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/[email protected]/ >>>>> Link: >>>>> https://nv-tegra.nvidia.com/gitweb/?p=linux-4.4.git;a=commit;h=c7bba40c6846fbf3eaad35c4472dcc7d8bbc02e5 >>>>> Inspired-by: Ben Dooks <[email protected]> >>>>> Signed-off-by: Dmitry Osipenko <[email protected]> >>>>> --- >>>>> >>>>> 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..
