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For situations in which we don't have a c0 counter register available,
we've been falling back to reading the c0 "random" register, which is
usually bounded by the amount of TLB entries and changes every other
cycle or so. This means it wraps extremely often. We can do better by
combining this fast-changing counter with a potentially slower-changing
counter from random_get_entropy_fallback() in the more significant bits.
This commit combines the two, taking into account that the changing bits
are in a different bit position depending on the CPU model. In addition,
we previously were falling back to 0 for ancient CPUs that Linux does
not support anyway; remove that dead path entirely.
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Tested-by: Maciej W. Rozycki <macro@orcam.me.uk>
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
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Fix the discrepancy between the two places we check for the CP0 counter
erratum in along with the incorrect comparison of the R4400 revision
number against 0x30 which matches none and consistently consider all
R4000 and R4400 processors affected, as documented in processor errata
publications[1][2][3], following the mapping between CP0 PRId register
values and processor models:
PRId | Processor Model
---------+--------------------
00000422 | R4000 Revision 2.2
00000430 | R4000 Revision 3.0
00000440 | R4400 Revision 1.0
00000450 | R4400 Revision 2.0
00000460 | R4400 Revision 3.0
No other revision of either processor has ever been spotted.
Contrary to what has been stated in commit ce202cbb9e0b ("[MIPS] Assume
R4000/R4400 newer than 3.0 don't have the mfc0 count bug") marking the
CP0 counter as buggy does not preclude it from being used as either a
clock event or a clock source device. It just cannot be used as both at
a time, because in that case clock event interrupts will be occasionally
lost, and the use as a clock event device takes precedence.
Compare against 0x4ff in `can_use_mips_counter' so that a single machine
instruction is produced.
References:
[1] "MIPS R4000PC/SC Errata, Processor Revision 2.2 and 3.0", MIPS
Technologies Inc., May 10, 1994, Erratum 53, p.13
[2] "MIPS R4400PC/SC Errata, Processor Revision 1.0", MIPS Technologies
Inc., February 9, 1994, Erratum 21, p.4
[3] "MIPS R4400PC/SC Errata, Processor Revision 2.0 & 3.0", MIPS
Technologies Inc., January 24, 1995, Erratum 14, p.3
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Fixes: ce202cbb9e0b ("[MIPS] Assume R4000/R4400 newer than 3.0 don't have the mfc0 count bug")
Cc: stable@vger.kernel.org # v2.6.24+
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
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Update to commit 9c9b415c50bc298ac61412dff856eae2f54889ee [MIPS:
Reimplement get_cycles().]
On systems were for whatever reasons we can't use the cycle counter, fall
back to the c0_random register as an entropy source. It has however a
very small range that makes it suitable for random_get_entropy only and
not get_cycles.
This optimised version compiles to 8 instructions in the fast path even in
the worst case of all the conditions to check being variable (including a
MFC0 move delay slot that is only required for very old processors):
828: 8cf90000 lw t9,0(a3)
828: R_MIPS_LO16 jiffies
82c: 40057800 mfc0 a1,c0_prid
830: 3c0200ff lui v0,0xff
834: 00a21024 and v0,a1,v0
838: 1040007d beqz v0,a30 <add_interrupt_randomness+0x22c>
83c: 3c030000 lui v1,0x0
83c: R_MIPS_HI16 cpu_data
840: 40024800 mfc0 v0,c0_count
844: 00000000 nop
848: 00409021 move s2,v0
84c: 8ce20000 lw v0,0(a3)
84c: R_MIPS_LO16 jiffies
On most targets the sequence will be shorter and on some it will reduce to
a single `MFC0 <reg>,c0_count', as all MIPS architecture (i.e. non-legacy
MIPS) processors require the CP0 Count register to be present.
The only known exception that reports MIPS architecture compliance, but
contrary to that lacks CP0 Count is the Ingenic JZ4740 thingy. For broken
platforms like that this code requires cpu_has_counter to be hardcoded to
0 (i.e. no variable setting is permitted) so as not to penalise all the
other good platforms out there.
The asm barrier is required so that the compiler does not pull any
potentially costly (cold cache!) `cpu_data' variable access into the fast
path.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: John Crispin <blogic@openwrt.org>
Cc: Andrew McGregor <andrewmcgr@gmail.com>
Cc: Dave Taht <dave.taht@bufferbloat.net>
Cc: Felix Fietkau <nbd@nbd.name>
Cc: Simon Kelley <simon@thekelleys.org.uk>
Cc: Jim Gettys <jg@freedesktop.org>
Cc: David Daney <ddaney@caviumnetworks.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6702/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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This essentially reverts commit efb9ca08b5a2374b29938cdcab417ce4feb14b54
(kernel.org) / 58020a106879a8b372068741c81f0015c9b0b96dbv [[MIPS] Change
get_cycles to always return 0.]
Most users of get_cycles() invoke it as a timing interface. That's why
in modern kernels it was never very much missed for. /dev/random however
uses get_cycles() in the how the jitter in the interrupt timing contains
some useful entropy.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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Jason A. Donenfeld
Maciej W. Rozycki
Maciej W. Rozycki
Ralf Baechle