BBB UART sometimes skips first byte and duplicates last one

[spanceac]

Hi,

We observed some erratic UART behavior on BBB using this kernel at tag 4.14.53-ti-r62.

Fortunately, this is very easy to reproduce.

Take a BBB and load the UART4 overlay:

enable_uboot_overlays=1
uboot_overlay_addr0=/boot/overlays/BB-UART4-00A0.dtbo

Connect the RX of a serial cable to the UART4 TX(pin P9_13) and open a terminal on this serial cable with 9600 8N1 settings.

On the BBB, run the following script that echoes some text in a loop on UART4:

while true; do
echo 123456789abcdefghijklmnopqrstuvxyz > /dev/ttyS4
done

The expected output is:

123456789abcdefghijklmnopqrstuvxyz
123456789abcdefghijklmnopqrstuvxyz
123456789abcdefghijklmnopqrstuvxyz
123456789abcdefghijklmnopqrstuvxyz

But after some time(usually less than 60s) you will see an output of this type:

123456789abcdefghijklmnopqrstuvxyz
123456789abcdefghijklmnopqrstuvxyz
23456789abcdefghijklmnopqrstuvxyz

123456789abcdefghijklmnopqrstuvxyz
123456789abcdefghijklmnopqrstuvxyz

You can notice how the first byte 1 disappeared and the last byte(new line) was duplicated.

Sometimes it happens that only the first byte is skipped but the last one is not duplicated, output becoming:

123456789abcdefghijklmnopqrstuvxyz
23456789abcdefghijklmnopqrstuvxyz
123456789abcdefghijklmnopqrstuvxyz
123456789abcdefghijklmnopqrstuvxyz

Do you know what could cause this and if there is an existing fix?

Thank you!

[spanceac]

I also want to mention that this problem is specific to the OMAP 8250 driver.

The other OMAP serial driver doesn't exhibit this problem.

The problem reproduces if we activate CONFIG_SERIAL_8250_OMAP in the kernel.

If instead we activate CONFIG_SERIAL_OMAP in the kernel, the problem will not reproduce.

[pdp7]

@spanceac @RobertCNelson is there a reason that CONFIG_SERIAL_OMAP can not just be used?

[pdp7]

@RobertCNelson so I should test this as we only support CONFIG_SERIAL_8250_OMAP in the current kernel builds, right?