I just traveled along the normal Chennai - Vellore - Hosur - Bangalore and it's overall drivable. The stretch from Hosur to Krishnagiri is full of diversions and road conditions aren't good either. From there on few diversions are in place, however the roads are okay with occasional bad patches. The road after Kancheepuram is actually getting worse and it's high time they re-laid it. If you can let us know which part of Chennai you'll be traveling into we can suggest the best route from there on.