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@xxxxxxxxxxxxxxx>
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;
--
