View source for Linux and interrupt latency (Axel)

===A real-word case: UART RX FIFO overrun===
[[Power_management_(Axel)#Interrupt_latency|This case]] is a typical example that is pretty hard to analyze because a lot of variables are involved and therefore it is extremely difficult to isolate cause-effect relationships. In other words, with the help of software-based tools only, it is not easy to find out which are the drivers that cause the UART RX interrupt latency to explode. Advanced debugging tools providing tracing capabilities such as [https://www.lauterbach.com/ Lauterbach TRACE32/PowerDebug] are extremely useful to perform such investigations. 

DAVE Embedded Systems' R&D team carried out such an analysis on a systems powered by the Axel Lite SoM. In this specific case, Lauterbach tools allowed our team to find out that the SD controller driver affects interrupt latency dramatically. This is due to 1ms delay that is implemented in the driver and that is issued after disabling the interrupts. In the system under investigation, the physical SD card interface is not provided with the card detect signal. Thus, the 1ms interval with disabled interrupt occurs on a regular basis because the polling mechanism used to detect the presence of the card. Consequently, the odds of a RX FIFO interrupt triggered in the middle of such delay are relatively high, causing the overrun issue.