Where should I place the series termination resistor for AVSBus?
You can add a series resistor at the sending end of the clock in order to control reflections but, you do need to route the PCB track at an impedance that somewhat matches the series resistor.

Or, you can use a parallel resistor at the receiving end: -



Given that a driver may not be able to supply enough current for (say) a 50 ohm termination, a lot of folk opt for the series arrangement. There is a further option - see picture (d) below: -



It works by dealing with the high frequency content of the clock pulses (i.e. the harmonics) and terminating them correctly. This means that "C" can be quite low (maybe 100 pF or less) and DC conditions are unaffected.

At 25 MHz, you might be interested in ensuring that the 7th harmonic is properly catered for i.e. up to 175 MHz. At this frequency, the wavelength is about 1.7 metres and usually, engineers say that if track lengths are less than one tenth the wavelength, then terminators can be avoided. This means tracks can be upto 17 cm long however, electrical signals travel at about 0.6 the speed of light so, this lowers that track length to about 10 cm.