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SubjectRe: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue granularity
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On 13/06/2019 22:08, Dmitry Osipenko wrote:
> 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);

I had a quick look at this, I am not sure that we want to call
tegra_dma_update_residual() here for cases where the dma_desc is on the
free_dma_desc list. In fact, couldn't this be simplified a bit for case
where the dma_desc is on the free list? In that case I believe that the
residual should always be 0.

Cheers
Jon

--
nvpublic

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