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

[Jon Hunter]

On 02/07/2019 13:27, Dmitry Osipenko wrote:
> 02.07.2019 15:04, Dmitry Osipenko ÐÐÑÐÑ:
>> 02.07.2019 14:56, Dmitry Osipenko ÐÐÑÐÑ:
>>> 02.07.2019 14:37, Dmitry Osipenko ÐÐÑÐÑ:
>>>> 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.
>>>>
>>>> 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..

I may have mentioned before that I don't like how this driver was
implemented. Ideally it should use the virt-dma helpers and avoid all
this nonsense. However, best to leave sleeping dogs lie.

Jon

-- 
nvpublic