drm/panthor: Use existing OPP table if present

On SoCs where the GPU's power-domain is in charge of setting performance
levels, the OPP table of the GPU node will have already been populated
during said power-domain's attach_dev operation.

To avoid initialising an OPP table twice, only set the OPP regulator and
the OPPs from DT if there's no OPP table present.

Reviewed-by: Chia-I Wu <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: Steven Price <[e-mail]>
Reviewed-by: Liviu Dudau <[e-mail]>
Signed-off-by: Nicolas Frattaroli <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Liviu Dudau <[e-mail]>

drivers/gpu/drm/panthor/panthor_devfreq.c

···
 	struct thermal_cooling_device *cooling;
 	struct device *dev = ptdev->base.dev;
 	struct panthor_devfreq *pdevfreq;
+	struct opp_table *table;
 	struct dev_pm_opp *opp;
 	unsigned long cur_freq;
 	unsigned long freq = ULONG_MAX;
···
 
 	ptdev->devfreq = pdevfreq;
 
-	ret = devm_pm_opp_set_regulators(dev, reg_names);
-	if (ret && ret != -ENODEV) {
-		if (ret != -EPROBE_DEFER)
-			DRM_DEV_ERROR(dev, "Couldn't set OPP regulators\n");
-		return ret;
-	}
+	/*
+	 * The power domain associated with the GPU may have already added an
+	 * OPP table, complete with OPPs, as part of the platform bus
+	 * initialization. If this is the case, the power domain is in charge of
+	 * also controlling the performance, with a set_performance callback.
+	 * Only add a new OPP table from DT if there isn't such a table present
+	 * already.
+	 */
+	table = dev_pm_opp_get_opp_table(dev);
+	if (IS_ERR_OR_NULL(table)) {
+		ret = devm_pm_opp_set_regulators(dev, reg_names);
+		if (ret && ret != -ENODEV) {
+			if (ret != -EPROBE_DEFER)
+				DRM_DEV_ERROR(dev, "Couldn't set OPP regulators\n");
+			return ret;
+		}
 
-	ret = devm_pm_opp_of_add_table(dev);
-	if (ret)
-		return ret;
+		ret = devm_pm_opp_of_add_table(dev);
+		if (ret)
+			return ret;
+	} else {
+		dev_pm_opp_put_opp_table(table);
+	}
 
 	spin_lock_init(&pdevfreq->lock);