ALSA: hda - Force to link down at runtime suspend on ATI/AMD HDMI

Henning Kühn reported that the discrete AMD GPU on his hybrid graphics
laptop no longer runtime-suspends due to the recent commit
07f4f97d7b ("vga_switcheroo: Use device link for HDA controller").

The root cause is that the HDMI codec on AMD GPU doesn't support
CLKSTOP and EPSS, which are currently mandatory for powering down the
HD-audio link at runtime suspend.  Because the HD-audio link is still
up, HD-audio controller driver blocks the transition to D3.

For addressing the regression, this patch adds a new flag to indicate
the forced link-down, and sets it for AMD HDMI codecs appropriately
in the codec driver.

Fixes: 07f4f97d7b ("vga_switcheroo: Use device link for HDA controller")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106957
Reported-by: Lukas Wunner <lukas@wunner.de>
Reported-and-tested-by: Henning Kühn <prg@cooco.de>
Cc: <stable@vger.kernel.org> # v4.17+
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/hda/patch_hdmi.c

@@ -3741,6 +3741,11 @@ static int patch_atihdmi(struct hda_codec *codec)
 
 	spec->chmap.channels_max = max(spec->chmap.channels_max, 8u);
 
+	/* AMD GPUs have neither EPSS nor CLKSTOP bits, hence preventing
+	 * the link-down as is.  Tell the core to allow it.
+	 */
+	codec->link_down_at_suspend = 1;
+
 	return 0;
 }