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Subject: Re: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue
 granularity
To:     Dmitry Osipenko <digetx@gmail.com>,
        Laxman Dewangan <ldewangan@nvidia.com>,
        Vinod Koul <vkoul@kernel.org>,
        Thierry Reding <thierry.reding@gmail.com>,
        Ben Dooks <ben.dooks@codethink.co.uk>
CC:     <dmaengine@vger.kernel.org>, <linux-tegra@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
References: <20190613210849.10382-1-digetx@gmail.com>
From:   Jon Hunter <jonathanh@nvidia.com>
Message-ID: <5fbe4374-cc9a-8212-017e-05f4dee64443@nvidia.com>
Date:   Fri, 14 Jun 2019 16:21:18 +0100
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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