I use DSI with external 25MHz osc. In some design I wanted to use
HSI (is seems supported wrt. ref.manual) but the DSI PLL doesn't
lock at all.
I use IDF=3 to have INFIN=16/3>4MHz and NDIV=125, thus Fvco=666MHz.
ODF=2 (/4) hence DSI clk is 166MHz. All is in correct range.
With HSE, the only difference is IDF=4...
Maybe, HSI is not stable enough ?