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? :)
