Created attachment 92262 [details]
Picture of a wrong timing(?)

With

commit 7424173698775ad90a039d8e00cbee333de536ec
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Tue Jan 14 10:45:51 2014 -0500

    drm/radeon/dp: sleep after powering up the display

    According to the DP 1.1 spec, the sink must power
    up within 1ms.  Noticed while reviewing Thierry's
    drm/dp patches.

    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --git a/drivers/gpu/drm/radeon/atombios_dp.c
b/drivers/gpu/drm/radeon/atombios_dp.c
index fb3ae07..ba7157a 100644
--- a/drivers/gpu/drm/radeon/atombios_dp.c
+++ b/drivers/gpu/drm/radeon/atombios_dp.c
@@ -671,9 +671,11 @@ static int radeon_dp_link_train_init(struct
radeon_dp_link_train_info *dp_info)
        u8 tmp;

        /* power up the sink */
-       if (dp_info->dpcd[0] >= 0x11)
+       if (dp_info->dpcd[0] >= 0x11) {
                radeon_write_dpcd_reg(dp_info->radeon_connector,
                                      DP_SET_POWER, DP_SET_POWER_D0);
+               usleep_range(1000, 2000);
+       }

        /* possibly enable downspread on the sink */
        if (dp_info->dpcd[3] & 0x1)

I got the attached image after two xrandr off/on cycles. After the next off/on
cycle the display worked. I did not notice such a behavior in my other tests.