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? Cheers Jon -- nvpublic
