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    SubjectRe: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue granularity
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    On 14/06/2019 16:21, Jon Hunter wrote:
    >
    > 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.

    Actually, no, it could be non-zero in the case the transfer is aborted.

    Jon

    --
    nvpublic

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