On Sun, Jun 20, 2010 at 01:48:03AM +0800, jjinfo@nudt.edu.cn wrote:

By now, we can confirm that the digital signal has arrived the wm8974 DAI interface. As per wm8974 manual, it is required to set some 8974 registers(such as enabling DAC, MONOOUT,etc) so as to get analogue signal.

However, the codec driver (wm8974.c) does not provide any routine to set those necessary registers. My question is, where and how are these

What makes you say that the driver has no support for these things? The driver provides automatic power management and application control for audio routing and gains. This control should be fairly complete. For example, the DAC power is controlled by line 221:

SND_SOC_DAPM_DAC("DAC", "HiFi Playback", WM8974_POWER3, 0, 0),

This will be enabled when a playback is active and the DAC is connected to an output.

outpu-related registers of audio codec chip got correctly set up? Are they set up by aplay via calling alsa library APIs? Must we provide additional help functions to do such initialization?

aplay only deals with streaming PCM data to the CODEC. You should use an application such as alsamixer or amixer to configure the CODEC into the desired state at runtime.