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Subject: Re: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue
 granularity
From:   Jon Hunter <jonathanh@nvidia.com>
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>
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References: <20190613210849.10382-1-digetx@gmail.com>
 <5fbe4374-cc9a-8212-017e-05f4dee64443@nvidia.com>
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Date:   Fri, 14 Jun 2019 16:24:18 +0100
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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