Comment 0 for 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 this patch, it appears that the read() command sometimes fails to receive a response, even when increasing the retry times. This could be due to our use of a spin lock to lock the UART port in the read() function, preventing us from calling read() immediately after a write() function.

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

To fix this we only need a small delay after the write() function to make it has time to clear its buffer.

[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.