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

[Andy Duan]

From: Philipp Puschmann <philipp.puschmann@xxxxxxxxx> Sent: Friday, September 20, 2019 3:06 PM
> 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:
> >> 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.

Okay, thank you for submit the SDMA/uart patch set.

> 
> Just for info: A single RX DMA period aka buffer can be filled with mutliple
> packets in regard of the upper layer, here BT.

Yes, that depends on system loading. 

> 
> 
> Regards,
> Philipp
> >
> > Andy
> >
> >>  static int imx_uart_start_rx_dma(struct imx_port *sport)  {
> >> --
> >> 2.23.0
> >