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Unlike TX interrupt, ST201 does not provide any mechanism to suppress RX interrupts. ste(4) can generate more than 70k RX interrupts under heavy RX traffics such that these excessive interrupts make system useless to process other useful things. Maybe this was the major reason why polling support code was introduced to ste(4). The STE_COUNTDOWN register provides a programmable counter that will generate an interrupt upon its expiration. We program STE_DMACTL register to use 3.2us clock rate to drive the counter register. Whenever ste(4) serves RX interrupt, the driver rearm the timer to expire after STE_IM_RX_TIMER_DEFAULT time and disables further generation of RX interrupts. This trick seems to work well and ste(4) generates less than 8k RX interrupts even under 64 bytes UDP torture test. Combined with TX interrupts, the total number of interrupts are less than 10k which looks reasonable on heavily loaded controller. The default RX interrupt moderation time is 150us. Users can change the value at any time with dev.ste.%d.int_rx_mod sysctl node. Setting it 0 effectively disables the RX interrupt moderation feature. Now we have both TX/RX interrupt moderation code so remove loop of interrupt handler which resulted in sub-optimal performance as well as more register accesses.
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