drm/panfrost: Make sure GPU is powered on when reading GPU_LATEST_FLUSH_ID

Bifrost devices do support the flush reduction feature, so on first job
submit we were trying to read the register while still powered off.

If the GPU is powered off, the feature doesn't bring any benefit, so
don't try to read.
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200611085900.49740-1-tomeu.vizoso@collabora.com

drivers/gpu/drm/panfrost/panfrost_gpu.c

@@ -10,6 +10,7 @@
 #include <linux/io.h>
 #include <linux/iopoll.h>
 #include <linux/platform_device.h>
+#include <linux/pm_runtime.h>
 
 #include "panfrost_device.h"
 #include "panfrost_features.h"
@@ -368,7 +369,16 @@ void panfrost_gpu_fini(struct panfrost_device *pfdev)
 
 u32 panfrost_gpu_get_latest_flush_id(struct panfrost_device *pfdev)
 {
-	if (panfrost_has_hw_feature(pfdev, HW_FEATURE_FLUSH_REDUCTION))
-		return gpu_read(pfdev, GPU_LATEST_FLUSH_ID);
+	u32 flush_id;
+
+	if (panfrost_has_hw_feature(pfdev, HW_FEATURE_FLUSH_REDUCTION)) {
+		/* Flush reduction only makes sense when the GPU is kept powered on between jobs */
+		if (pm_runtime_get_if_in_use(pfdev->dev)) {
+			flush_id = gpu_read(pfdev, GPU_LATEST_FLUSH_ID);
+			pm_runtime_put(pfdev->dev);
+			return flush_id;
+		}
+	}
+
 	return 0;
 }