Avoid UART6 Rx FIFO overrun issue by disabling DMA while in low-latency mode.

[bander9289]

Based on:

http://forum.embeddedarm.com/showthread.php?1170-UART-Rx-Fifo-Overrun-Error

[bander9289]

@fmntf Interesting ... This change was required for UART6 to work for both myself and 1chi. 1chi wasn't fully supportive of the implementation, but he recognized that this change made it work, as at least one other forum member noted as well.

Which kernel release did you see it working out of the box on? If I was building out of the incorrect branch, which is the correct branch? The '3.14-1.0.x-udoo' branch is what is specified on the wiki:

http://www.udoo.org/docs-neo/Kernel_Hacking/Get_the_sources.html

[fmntf]

Hi. Yes, 3.14-1.0.x-udoo is the right branch. Please note there are some changes on the kernel (mostly on the defconfig) between the latest December release and the current branch. I have tested UART6 today and it works without this patch. I double tested it by putting printk in both the branches of the if.

[bander9289]

Good. I'm glad it works without this patch. I didn't realize that you were talking about changes made since December.

[fmntf]

Actually I don't know if the commits in December-January fixed the serial port or not. I did not try to build an older version, today I just tried to build the kernel and found the serial working.

Could you please confirm that it is working for you too?

[modjo756]

Hi Francesco, with udoobuntu2Rc2 it's necessary to add this patch to have Rx functional (tested last week with a fresh install)

[fmntf]

Which patch? The one in this pull request or the one from @graugans? @ektor5 , since you are mantaining this kernel, please have a look and fix this.

[graugans]

The patch is not from me. I have to check when back home

[modjo756]

This is the patch that i use with yocto that working fine :

Index:

git/drivers/tty/serial/imx.c

--- git.orig/drivers/tty/serial/imx.c 2016-05-12 15:30:47.840863698 +0200 +++ git/drivers/tty/serial/imx.c 2016-05-12 15:35:28.120858578 +0200 @@ -1221,7 +1221,8 @@

/* Can we enable the DMA support? */
if (is_imx6q_uart(sport) && !uart_console(port)

• && !sport->dma_is_inited)
• && !sport->dma_is_inited

• && !sport->port.flags == UPF_LOW_LATENCY)
  imx_uart_dma_init(sport);

  if (sport->dma_is_inited) {

[fmntf]

The if is_imx6q_uart fails on imx6sx. (Try to put a printk inside the if.) This patch is worthless for UDOO Neo, isn't it?

[modjo756]

If is_imx6q_uart fails on imx6sx, what is the good instruction ?

[fmntf]

That's not the point. This pull requests is about fixing UART6 on Neo, using code that will not be executed on Neo. I tested UART6 (after exporting it in the device tree editor) and it works for me.

I will definitely close this pull request (because is worthless on Neo). If you find other issues on UART6, please open a separate bug with instructions on how to reproduce it. Thank you!