mips: use fallback for random_get_entropy() instead of just c0 random

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>

arch/mips/include/asm/timex.h

@@ -76,25 +76,24 @@ static inline cycles_t get_cycles(void)
 	else
 		return 0;	/* no usable counter */
 }
+#define get_cycles get_cycles
 
 /*
  * Like get_cycles - but where c0_count is not available we desperately
  * use c0_random in an attempt to get at least a little bit of entropy.
- *
- * R6000 and R6000A neither have a count register nor a random register.
- * That leaves no entropy source in the CPU itself.
  */
 static inline unsigned long random_get_entropy(void)
 {
-	unsigned int prid = read_c0_prid();
-	unsigned int imp = prid & PRID_IMP_MASK;
+	unsigned int c0_random;
 
-	if (can_use_mips_counter(prid))
+	if (can_use_mips_counter(read_c0_prid()))
 		return read_c0_count();
-	else if (likely(imp != PRID_IMP_R6000 && imp != PRID_IMP_R6000A))
-		return read_c0_random();
+
+	if (cpu_has_3kex)
+		c0_random = (read_c0_random() >> 8) & 0x3f;
 	else
-		return 0;	/* no usable register */
+		c0_random = read_c0_random() & 0x3f;
+	return (random_get_entropy_fallback() << 6) | (0x3f - c0_random);
 }
 #define random_get_entropy random_get_entropy