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

[Philipp Puschmann]

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:
>> 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
>