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

Dmitry Osipenko
2019-07-02T11:37:16Z

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? :)