[4/6] dma: tegra: add accurate reporting of dma state

[Dmitry Osipenko <digetx@gmail.com>]

On 31.10.2018 19:03, Ben Dooks wrote:
> The tx_status callback does not report the state of the transfer
> beyond complete segments. This causes problems with users such as
> ALSA when applications want to know accurately how much data has
> been moved.
> 
> This patch addes a function tegra_dma_update_residual() to query
> the hardware and modify the residual information accordinly. It
> takes into account any hardware issues when trying to read the
> state, such as delays between finishing a buffer and signalling
> the interrupt.
> 
> Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>

Hello Ben,

Thank you very much for the patch! It fixes "farting sound" for chromium-browser and applications that use chromium-engine (tested on Tegra20) because apparently it tries to use low latency for everything and audio buffer is constantly underflowing without more detailed DMA-progress reporting. See couple more comments below.

> ---
>  drivers/dma/tegra20-apb-dma.c | 94 ++++++++++++++++++++++++++++++++---
>  1 file changed, 87 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
> index 4f7d1e576d03..3fa3a1ac4f57 100644
> --- a/drivers/dma/tegra20-apb-dma.c
> +++ b/drivers/dma/tegra20-apb-dma.c
> @@ -802,12 +802,96 @@ 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 = 0x0;

There is no need to initialize "status" variable.

> +	unsigned long wcount;
> +	unsigned long ahbptr;
> +	unsigned long tmp = 0x0;
> +	unsigned int result;
> +	int retries = TEGRA_APBDMA_BURST_COMPLETE_TIME * 10;
> +	int done;
> +
> +	/* if we're not the current request, then don't alter the residual */
> +	if (sg_req != list_first_entry(&tdc->pending_sg_req,
> +				       struct tegra_dma_sg_req, node)) {
> +		result = residual;
> +		ahbptr = 0xffffffff;
> +		goto done;
> +	}
> +
> +	/* loop until we have a reliable result for residual */
> +	do {
> +		ahbptr = tdc_read(tdc, TEGRA_APBDMA_CHAN_AHBPTR);
> +		status = tdc_read(tdc, TEGRA_APBDMA_CHAN_STATUS);

> +		tmp =  tdc_read(tdc, 0x08);	/* total count for debug */

Register 0x08 (DMA_BYTE_STA) doesn't present on Tegra20 and "tmp" isn't used anywhere in the code. Please remove it entirely.

> +
> +		/* check status, if channel isn't busy then skip */
> +		if (!(status & TEGRA_APBDMA_STATUS_BUSY)) {
> +			result = residual;
> +			break;
> +		}

If "BUSY" is unset, doesn't this mean that transaction could be completed already? I don't quite understand why you want to skip here.

> +
> +		/* if we've got an interrupt pending on the channel, don't
> +		 * try and deal with the residue as the hardware has likely
> +		 * moved on to the next buffer. return all data moved.
> +		 */
> +		if (status & TEGRA_APBDMA_STATUS_ISE_EOC) {
> +			result = residual - sg_req->req_len;
> +			break;
> +		}
> +
> +		if (tdc->tdma->chip_data->support_separate_wcount_reg)
> +			wcount = tdc_read(tdc, TEGRA_APBDMA_CHAN_WORD_TRANSFER);
> +		else
> +			wcount = status;
> +
> +		/* If the request is at the full point, then there is a
> +		 * chance that we have read the status register in the
> +		 * middle of the hardware reloading the next buffer.
> +		 *
> +		 * The sequence seems to be at the end of the buffer, to
> +		 * load the new word count before raising the EOC flag (or
> +		 * changing the ping-pong flag which could have also been
> +		 * used to determine a new buffer). This  means there is a
                               two whitespaces here-----|                      

> +		 * small window where we cannot determine zero-done for the
> +		 * current buffer, or moved to next buffer.
> +		 *

> +		 * If done shows 0, then retry the load, as it may hit the
> +		 * above hardware race. We will either get a new value which
> +		 * is from the first buffer, or we get an EOC (new buffer)
> +		 * or both a new value and an EOC...

I think we just need to wait 20usec after reading out "words count" and then re-check interrupt status, so transfer is done if interrupt is set and otherwise "words count" value is actual and reliable.

