On 02/07/2019 13:54, Jon Hunter wrote: > > 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@xxxxxxxxxxxxxxx/ >>>> Link: https://nv-tegra.nvidia.com/gitweb/?p=linux-4.4.git;a=commit;h=c7bba40c6846fbf3eaad35c4472dcc7d8bbc02e5 >>>> Inspired-by: Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx> >>>> Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx> >>>> --- >>>> >>>> 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. s/I see/I don't see/ Jon -- nvpublic
