gpu: ipu-v3: prg: wait for double buffers to be filled on channel startup

Wait for both double buffer to be filled when first starting a channel.
This makes channel startup a lot more reliable, probably because it allows
the internal state machine to settle before the requests from the IPU are
coming in.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
[p.zabel@pengutronix.de: rebased before switch to runtime PM]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

drivers/gpu/ipu-v3/ipu-prg.c

@@ -14,6 +14,7 @@
 #include <drm/drm_fourcc.h>
 #include <linux/clk.h>
 #include <linux/err.h>
+#include <linux/iopoll.h>
 #include <linux/mfd/syscon.h>
 #include <linux/mfd/syscon/imx6q-iomuxc-gpr.h>
 #include <linux/module.h>
@@ -329,6 +330,12 @@ int ipu_prg_channel_configure(struct ipuv3_channel *ipu_chan,
 	val = IPU_PRG_REG_UPDATE_REG_UPDATE;
 	writel(val, prg->regs + IPU_PRG_REG_UPDATE);
 
+	/* wait for both double buffers to be filled */
+	readl_poll_timeout(prg->regs + IPU_PRG_STATUS, val,
+			   (val & IPU_PRG_STATUS_BUFFER0_READY(prg_chan)) &&
+			   (val & IPU_PRG_STATUS_BUFFER1_READY(prg_chan)),
+			   5, 1000);
+
 	clk_disable_unprepare(prg->clk_ipg);
 
 	chan->enabled = true;