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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 15:27:31 +0300
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02.07.2019 15:04, Dmitry Osipenko пишет:
> 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;
>  	}
>  }
> 

BTW, I don't understand the logic of having multiple SGs for a cyclic transfer. It looks
like driver is doing insane thing by trying to substitute an in-fly SG with another one..