From Chengdu over Aba, Garze to Derge. Especially the last part after Garze is fantastic as you cycle along the Chola Mountains and finally over them. There are many other option for the start. The one that I have chosen is certainly not the shortest... Very nice is the route over the Balang Shan pass and Danba, see here.
As the whole route is outside of the TAR it is mostly possible without problems.
Mostly paved, just over the passes sometimes a couple of km are dirt road. On the 317 the asphalt is often very bumpy because of the heavy trucks. On the smaller roads that I took the pavement was much better!
There are quite a lot of towns along the way with accommodation and good shops. Camping is always possible.