define how much we
* adjust the frequency correction for a given offset in PLL mode.