In xorg 7.0, the open source driver works fine at color depth 16.  However, after I upgraded to 7.5, using the same Driver settings, the screen will go unreasonably dim.

As suggested in other forum (i.e. https://bugzilla.redhat.com/show_bug.cgi?id=554967), I have to disable the gamma set for color depth 16, as follows:

static void
radeon_crtc_gamma_set(xf86CrtcPtr crtc, uint16_t *red, uint16_t *green,
                      uint16_t *blue, int size)
{
    RADEONCrtcPrivatePtr radeon_crtc = crtc->driver_private;
    ScrnInfoPtr         pScrn = crtc->scrn;
    int i, j;

    if (0/*pScrn->depth == 16*/) {
        for (i = 0; i < 64; i++) {
            if (i <= 31) {
                for (j = 0; j < 8; j++) {
                    radeon_crtc->lut_r[i * 8 + j] = red[i] >> 6;
                    radeon_crtc->lut_b[i * 8 + j] = blue[i] >> 6;
                }
            }
...