Linux Kernel UART Hibernation Warning Issue

A warning related to clock management has been observed in the Linux kernel's handling of non-console UART instances during system hibernation. This issue arises because the .thaw_noirq hook is missing, leading to an unbalanced clock state. When the system hibernates, the power management core calls various hooks in a specific sequence. Without the .thaw_noirq hook, the clock is disabled prematurely, causing a warning to be logged. The missing hook can be added to restore the correct sequence and eliminate the warning.

Impact

The absence of the .thaw_noirq hook can lead to improper clock management, causing warnings during system hibernation. This could potentially disrupt the normal power management process, although it does not appear to cause any severe functional issues.

Reproduction

To reproduce this issue, observe the system's behavior during hibernation while a non-console UART instance is active. The warning about the uart3_root_clk being already disabled will be displayed, indicating that the clock was turned off without being properly re-enabled first. This unbalanced handling is what triggers the warning.

Remediation

The issue can be fixed by adding the .thaw_noirq hook to the UART's power management operations. This adjustment ensures that the clock is correctly managed during the hibernation process, preventing the warning from occurring.