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Subject: Re: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue
 granularity
To:     Jon Hunter <jonathanh@nvidia.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>
 <5fbe4374-cc9a-8212-017e-05f4dee64443@nvidia.com>
 <7ab96aa5-0be2-dc01-d187-eb718093eb99@nvidia.com>
From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <840fcf60-8e24-ff44-a816-ef63a5f18652@gmail.com>
Date:   Fri, 14 Jun 2019 19:44:07 +0300
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14.06.2019 18:24, Jon Hunter пишет:
> 
> 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.

Looks like everything should be fine as-is.

BTW, it's a bit hard to believe that there is any real benefit from the
free_dma_desc list at all, maybe worth to just remove it?