Re: [PATCH v3] dmaengine: tegra-apb: Support per-burst residue granularity

[Jon Hunter]

On 02/07/2019 15:41, Dmitry Osipenko wrote:
> 02.07.2019 16:41, Jon Hunter ÐÐÑÐÑ:
>>
>> On 02/07/2019 14:22, Dmitry Osipenko wrote:
>>> 02.07.2019 15:54, Jon Hunter ÐÐÑÐÑ:
>>>>
>>>> On 02/07/2019 12:37, Dmitry Osipenko wrote:
>>>>> 02.07.2019 14:20, Jon Hunter ÐÐÑÐÑ:
>>>>>>
>>>>>> 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@xxxxxxxxxxxxxxx/
>>>>>>> Link: https://nv-tegra.nvidia.com/gitweb/?p=linux-4.4.git;a=commit;h=c7bba40c6846fbf3eaad35c4472dcc7d8bbc02e5
>>>>>>> Inspired-by: Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx>
>>>>>>> Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx>
>>>>>>> ---
>>>>>>>
>>>>>>> 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.
>>>>
>>>> I see how you know that the EOC is set. Anyway, you check if the EOC is
>>>> set before and if so return sg_req->req_len prior to this test.
>>>>
>>>> Maybe I am missing something, but what happens if we are mid block when
>>>> dmaengine_tx_status() is called? That happen asynchronously right?
>>>
>>>
>>> Do you mean asynchronously in regards to the ISR? Or something else?
>>
>> In the sense that the client can call dmaengine_tx_status() at anytime
>> to check the status of a transfer.
> 
> Should be alright, I think this patch covers all of possible cases:
> 
> 1) Start of transfer, when wcount=0.
> 2) Middle of transfer, when wcount!=0.
> 3) End of transfer, when wcount=0.
> 4) End of transfer, when wcount!=0 and EOC is set.

I think I see my problem I read 'wcount < sg_req->req_len' and NOT
'wcount < sg_req->words_xferred'. So yes this should be fine.

Jon

-- 
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