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Subject: Re: [PATCH v3] 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: <20190627194728.8948-1-digetx@gmail.com>
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Date:   Tue, 2 Jul 2019 15:04:05 +0300
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02.07.2019 14:56, Dmitry Osipenko пишет:
> 02.07.2019 14:37, Dmitry Osipenko пишет:
>> 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? :)
>>
> 
> Ah, probably I was too focused on audio playback use-case. If it's a free-running
> transfer, then that case of a wraparound seems should be legit.. hmm.
> 

Looks like that will work:

diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
index 71473eda28ee..201c693b11f5 100644
--- a/drivers/dma/tegra20-apb-dma.c
+++ b/drivers/dma/tegra20-apb-dma.c
@@ -648,6 +648,8 @@ static void handle_cont_sngl_cycle_dma_done(struct tegra_dma_channel *tdc,
 		st = handle_continuous_head_request(tdc, sgreq, to_terminate);
 		if (!st)
 			dma_desc->dma_status = DMA_ERROR;
+	} else {
+		sg_req->words_xferred = 0;
 	}
 }