It's a bit impractical to enable hardware's clock at the time of DMA
channel's allocation because most of DMA client drivers allocate DMA
channel at the time of the driver's probing, and thus, DMA clock is kept
always-enabled in practice, defeating the whole purpose of runtime PM.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
---
 drivers/dma/tegra20-apb-dma.c | 47 ++++++++++++++++++++++++-----------
 1 file changed, 32 insertions(+), 15 deletions(-)

diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
index 22b88ccff05d..0ee28d8e3c96 100644
--- a/drivers/dma/tegra20-apb-dma.c
+++ b/drivers/dma/tegra20-apb-dma.c

@@ -436,6 +436,8 @@ static void tegra_dma_stop(struct tegra_dma_channel *tdc)
 		tdc_write(tdc, TEGRA_APBDMA_CHAN_STATUS, status);
 	}
 	tdc->busy = false;
+
+	pm_runtime_put(tdc->tdma->dev);
 }
 
 static void tegra_dma_start(struct tegra_dma_channel *tdc,

@@ -500,18 +502,25 @@ static void tegra_dma_configure_for_next(struct tegra_dma_channel *tdc,
 	tegra_dma_resume(tdc);
 }
 
-static void tdc_start_head_req(struct tegra_dma_channel *tdc)
+static bool tdc_start_head_req(struct tegra_dma_channel *tdc)
 {
 	struct tegra_dma_sg_req *sg_req;
+	int err;
 
 	if (list_empty(&tdc->pending_sg_req))
-		return;
+		return false;
+
+	err = pm_runtime_get_sync(tdc->tdma->dev);
+	if (WARN_ON_ONCE(err < 0))
+		return false;
 
 	sg_req = list_first_entry(&tdc->pending_sg_req, typeof(*sg_req), node);
 	tegra_dma_start(tdc, sg_req);
 	sg_req->configured = true;
 	sg_req->words_xferred = 0;
 	tdc->busy = true;
+
+	return true;
 }
 
 static void tdc_configure_next_head_desc(struct tegra_dma_channel *tdc)

@@ -615,6 +624,8 @@ static void handle_once_dma_done(struct tegra_dma_channel *tdc,
 	}
 	list_add_tail(&sgreq->node, &tdc->free_sg_req);
 
+	pm_runtime_put(tdc->tdma->dev);
+
 	/* Do not start DMA if it is going to be terminate */
 	if (to_terminate || list_empty(&tdc->pending_sg_req))
 		return;

@@ -730,9 +741,7 @@ static void tegra_dma_issue_pending(struct dma_chan *dc)
 		dev_err(tdc2dev(tdc), "No DMA request\n");
 		goto end;
 	}
-	if (!tdc->busy) {
-		tdc_start_head_req(tdc);
-
+	if (!tdc->busy && tdc_start_head_req(tdc)) {
 		/* Continuous single mode: Configure next req */
 		if (tdc->cyclic) {
 			/*

@@ -775,6 +784,13 @@ static int tegra_dma_terminate_all(struct dma_chan *dc)
 	else
 		wcount = status;
 
+	/*
+	 * tegra_dma_stop() will drop the RPM's usage refcount, but
+	 * tegra_dma_resume() touches hardware and thus we should keep
+	 * the DMA clock active while it's needed.
+	 */
+	pm_runtime_get(tdc->tdma->dev);
+