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

Jon Hunter
2019-07-02T12:56:58Z

On 02/07/2019 13:27, Dmitry Osipenko wrote:
> 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..

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

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