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

Jon Hunter
2019-07-02T15:27:13Z

On 02/07/2019 15:41, Dmitry Osipenko wrote:
> 02.07.2019 16:41, Jon Hunter пишет:
>>
>> On 02/07/2019 14:22, Dmitry Osipenko wrote:
>>> 02.07.2019 15:54, Jon Hunter пишет:
>>>>
>>>> On 02/07/2019 12:37, Dmitry Osipenko wrote:
>>>>> 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.
>>>>
>>>> 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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