ARM Cortex-A53 System Counter and Kernel Boot Timing Discrepancy
The issue revolves around a significant discrepancy between the kernel boot timestamps reported by the ARM Cortex-A53-based Rockchip RK3568 board and the actual boot time measured using an external device (iPhone). The kernel reports a boot time of 10 seconds via dmesg, while the actual boot time, measured manually, is approximately 3 minutes. This discrepancy suggests a misalignment between the system counter configuration and the kernel’s timing mechanisms.
The ARM Cortex-A53 processor, part of the ARMv8-A architecture, relies on the system counter (CNTFRQ_EL0) for timing. The system counter is a high-resolution timer that provides a consistent timebase for the system. In this case, the system counter frequency is set to 20 MHz, as confirmed by the dmesg output: arch_timer: cp15 timer(s) running at 20.00MHz (phys). However, the kernel’s reported boot time does not align with the real-world elapsed time, indicating a potential issue with how the system counter is being utilized during the boot process.
The kernel boot process involves several stages, including firmware initialization, bootloader execution, and finally, the Linux kernel initialization. The dmesg timestamps are generated by the kernel once it starts executing, specifically after the start_kernel() function is called. This means that any time spent in the firmware or bootloader stages is not accounted for in the dmesg output. However, the discrepancy of 2 minutes and 50 seconds (3 minutes actual vs. 10 seconds reported) is too large to be explained solely by the firmware and bootloader execution time. This suggests that the system counter configuration or its interaction with the kernel is not functioning as expected.
System Counter Frequency Misconfiguration and Firmware Boot Overhead
The primary cause of the timing discrepancy lies in the misconfiguration of the system counter (CNTFRQ_EL0) and the potential overhead introduced by the firmware and bootloader stages. The system counter frequency is set to 20 MHz, which should provide a high-resolution timebase for the kernel. However, if the system counter is not properly synchronized with the kernel’s timing mechanisms, the reported timestamps will be inaccurate.
One possible cause is that the system counter is not initialized or started until after the kernel begins execution. This would result in the kernel missing a significant portion of the boot time, as the counter would not be incrementing during the firmware and bootloader stages. Additionally, if the system counter frequency is not correctly propagated to the kernel, the kernel may misinterpret the counter values, leading to incorrect timestamp calculations.
Another potential cause is the overhead introduced by the firmware and bootloader. The Rockchip RK3568 board uses a complex firmware stack, which may include stages such as ARM Trusted Firmware (ATF) and U-Boot. These stages can take a significant amount of time to execute, especially if they involve hardware initialization, security checks, or loading large binaries from storage. If the kernel does not account for this overhead, the reported boot time will be significantly shorter than the actual elapsed time.
Finally, there may be issues with the kernel’s timing subsystem itself. The ARM architecture relies on the Generic Timer, which is part of the ARMv8-A architecture. The Generic Timer is driven by the system counter, and any misconfiguration of the timer or counter can lead to incorrect timing calculations. For example, if the kernel is not properly configured to use the Generic Timer, it may fall back to a less accurate timing source, leading to discrepancies in the reported timestamps.
Correcting System Counter Configuration and Kernel Timing Mechanisms
To resolve the timing discrepancy, several steps must be taken to ensure that the system counter and kernel timing mechanisms are correctly configured and synchronized.
First, verify that the system counter frequency is correctly set and propagated to the kernel. The system counter frequency is set using the CNTFRQ_EL0 register, and this value must be correctly passed to the kernel during boot. This can be done by checking the bootloader (e.g., U-Boot) configuration to ensure that the CNTFRQ_EL0 value is correctly set and passed to the kernel via the device tree or boot arguments. If the frequency is not correctly set, the kernel will misinterpret the counter values, leading to incorrect timestamps.
Second, ensure that the system counter is started as early as possible in the boot process. The system counter should be initialized and started by the firmware or bootloader, before the kernel begins execution. This will ensure that the counter is incrementing throughout the entire boot process, including the firmware and bootloader stages. If the counter is not started until after the kernel begins execution, the kernel will miss a significant portion of the boot time, leading to inaccurate timestamps.
Third, investigate the firmware and bootloader overhead. The firmware and bootloader stages can take a significant amount of time to execute, especially on complex boards like the Rockchip RK3568. To reduce this overhead, optimize the firmware and bootloader configuration to minimize unnecessary delays. For example, disable unnecessary hardware initialization or security checks that are not required for the specific use case. Additionally, ensure that the firmware and bootloader are using the latest versions, as newer versions may include performance improvements or bug fixes that reduce boot time.
Fourth, verify that the kernel is correctly configured to use the Generic Timer. The Generic Timer is the preferred timing source for ARMv8-A architectures, and it provides high-resolution timing that is synchronized with the system counter. Ensure that the kernel is configured to use the Generic Timer by checking the kernel configuration (e.g., .config file) for the CONFIG_ARM_ARCH_TIMER option. If this option is not enabled, the kernel may fall back to a less accurate timing source, leading to incorrect timestamps.
Finally, if the above steps do not resolve the issue, consider adding additional logging to the firmware, bootloader, and kernel to track the timing of each stage of the boot process. This will help identify where the discrepancy is occurring and provide more detailed information for further troubleshooting. For example, add logging to the firmware and bootloader to record the start and end times of each stage, and compare these timestamps with the kernel’s dmesg output to identify any gaps or inconsistencies.
By following these steps, the timing discrepancy between the kernel boot timestamps and the actual boot time can be resolved, ensuring accurate and reliable timing for the ARM Cortex-A53-based Rockchip RK3568 board.
