drm/radeon/dpm: make sure dc performance level limits are valid (BTC-SI) (v2)

Check to make sure the dc limits are valid before using them.
Some systems may not have a dc limits table.  In that case just
use the ac limits.  This fixes hangs on systems when the power
state is changed when on battery (dc) due to invalid performance
state parameters.

Should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=68708

v2: fix up limits in dpm_init()

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
diff --git a/drivers/gpu/drm/radeon/btc_dpm.c b/drivers/gpu/drm/radeon/btc_dpm.c
index 9953e1f..084e694 100644
--- a/drivers/gpu/drm/radeon/btc_dpm.c
+++ b/drivers/gpu/drm/radeon/btc_dpm.c
@@ -2699,6 +2699,12 @@
 	else
 		rdev->pm.dpm.dyn_state.sclk_mclk_delta = 10000;
 
+	/* make sure dc limits are valid */
+	if ((rdev->pm.dpm.dyn_state.max_clock_voltage_on_dc.sclk == 0) ||
+	    (rdev->pm.dpm.dyn_state.max_clock_voltage_on_dc.mclk == 0))
+		rdev->pm.dpm.dyn_state.max_clock_voltage_on_dc =
+			rdev->pm.dpm.dyn_state.max_clock_voltage_on_ac;
+
 	return 0;
 }