Re: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue granularity
From: Ben Dooks
Date: Wed Jun 19 2019 - 09:00:58 EST
On 19/06/2019 12:10, Dmitry Osipenko wrote:
19.06.2019 13:55, Jon Hunter ÐÐÑÐÑ:
On 19/06/2019 11:27, Dmitry Osipenko wrote:
19.06.2019 13:04, Jon Hunter ÐÐÑÐÑ:
On 19/06/2019 00:27, Dmitry Osipenko wrote:
19.06.2019 1:22, Ben Dooks ÐÐÑÐÑ:
On 13/06/2019 22:08, 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 chromiuim web browser. The patch is
based on the original work that was made by Ben Dooks [1]. It was tested
on Tegra20 and Tegra30 devices.
[1] https://lore.kernel.org/lkml/20190424162348.23692-1-ben.dooks@xxxxxxxxxxxxxxx/
Inspired-by: Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx>
Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx>
---
 drivers/dma/tegra20-apb-dma.c | 35 ++++++++++++++++++++++++++++-------
 1 file changed, 28 insertions(+), 7 deletions(-)
diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
index 79e9593815f1..c5af8f703548 100644
--- a/drivers/dma/tegra20-apb-dma.c
+++ b/drivers/dma/tegra20-apb-dma.c
@@ -797,12 +797,36 @@ static int tegra_dma_terminate_all(struct dma_chan *dc)
ÂÂÂÂÂ return 0;
 }
 +static unsigned int tegra_dma_update_residual(struct tegra_dma_channel *tdc,
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ struct tegra_dma_sg_req *sg_req,
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ struct tegra_dma_desc *dma_desc,
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unsigned int residual)
+{
+ÂÂÂ unsigned long status, wcount = 0;
+
+ÂÂÂ if (!list_is_first(&sg_req->node, &tdc->pending_sg_req))
+ÂÂÂÂÂÂÂ return residual;
+
+ÂÂÂ if (tdc->tdma->chip_data->support_separate_wcount_reg)
+ÂÂÂÂÂÂÂ wcount = tdc_read(tdc, TEGRA_APBDMA_CHAN_WORD_TRANSFER);
+
+ÂÂÂ status = tdc_read(tdc, TEGRA_APBDMA_CHAN_STATUS);
+
+ÂÂÂ if (!tdc->tdma->chip_data->support_separate_wcount_reg)
+ÂÂÂÂÂÂÂ wcount = status;
+
+ÂÂÂ if (status & TEGRA_APBDMA_STATUS_ISE_EOC)
+ÂÂÂÂÂÂÂ return residual - sg_req->req_len;
+
+ÂÂÂ return residual - get_current_xferred_count(tdc, sg_req, wcount);
+}
I am unfortunately nowhere near my notes, so can't completely
review this. I think the complexity of my patch series is due
to an issue with the count being updated before the EOC IRQ
is actually flagged (and most definetly before it gets to the
CPU IRQ handler).
The test system I was using, which i've not really got any
access to at the moment would show these internal inconsistent
states every few hours, however it was moving 48kHz 8ch 16bit
TDM data.
Thanks for looking into this, I am not sure if I am going to
get any time to look into this within the next couple of
months.
I'll try to add some debug checks to try to catch the case where count is updated before EOC
is set. Thank you very much for the clarification of the problem. So far I haven't spotted
anything going wrong.
Jon / Laxman, are you aware about the possibility to get such inconsistency of words count
vs EOC? Assuming the cyclic transfer mode.
I can't say that I am. However, for the case of cyclic transfer, given
that the next transfer is always programmed into the registers before
the last one completes, I could see that by the time the interrupt is
serviced that the DMA has moved on to the next transfer (which I assume
would reset the count).
Interestingly, our downstream kernel implemented a change to avoid the
count appearing to move backwards. I am curious if this also works,
which would be a lot simpler that what Ben has implemented and may
mitigate that race condition that Ben is describing.
Cheers
Jon
[0]
https://nv-tegra.nvidia.com/gitweb/?p=linux-4.4.git;a=commit;h=c7bba40c6846fbf3eaad35c4472dcc7d8bbc02e5
The downstream patch doesn't check for EOC and has no comments about it, so it's hard to
tell if it's intentional. Secondly, looks like the downstream patch is mucked up because it
doesn't check whether the dma_desc is *the active* transfer and not a pending!
I agree that it should check to see if it is active. I assume that what
this patch is doing is not updating the dma position if it appears to
have gone backwards, implying we have moved on to the next buffer. Yes
this is still probably not as accurate as Ben's implementation because
most likely we have finished that transfer and this patch would report
that it is not quite finished.
If Ben's patch works for you then why not go with this?
Because I'm doubtful that it is really the case and not something else. It will be very odd
if hardware updates words count and sets EOC asynchronously, I'd call it as a faulty design
and thus a bug that need to worked around in software if that's really happening.
Unfortunately someone designed hardware which does not update all the
state in one go. Find the designer and make them explain why they did
this.
--
Ben Dooks http://www.codethink.co.uk/
Senior Engineer Codethink - Providing Genius
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