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Subject: Re: [PATCH v3] 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: <20190627194728.8948-1-digetx@gmail.com>
 <dab25158-272c-a18f-a858-433f7f9000e0@nvidia.com>
From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <3a5403fe-b81f-993c-e7c0-407387e001d9@gmail.com>
Date:   Tue, 2 Jul 2019 14:37:10 +0300
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02.07.2019 14:20, Jon Hunter пишет:
> 
> On 27/06/2019 20:47, 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 chromium web browser. The patch is
>> based on the original work that was made by Ben Dooks and a patch from
>> downstream kernel. It was tested on Tegra20 and Tegra30 devices.
>>
>> Link: https://lore.kernel.org/lkml/20190424162348.23692-1-ben.dooks@codethink.co.uk/
>> Link: https://nv-tegra.nvidia.com/gitweb/?p=linux-4.4.git;a=commit;h=c7bba40c6846fbf3eaad35c4472dcc7d8bbc02e5
>> Inspired-by: Ben Dooks <ben.dooks@codethink.co.uk>
>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>> ---
>>
>> Changelog:
>>
>> v3:  Added workaround for a hardware design shortcoming that results
>>      in a words counter wraparound before end-of-transfer bit is set
>>      in a cyclic mode.
>>
>> 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 | 69 +++++++++++++++++++++++++++++++----
>>  1 file changed, 62 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
>> index 79e9593815f1..71473eda28ee 100644
>> --- a/drivers/dma/tegra20-apb-dma.c
>> +++ b/drivers/dma/tegra20-apb-dma.c
>> @@ -152,6 +152,7 @@ struct tegra_dma_sg_req {
>>  	bool				last_sg;
>>  	struct list_head		node;
>>  	struct tegra_dma_desc		*dma_desc;
>> +	unsigned int			words_xferred;
>>  };
>>  
>>  /*
>> @@ -496,6 +497,7 @@ static void tegra_dma_configure_for_next(struct tegra_dma_channel *tdc,
>>  	tdc_write(tdc, TEGRA_APBDMA_CHAN_CSR,
>>  				nsg_req->ch_regs.csr | TEGRA_APBDMA_CSR_ENB);
>>  	nsg_req->configured = true;
>> +	nsg_req->words_xferred = 0;
>>  
>>  	tegra_dma_resume(tdc);
>>  }
>> @@ -511,6 +513,7 @@ static void tdc_start_head_req(struct tegra_dma_channel *tdc)
>>  					typeof(*sg_req), node);
>>  	tegra_dma_start(tdc, sg_req);
>>  	sg_req->configured = true;
>> +	sg_req->words_xferred = 0;
>>  	tdc->busy = true;
>>  }
>>  
>> @@ -797,6 +800,61 @@ 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;
>> +
>> +	wcount = get_current_xferred_count(tdc, sg_req, wcount);
>> +
>> +	if (!wcount) {
>> +		/*
>> +		 * If wcount wasn't ever polled for this SG before, then
>> +		 * simply assume that transfer hasn't started yet.
>> +		 *
>> +		 * Otherwise it's the end of the transfer.
>> +		 *
>> +		 * The alternative would be to poll the status register
>> +		 * until EOC bit is set or wcount goes UP. That's so
>> +		 * because EOC bit is getting set only after the last
>> +		 * burst's completion and counter is less than the actual
>> +		 * transfer size by 4 bytes. The counter value wraps around
>> +		 * in a cyclic mode before EOC is set(!), so we can't easily
>> +		 * distinguish start of transfer from its end.
>> +		 */
>> +		if (sg_req->words_xferred)
>> +			wcount = sg_req->req_len - 4;
>> +
>> +	} else if (wcount < sg_req->words_xferred) {
>> +		/*
>> +		 * This case shall not ever happen because EOC bit
>> +		 * must be set once next cyclic transfer is started.
> 
> I am not sure I follow this and why this condition cannot happen for
> cyclic transfers. What about non-cyclic transfers?

It cannot happen because the EOC bit will be set in that case. The counter wraps
around when the transfer of a last burst happens, EOC bit is guaranteed to be set
after completion of the last burst. That's my observation after a thorough testing,
it will be very odd if EOC setting happened completely asynchronously.

For a non-cyclic transfers it doesn't matter.. because they are not cyclic and thus
counter will be stopped by itself. It will be a disaster if all of sudden a
non-cyclic transfer becomes cyclic, don't you think so? :)