On OpenPOWER POWER9 systems, we typically talk to the flash chips that hold firmware for the host (i.e. the POWER9) processor through a daemon running on the BMC (aka service processor) rather than directly. We have host firmware map “windows” on the LPC bus to parts of the flash chip. This flash chip can in fact be a virtual one, constructed dynamically from files on the BMC. Since we’re mapping windows into this flash address space, we have some knowledge as to what IO the host is doing to/from the pnor. We can use this to output data in the blktrace format and feed into existing tools used to analyze IO patterns. So, with a bit of learning of the data format and learning how to drive the various tools, I was ready to patch the BMC daemon (mboxbridge) to get some data out.[…]

https://www.flamingspork.com/blog/2018/11/19/tracing-flash-reads-and-writes-during-boot/