[PATCH v1] dmaengine: tegra-apb: Support per-burst residue granularity

[Dmitry Osipenko <digetx@gmail.com>]

Tegra's APB DMA engine updates words counter after each transferred burst
of data, hence it can report transfer's residual with more fidelity which
may be required in cases like audio playback. In particular this fixes
audio stuttering during playback in a chromiuim web browser. The patch is
based on the original work that was made by Ben Dooks [1]. It was tested
on Tegra20 and Tegra30 devices.

[1] https://lore.kernel.org/lkml/20190424162348.23692-1-ben.dooks@codethink.co.uk/

Inspired-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
---
 drivers/dma/tegra20-apb-dma.c | 35 ++++++++++++++++++++++++++++-------
 1 file changed, 28 insertions(+), 7 deletions(-)

diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
index 79e9593815f1..c5af8f703548 100644
--- a/drivers/dma/tegra20-apb-dma.c
+++ b/drivers/dma/tegra20-apb-dma.c
@@ -797,12 +797,36 @@ static int tegra_dma_terminate_all(struct dma_chan *dc)
 	return 0;
 }
 
+static unsigned int tegra_dma_update_residual(struct tegra_dma_channel *tdc,
+					      struct tegra_dma_sg_req *sg_req,
+					      struct tegra_dma_desc *dma_desc,
+					      unsigned int residual)
+{
+	unsigned long status, wcount = 0;
+
+	if (!list_is_first(&sg_req->node, &tdc->pending_sg_req))
+		return residual;
+
+	if (tdc->tdma->chip_data->support_separate_wcount_reg)
+		wcount = tdc_read(tdc, TEGRA_APBDMA_CHAN_WORD_TRANSFER);
+
+	status = tdc_read(tdc, TEGRA_APBDMA_CHAN_STATUS);
+
+	if (!tdc->tdma->chip_data->support_separate_wcount_reg)
+		wcount = status;
+
+	if (status & TEGRA_APBDMA_STATUS_ISE_EOC)
+		return residual - sg_req->req_len;
+
+	return residual - get_current_xferred_count(tdc, sg_req, wcount);
+}
+
 static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
 	dma_cookie_t cookie, struct dma_tx_state *txstate)
 {
 	struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
+	struct tegra_dma_sg_req *sg_req = NULL;
 	struct tegra_dma_desc *dma_desc;
-	struct tegra_dma_sg_req *sg_req;
 	enum dma_status ret;
 	unsigned long flags;
 	unsigned int residual;
@@ -838,6 +862,8 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
 		residual = dma_desc->bytes_requested -
 			   (dma_desc->bytes_transferred %
 			    dma_desc->bytes_requested);
+		residual = tegra_dma_update_residual(tdc, sg_req, dma_desc,
+						     residual);
 		dma_set_residue(txstate, residual);
 	}
 
@@ -1441,12 +1467,7 @@ static int tegra_dma_probe(struct platform_device *pdev)
 		BIT(DMA_SLAVE_BUSWIDTH_4_BYTES) |
 		BIT(DMA_SLAVE_BUSWIDTH_8_BYTES);
 	tdma->dma_dev.directions = BIT(DMA_DEV_TO_MEM) | BIT(DMA_MEM_TO_DEV);
-	/*
-	 * XXX The hardware appears to support
-	 * DMA_RESIDUE_GRANULARITY_BURST-level reporting, but it's
-	 * only used by this driver during tegra_dma_terminate_all()
-	 */
-	tdma->dma_dev.residue_granularity = DMA_RESIDUE_GRANULARITY_SEGMENT;
+	tdma->dma_dev.residue_granularity = DMA_RESIDUE_GRANULARITY_BURST;
 	tdma->dma_dev.device_config = tegra_dma_slave_config;
 	tdma->dma_dev.device_terminate_all = tegra_dma_terminate_all;
 	tdma->dma_dev.device_tx_status = tegra_dma_tx_status;
-- 
2.22.0