PR/57280 CVS commit: src/sys/arch/mips/cavium/dev

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

The following reply was made to PR port-mips/57280; it has been noted by GNATS.

From: "Taylor R Campbell" <riastradh%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/57280 CVS commit: src/sys/arch/mips/cavium/dev
Date: Tue, 21 Mar 2023 22:07:29 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Tue Mar 21 22:07:29 UTC 2023
 
 Modified Files:
 	src/sys/arch/mips/cavium/dev: octeon_rnm.c
 
 Log Message:
 octrnm(4): Raise delay on startup.
 
 According to CN50XX-HRM-V0.99E and CN78XX-HM-0.99E:
 
    The entropy is provided by the jitter of 125 of 128 free-running
    oscillators XORed into a 128-bit LFSR.  The LFSR accumulates entropy
    over 81 cycles, after which it is fed into a SHA-1 engine.
    [...]
    The SHA-1 engine runs once every 81 cycles.
    [...]
    The hardware produces new 64-bit random number every 81 cycles.
 
 The last sentence means that we only need to wait 81 cycles _between_
 consecutive SHA-1 outputs (which isn't relevant anyway because we
 reconfigure it into raw mode later), but the first two quotes might
 mean that we need to wait 81+81 cycles for the _first_ output to be
 produced on boot when running the self-test.
 
 Now, in this case, the self-test is run with the LFSR unhooked, by
 clearing the RNM_CTL_STATUS[ENT_EN] bit, so that SHA-1 is computed
 from a known input -- this is really just paranoia to make sure that
 _some_ functions of the device (which is conjured out of thin air at
 a fixed virtual address, with no firmware bindings to guide us)
 behave as we expect.
 
 And it's not clear if it really does take 81+81 cycles for the first
 SHA-1 output to appear when the LFSR isn't feeding into it anyway.
 But experimentally, delay of 81+81 cycles seems to work whereas a
 delay of only 81 cycles crashes.
 
 PR kern/57280
 
 XXX pullup-10
 XXX pullup-9
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.15 -r1.16 src/sys/arch/mips/cavium/dev/octeon_rnm.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.