[PATCH 0/1][SRU][M] dell-uart-backlight fails to communicate

[AceLan Kao]

From: "Chia-Lin Kao (AceLan)" <acelan.kao at canonical.com>

BugLink: https://bugs.launchpad.net/bug/2035299

[Impact]
Boots up the system and sometimes we can see below messages and that leads
to backlight driver doesn't get loaded.

[ 86.424880] dell_uart_backlight:dell_uart_get_display_mode:tx: 6a 10 85
[ 86.603936] [7439] dell_uart_backlight:dell_uart_get_display_mode:rx: NULL
[ 87.596047] dell_uart_backlight:dell_uart_get_display_mode:tx: 6a 10 85
[ 87.775209] [7439] dell_uart_backlight:dell_uart_get_display_mode:rx: NULL

[Fix]
After applying the patch below, it seems that the read() command may fail
to receive a response, even when increasing the retry times. It never gets
a response when it fails.

fbf84fb368923 ("UBUNTU: SAUCE: platform/x86: dell-uart-backlight: replace chars_in_buffer() with flush_chars()")

To fix this, try adding a small delay after the write() function as a
workaround.

[Test case]
1. Add dell_uart_backlight.dyndbg="+pt" to the kernel cmdline
2. Boots up the system and check dmesg
3. Make sure there is no dell_uart_backlight rx NULL message in dmesg
4. Try to unload/reload dell_uart_backlight couple times and check dmesg again

[Where problems could occur]
sleep for 1ms is pretty short that it's not possible to lead to messages loss.

Chia-Lin Kao (AceLan) (1):
  UBUNTU: SAUCE: platform/x86: dell-uart-backlight: add small delay
    after write command

 drivers/platform/x86/dell/dell-uart-backlight.c | 5 +++++
 1 file changed, 5 insertions(+)

-- 
2.34.1