Re: [PATCH v2] dmaengine: tegra-apb: Support per-burst residue granularity

Dmitry Osipenko
2019-06-25T23:44:45Z

21.06.2019 0:58, Dmitry Osipenko пишет:
> 20.06.2019 11:38, Jon Hunter пишет:
>>
>> On 18/06/2019 12:50, 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 chromiuim web browser. The patch is
>>> based on the original work that was made by Ben Dooks. It was tested on
>>> Tegra20 and Tegra30 devices.
>>>
>>> Link: https://lore.kernel.org/lkml/20190424162348.23692-1-ben.dooks@codethink.co.uk/
>>> Inspired-by: Ben Dooks <ben.dooks@codethink.co.uk>
>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>>> ---
>>>
>>> Changelog:
>>>
>>> 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 | 33 ++++++++++++++++++++++++++-------
>>> 1 file changed, 26 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
>>> index 79e9593815f1..fed18bc46479 100644
>>> --- a/drivers/dma/tegra20-apb-dma.c
>>> +++ b/drivers/dma/tegra20-apb-dma.c
>>> @@ -797,6 +797,28 @@ 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;
>>> +
>>> + return get_current_xferred_count(tdc, sg_req, wcount);
>>> +}
>>> +
>>> static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>> dma_cookie_t cookie, struct dma_tx_state *txstate)
>>> {
>>> @@ -806,6 +828,7 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>> enum dma_status ret;
>>> unsigned long flags;
>>> unsigned int residual;
>>> + unsigned int bytes = 0;
>>>
>>> ret = dma_cookie_status(dc, cookie, txstate);
>>> if (ret == DMA_COMPLETE)
>>> @@ -825,6 +848,7 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>> list_for_each_entry(sg_req, &tdc->pending_sg_req, node) {
>>> dma_desc = sg_req->dma_desc;
>>> if (dma_desc->txd.cookie == cookie) {
>>> + bytes = tegra_dma_sg_bytes_xferred(tdc, sg_req);
>>> ret = dma_desc->dma_status;
>>> goto found;
>>> }
>>> @@ -836,7 +860,7 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>> found:
>>> if (dma_desc && txstate) {
>>> residual = dma_desc->bytes_requested -
>>> - (dma_desc->bytes_transferred %
>>> + ((dma_desc->bytes_transferred + bytes) %
>>> dma_desc->bytes_requested);
>>> dma_set_residue(txstate, residual);
>>> }
>>> @@ -1441,12 +1465,7 @@ static int tegra_dma_probe(struct platform_device *pdev)
>>> BIT(DMA_SLAVE_BUSWIDTH_4_BYTES) |
>>> BIT(DMA_SLAVE_BUSWIDTH_8_BYTES);
>>> tdma->dma_dev.directions = BIT(DMA_DEV_TO_MEM) | BIT(DMA_MEM_TO_DEV);
>>> - /*
>>> - * XXX The hardware appears to support
>>> - * DMA_RESIDUE_GRANULARITY_BURST-level reporting, but it's
>>> - * only used by this driver during tegra_dma_terminate_all()
>>> - */
>>> - tdma->dma_dev.residue_granularity = DMA_RESIDUE_GRANULARITY_SEGMENT;
>>> + tdma->dma_dev.residue_granularity = DMA_RESIDUE_GRANULARITY_BURST;
>>> tdma->dma_dev.device_config = tegra_dma_slave_config;
>>> tdma->dma_dev.device_terminate_all = tegra_dma_terminate_all;
>>> tdma->dma_dev.device_tx_status = tegra_dma_tx_status;
>>>
>>
>> Looks good to me. Thanks for fixing!
>>
>> Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
>
> Thanks to you too for the review :)
>

Please hold on this patch. I took another look at this and spotted unfortunate typo
in the tests that I wrote and indeed wraparound happens before EOC bit is set, just
like Ben told :(

I'll be looking at this all again during next days. Looks like the EOC bit is set
after a burst completion once hardware counter is reloaded.