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Subject: Re: [PATCH v2] dmaengine: tegra-apb: Support per-burst residue
 granularity
From:   Dmitry Osipenko <digetx@gmail.com>
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: <20190618115035.29250-1-digetx@gmail.com>
 <8f48fcba-df7c-a313-2f84-0fa896e4ccec@nvidia.com>
 <8470f28f-b903-c6ed-23bd-0cbd130f0798@gmail.com>
Message-ID: <81648813-3a19-d390-fa9a-f2e0bf4e4d72@gmail.com>
Date:   Wed, 26 Jun 2019 02:44:38 +0300
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21.06.2019 0:58, Dmitry Osipenko пишет:
> 20.06.2019 11:38, Jon Hunter пишет:
>>
>> On 18/06/2019 12:50, 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. It was tested on
>>> Tegra20 and Tegra30 devices.
>>>
>>> Link: 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>
>>> ---
>>>
>>> Changelog:
>>>
>>> v2:  Addressed review comments made by Jon Hunter to v1. We won't try
>>>      to get words count if dma_desc is on free list as it will result
>>>      in a NULL dereference because this case wasn't handled properly.
>>>
>>>      The residual value is now updated properly, avoiding potential
>>>      integer overflow by adding the "bytes" to the "bytes_transferred"
>>>      instead of the subtraction.
>>>
>>>  drivers/dma/tegra20-apb-dma.c | 33 ++++++++++++++++++++++++++-------
>>>  1 file changed, 26 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
>>> index 79e9593815f1..fed18bc46479 100644
>>> --- a/drivers/dma/tegra20-apb-dma.c
>>> +++ b/drivers/dma/tegra20-apb-dma.c
>>> @@ -797,6 +797,28 @@ static int tegra_dma_terminate_all(struct dma_chan *dc)
>>>  	return 0;
>>>  }
>>>  
>>> +static unsigned int tegra_dma_sg_bytes_xferred(struct tegra_dma_channel *tdc,
>>> +					       struct tegra_dma_sg_req *sg_req)
>>> +{
>>> +	unsigned long status, wcount = 0;
>>> +
>>> +	if (!list_is_first(&sg_req->node, &tdc->pending_sg_req))
>>> +		return 0;
>>> +
>>> +	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 sg_req->req_len;
>>> +
>>> +	return 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)
>>>  {
>>> @@ -806,6 +828,7 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>>  	enum dma_status ret;
>>>  	unsigned long flags;
>>>  	unsigned int residual;
>>> +	unsigned int bytes = 0;
>>>  
>>>  	ret = dma_cookie_status(dc, cookie, txstate);
>>>  	if (ret == DMA_COMPLETE)
>>> @@ -825,6 +848,7 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>>  	list_for_each_entry(sg_req, &tdc->pending_sg_req, node) {
>>>  		dma_desc = sg_req->dma_desc;
>>>  		if (dma_desc->txd.cookie == cookie) {
>>> +			bytes = tegra_dma_sg_bytes_xferred(tdc, sg_req);
>>>  			ret = dma_desc->dma_status;
>>>  			goto found;
>>>  		}
>>> @@ -836,7 +860,7 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>>  found:
>>>  	if (dma_desc && txstate) {
>>>  		residual = dma_desc->bytes_requested -
>>> -			   (dma_desc->bytes_transferred %
>>> +			   ((dma_desc->bytes_transferred + bytes) %
>>>  			    dma_desc->bytes_requested);
>>>  		dma_set_residue(txstate, residual);
>>>  	}
>>> @@ -1441,12 +1465,7 @@ static int tegra_dma_probe(struct platform_device *pdev)
>>>  		BIT(DMA_SLAVE_BUSWIDTH_4_BYTES) |
>>>  		BIT(DMA_SLAVE_BUSWIDTH_8_BYTES);
>>>  	tdma->dma_dev.directions = BIT(DMA_DEV_TO_MEM) | BIT(DMA_MEM_TO_DEV);
>>> -	/*
>>> -	 * XXX The hardware appears to support
>>> -	 * DMA_RESIDUE_GRANULARITY_BURST-level reporting, but it's
>>> -	 * only used by this driver during tegra_dma_terminate_all()
>>> -	 */
>>> -	tdma->dma_dev.residue_granularity = DMA_RESIDUE_GRANULARITY_SEGMENT;
>>> +	tdma->dma_dev.residue_granularity = DMA_RESIDUE_GRANULARITY_BURST;
>>>  	tdma->dma_dev.device_config = tegra_dma_slave_config;
>>>  	tdma->dma_dev.device_terminate_all = tegra_dma_terminate_all;
>>>  	tdma->dma_dev.device_tx_status = tegra_dma_tx_status;
>>>
>>
>> Looks good to me. Thanks for fixing!
>>
>> Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
> 
> Thanks to you too for the review :)
> 

Please hold on this patch. I took another look at this and spotted unfortunate typo
in the tests that I wrote and indeed wraparound happens before EOC bit is set, just
like Ben told :(

I'll be looking at this all again during next days. Looks like the EOC bit is set
after a burst completion once hardware counter is reloaded.