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.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. > > 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.
