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

Dmitry Osipenko
2019-07-02T12:27:38Z

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..