Re: [EXT] [PATCH v3] serial: imx: adapt rx buffer and dma periods

[Adam Ford]

On Fri, Sep 20, 2019 at 2:06 AM Philipp Puschmann
<philipp.puschmann@xxxxxxxxx> wrote:
>
> Hi Andy,
>
> Am 20.09.19 um 05:42 schrieb Andy Duan:
> > From: Philipp Puschmann <philipp.puschmann@xxxxxxxxx> Sent: Thursday, September 19, 2019 10:51 PM
> >> Using only 4 DMA periods for UART RX is very few if we have a high frequency
> >> of small transfers - like in our case using Bluetooth with many small packets
> >> via UART - causing many dma transfers but in each only filling a fraction of a
> >> single buffer. Such a case may lead to the situation that DMA RX transfer is
> >> triggered but no free buffer is available. When this happens dma channel ist
> >> stopped - with the patch
> >> "dmaengine: imx-sdma: fix dma freezes" temporarily only - with the possible
> >> consequences that:

I have an i.MX6Q with Wl1837MOD on UART 2 with flow control, and I am
getting Bluetooth transfer timeouts.
(see imx6-logicpd-som.dtsi)

On top of 5.3.1, I have installed:

dmaengine: imx-sdma: fix buffer ownership
dmaengine: imx-sdma: fix dma freezes
dmaengine: imx-sdma: drop redundant variable
dmaengine: imx-sdma: fix kernel hangs with SLUB slab allocator
serial: imx: adapt rx buffer and dma periods

and I still get timeouts:

[   66.632006] Bluetooth: hci0: command 0xff36 tx timeout
[   76.790499] Bluetooth: hci0: command 0x1001 tx timeout
[   87.110488] Bluetooth: hci0: command 0xff36 tx timeout
[   97.270507] Bluetooth: hci0: command 0x1001 tx timeout
[  107.590457] Bluetooth: hci0: command 0xff36 tx timeout
[  117.750477] Bluetooth: hci0: command 0x1001 tx timeout
[  226.390499] Bluetooth: hci0: command 0xfe38 tx timeout
[  231.590735] Bluetooth: hci0: command tx timeout

I did a bisect and found the start of my problems came from

361deb7243d2 ("dmaengine: dmatest: wrap src & dst data into a struct")

This happened sometime between v5.0 and v5.1

Is there a patch I missed somewhere?  Do I need to change my device
tree configuration somehow to allocate the proper DMA memory?



> >> with disabled hw flow control:
> >>   If enough data is incoming on UART port the RX FIFO runs over and
> >>   characters will be lost. What then happens depends on upper layer.
> >>
> >> with enabled hw flow control:
> >>   If enough data is incoming on UART port the RX FIFO reaches a level
> >>   where CTS is deasserted and remote device sending the data stops.
> >>   If it fails to stop timely the i.MX' RX FIFO may run over and data
> >>   get lost. Otherwise it's internal TX buffer may getting filled to
> >>   a point where it runs over and data is again lost. It depends on
> >>   the remote device how this case is handled and if it is recoverable.
> >>
> >> Obviously we want to avoid having no free buffers available. So we decrease
> >> the size of the buffers and increase their number and the total buffer size.
> >>
> >> Signed-off-by: Philipp Puschmann <philipp.puschmann@xxxxxxxxx>
> >> Reviewed-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
> >> ---
> >>
> >> Changelog v3:
> >>  - enhance description
> >>
> >> Changelog v2:
> >>  - split this patch from series "Fix UART DMA freezes for iMX6"
> >>  - add Reviewed-by tag
> >>
> >>  drivers/tty/serial/imx.c | 5 ++---
> >>  1 file changed, 2 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c index
> >> 87c58f9f6390..51dc19833eab 100644
> >> --- a/drivers/tty/serial/imx.c
> >> +++ b/drivers/tty/serial/imx.c
> >> @@ -1034,8 +1034,6 @@ static void imx_uart_timeout(struct timer_list *t)
> >>         }
> >>  }
> >>
> >> -#define RX_BUF_SIZE    (PAGE_SIZE)
> >> -
> >>  /*
> >>   * There are two kinds of RX DMA interrupts(such as in the MX6Q):
> >>   *   [1] the RX DMA buffer is full.
> >> @@ -1118,7 +1116,8 @@ static void imx_uart_dma_rx_callback(void
> >> *data)  }
> >>
> >>  /* RX DMA buffer periods */
> >> -#define RX_DMA_PERIODS 4
> >> +#define RX_DMA_PERIODS 16
> >> +#define RX_BUF_SIZE    (PAGE_SIZE / 4)
> >>
> > Why to decrease the DMA RX buffer size here ?
> >
> > The current DMA implementation support DMA cyclic mode, one SDMA BD receive one Bluetooth frame can
> > bring better performance.
> > As you know, for L2CAP, a maximum transmission unit (MTU) associated with the largest Baseband payload
> > is 341 bytes for DH5 packets.
> >
> > So I suggest to increase RX_BUF_SIZE along with RX_DMA_PERIODS to feasible value.
>
> I debugged and developed this patches on a system with a 4.15 kernel. When prepared for upstream i have adapted
> some details and missed a important thing here. It should say:
>
> +#define RX_BUF_SIZE    (RX_DMA_PERIODS * PAGE_SIZE / 4)
>
> Yes, i wanted to increase the total buffer size too, even wrote it in the description.
> I will prepare a version 4, thanks for the hint.
>
> Just for info: A single RX DMA period aka buffer can be filled with mutliple packets in regard of the upper layer, here BT.
>
>
> Regards,
> Philipp
> >
> > Andy
> >
> >>  static int imx_uart_start_rx_dma(struct imx_port *sport)  {
> >> --
> >> 2.23.0
> >
>
>
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