IMX6ULL UART: Missed DMA callback in high speed transfers

[arnoutdiels]

Hello,

We have been running into an issue with the ttymxc (uart) on the IMX6ULL on linux on 1Mbaud communcation.

We had to backport the v5.2 mxc.c serial driver and imx-sdma.c driver to our v4.9 branch to fix a number of issues, but now this works stable.

With the following exception:

- When two 1Mbaud transfers occur very close to each other ( in our case, a transfer of 9 bytes, followed by 100us idle, then 256bytes), the imx_uart_dma_rx_callback is only called once (I checked the icount.rx), and only has the first 9 bytes inside.

- When the same sequence is sent again, we suddenly receive the other 256 bytes + 9 bytes of the next sequence. (the new 256 bytes again are stored in a DMA buffer, but are not flushed out).

For now, we work around this by simply making sure the idle time between transfers > 200us, but this is not ideal.

Is this a known issue? 

Thanks in advance for any feedback.

Kind regards,

Arnout

[Yuri]

Hello,

  Below are some comments regarding the issue.

1.

   It is recommended to use flow control (RTS/CTS) for UART with DMA  to avoid

data lost.

2.

   DMA transfers data as bursts, containing some minimal (threshold) number of bytes.

Data will be transferred when threshold  is achieved.   

Have a great day,
Yuri

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