• Maciej W. Rozycki's avatar
    MIPS: Implement random_get_entropy with CP0 Random · 06947aaa
    Maciej W. Rozycki authored
    Update to commit 9c9b415c [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: default avatarRalf Baechle <ralf@linux-mips.org>
    Signed-off-by: default avatarMaciej 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: default avatarRalf Baechle <ralf@linux-mips.org>
    06947aaa
cpu-probe.c 29.5 KB