> +		 */
> +		done = get_current_xferred_count(tdc, sg_req, wcount);
> +		if (done != 0) {
> +			result = residual - done;
> +			break;
> +		}
> +
> +		ndelay(100);

There is no ndelay() on ARM, hence your 20usec timeout is 200usec. Please use udelay().

> +	} while (--retries > 0);
> +
> +	if (retries <= 0) {
> +		dev_err(tdc2dev(tdc), "timeout waiting for dma load\n");
> +		result = residual;
> +	}
> +
> +done:	

Please rename goto label as it duplicates local variable name.

> +	dev_dbg(tdc2dev(tdc), "residual: req %08lx, ahb@%08lx, wcount %08lx, done %d\n",
> +		 sg_req->ch_regs.ahb_ptr, ahbptr, wcount, done);

Whitespace just after tab not needed.
 
> +
> +	return result;
> +}
> +
>  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_desc *dma_desc;
> -	struct tegra_dma_sg_req *sg_req;
> +	struct tegra_dma_sg_req *sg_req = NULL;
>  	enum dma_status ret;
>  	unsigned long flags;
>  	unsigned int residual;
> @@ -843,6 +927,7 @@ 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);
>  	}
>  
> @@ -1436,12 +1521,7 @@ 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;
> 

Summarizing all of the comments above, patch may look like this:

diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
index 9a558e30c461..956e8130c059 100644
--- a/drivers/dma/tegra20-apb-dma.c
+++ b/drivers/dma/tegra20-apb-dma.c
@@ -799,12 +799,73 @@ 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;
+	unsigned int result;
+	int done;
+
+	/* if we're not the current request, then don't alter the residual */
+	if (sg_req != list_first_entry(&tdc->pending_sg_req,
+				       struct tegra_dma_sg_req, node))
+		return residual;
+
+	status = tdc_read(tdc, TEGRA_APBDMA_CHAN_STATUS);
+
+	/* if we've got an interrupt pending on the channel, don't
+	 * try and deal with the residue as the hardware has likely
+	 * moved on to the next buffer. return all data moved.
+	 */
+	if (status & TEGRA_APBDMA_STATUS_ISE_EOC) {
+		result = residual - sg_req->req_len;
+		goto out;
+	}
+
+	if (tdc->tdma->chip_data->support_separate_wcount_reg)
+		status = tdc_read(tdc, TEGRA_APBDMA_CHAN_WORD_TRANSFER);
+
+	/*
+	 * If the request is at the full point, then there is a
+	 * chance that we have read the status register in the
+	 * middle of the hardware reloading the next buffer.
+	 *
+	 * The sequence seems to be at the end of the buffer, to
+	 * load the new word count before raising the EOC flag (or
+	 * changing the ping-pong flag which could have also been
+	 * used to determine a new buffer). This  means there is a
+	 * small window where we cannot determine zero-done for the
+	 * current buffer, or moved to next buffer.
+	 */
+	done = get_current_xferred_count(tdc, sg_req, status);
+
+	udelay(TEGRA_APBDMA_BURST_COMPLETE_TIME);
+
+	status = tdc_read(tdc, TEGRA_APBDMA_CHAN_STATUS);
+
+	if (status & TEGRA_APBDMA_STATUS_ISE_EOC)
+		result = residual - sg_req->req_len;
+	else
+		result = residual - done;
+out:
+#ifdef DEBUG
+	ahbptr = tdc_read(tdc, TEGRA_APBDMA_CHAN_AHBPTR);
+
+	dev_dbg(tdc2dev(tdc), "residual: req %08lx, ahb@%08lx, wcount %08lx, done %d\n",
+		sg_req->ch_regs.ahb_ptr, ahbptr, wcount, done);
+#endif
+
+	return result;
+}
+
 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_desc *dma_desc;
-	struct tegra_dma_sg_req *sg_req;
+	struct tegra_dma_sg_req *sg_req = NULL;
 	enum dma_status ret;
 	unsigned long flags;
 	unsigned int residual;
@@ -840,6 +901,7 @@ 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);
 	}
 
@@ -1433,12 +1495,7 @@ 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;