Completion wait timeout error thrown by i2c-ismt.c driver

[Rahul Gore (Nokia)]

Hi,

I have come across the following problem:

Error:
After pasting ~130kb text in console, multiple "ismt_smbus 0000:00:12.0: completion wait timed out" messages are seen in dmesg.

Configuration:
1. Custom Debian-based operating system based on Linux kernel 6.1.25-13-amd64.
2. Sysrq is set to 16.
3. Following peripherals are present:
a. Intel Corporation Atom Processor C3000 Series SMBus Contoller (Driver i2c_ismt).
b. Serial controller: Intel Corporation Atom Processor C3000 Series HSUART Controller (Driver 8250_mid)
4. The original baudrate of the serial controller is 115200, however, we configure its baudrate to 57600.
5. Model name: Intel(R) Atom(TM) CPU C3558 @ 2.20GHz; Number of cores = 4; RAM = 16GB.

Additional observations:
1. The paste operation takes around 61 minutes.
2. Number of interrupts on serial controller during this time is around 175000.
3. Number of interrupts on SMBus controller during this time is around 346000.
4. When sysrq is set to 0, the problem does not occur.
5. The interrupts on the IRQ lines for serial driver and SMBus driver are serviced by the same core.
6. If I ensure that the interrupts on the serial and SMBus driver are managed by different cores, the problem does not occur.
7. During the paste process, the CPU usage of the core (that handles both interrupts) goes to 100%, then suddenly drops to 0%, before returning to normal values (around 90%).
8. If the baudrate is set to 115200 (original value), the problem does not occur.

I studied the source code for the serial driver. I could not find anything which may have caused other interrupts to get blocked.

I would appreciate any direction and/or explanation for this behavior. Please let me know if any additional information is needed from my end.

Regards,
Rahul

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