On 02/07/2019 13:27, Dmitry Osipenko wrote: > 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.do...@codethink.co.uk/ >>>>>> Link: >>>>>> https://nv-tegra.nvidia.com/gitweb/?p=linux-4.4.git;a=commit;h=c7bba40c6846fbf3eaad35c4472dcc7d8bbc02e5 >>>>>> Inspired-by: Ben Dooks <ben.do...@codethink.co.uk> >>>>>> Signed-off-by: Dmitry Osipenko <dig...@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..
I may have mentioned before that I don't like how this driver was implemented. Ideally it should use the virt-dma helpers and avoid all this nonsense. However, best to leave sleeping dogs lie. Jon -- nvpublic
