Regarding the way 2.4.19 handles the 4xx watchdog: The 4xx timer interrupt, the PIT, is the lowest priority exception (18). All the external IRQs come in at a higher priority (16). This is opposite of x86, where the timer ISR is highest priority (0).
This would seem to negate the effectiveness of drivers which implement "work limits" in their ISRs (like many ethernet drivers). These drivers do an RFI after doing a certain amount of work, leaving the interrupt pending and returning later to finish up. In the case of 4xx, a pending PIT interrupt would not be serviced due to its lower priority. In the case of heavy IRQ load, the watchdog can expire. Has anyone considered implementing the 4xx watchdog interrupt itself as the place to refresh the watchdog? We could still use the existing method of a heartbeat counter decremented each time, but just do the refreshing in a watchdog ISR rather than the timer ISR. This way the watchdog could be reliably refreshed even during heavy IRQ loads. Any comments? -Brian ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/
