  We study the spin dynamics in a high-mobility two dimensional electron gas
(2DEG) with generic spin-orbit interactions (SOIs). We derive a set of spin
dynamic equations which capture the purely exponential to the damped
oscillatory spin evolution modes observed in different regimes of SOI strength.
Hence we provide a full treatment of the D'yakonov-Perel's mechanism by using
the microscopic linear response theory from the weak to the strong SOI limit.
We show that the damped oscillatory modes appear when the electron scattering
time is larger than half of the spin precession time due to the SOI, in
agreement with recent observations. We propose a new way to measure the
scattering time and the relative strength of Rashba and linear Dresselhaus SOIs
based on these modes and optical grating experiments. We discuss the physical
interpretation of each of these modes in the context of Rabi oscillation.
