Hi Adam,

Am 23.09.19 um 16:55 schrieb Adam Ford:

On Mon, Sep 23, 2019 at 8:58 AM Philipp Puschmann
[off-list ref] wrote:

quoted

For some years and since many kernel versions there are reports that
RX UART DMA channel stops working at one point. So far the usual
workaround was to disable RX DMA. This patches fix the underlying
problem.

When a running sdma script does not find any usable destination buffer
to put its data into it just leads to stopping the channel being
scheduled again. As solution we manually retrigger the sdma script for
this channel and by this dissolve the freeze.

While this seems to work fine so far, it may come to buffer overruns
when the channel - even temporary - is stopped. This case has to be
addressed by device drivers by increasing the number of DMA periods.

This patch series was tested with the current kernel and backported to
kernel 4.15 with a special use case using a WL1837MOD via UART and
provoking the hanging of UART RX DMA within seconds after starting a
test application. It resulted in well known
  "Bluetooth: hci0: command 0x0408 tx timeout"
errors and complete stop of UART data reception. Our Bluetooth traffic
consists of many independent small packets, mostly only a few bytes,
causing high usage of periods.

Using the 4.19.y branch, this seems to working just fine for me with an i.MX6Q
with WL1837MOD Bluetooth connected to UART2.  I am still seeing some
timeouts with 5.3, but I'm going to continue to run some tests.

Thanks for testing.
With my local setup i still have very few tx timeouts too. But i think they have a different
cause and especially different consequences. When the problem addressed by this series
appear you get a whole bunch of tx timeouts (and maybe errors from Bluetooth
layer) and monitoring received Bluetooth packets with hciconfig shows a
complete freeze of rx counter. Only resetting the hci_uart driver and the wl1837mon then helps.
With these patches applied the rx data shold still coming in even if a single or
multiple tx timeout error happen. I'm not sure where the error comes from and what the
consequences for the Bluetooth layer are.

Regards,
Philipp

Tested-by: Adam Ford <redacted> #imx6q w/ 4.19 Kernel

quoted

Signed-off-by: Philipp Puschmann <redacted>
Reviewed-by: Fugang Duan <redacted>

---

Changelog v5:
 - join with patch version from Jan Luebbe
 - adapt comments and patch descriptions
 - add Reviewed-by

Changelog v4:
 - fixed the fixes tags

Changelog v3:
 - fixes typo in dma_wmb
 - add fixes tags

Changelog v2:
 - adapt title (this patches are not only for i.MX6)
 - improve some comments and patch descriptions
 - add a dma_wb() around BD_DONE flag
 - add Reviewed-by tags
 - split off  "serial: imx: adapt rx buffer and dma periods"

Philipp Puschmann (3):
  dmaengine: imx-sdma: fix buffer ownership
  dmaengine: imx-sdma: fix dma freezes
  dmaengine: imx-sdma: drop redundant variable

 drivers/dma/imx-sdma.c | 37 +++++++++++++++++++++++++++----------
 1 file changed, 27 insertions(+), 10 deletions(-)

--
2.23.0


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