Example code does not match comment

[jrahmeh]

In the sample code for synchronizing MTIME registers, the code sequence for computing T3 does not match the comment. The comment says: "T3 = T1 - ((T0 + T2) / 2)". The code actually computes: "T3 = T1 - (T0/2 + T2/2)". For odd values in T0 and T2, these expressions produce different results. For example if T0 and T2 are both 3, then (T0+T2)/2 is 3, while (T0/2 + T2/2) is 2.

[bgrayson-mips]

Even worse, the code does not actually provide the required guarantee, that after synchronization the two MTIME values are <1 count away from each other. Take this scenario:

• a0's MTIME is one tick ahead for both readings, and is an odd number, say "5"
• a1's MTIME is not, i.e. holds the value "4".
• so we need to decrement a0's MTIME by one if we want to truly synchronize
• But we first take both readings of "5", right-shift them by 1 to obtain "2", add them together to get "4", and subtract to get an offset of 0.
• End result: no change to a0's MTIME, so it remains 1 tick ahead of a1's MTIME.

I am not sure if "synchronized to within one MTIME tick period" means strictly-less-than-one tick, or just "less than two MTIME tick periods". If the former, the code should be fixed.