This project implements a driver for Winbond W25N01GVxxIG/IT flash chips. Because there are no embedded-hal traits for QSPI, for now I've directly added a dependency to the stm32l4xx-hal library that I'll personally be using to interface with the flash chips. I may write a temporary QSPI trait in the future, and ideally I'll shift the library to use any QSPI traits from embedded-hal when (if) they come out. Until then feel free to adapt the library to your own needs.
Some basic examples can be found in the examples folder. write_read writes a couple values to the first page of the first block and reads it back via semihosting. validate continually writes and reads back pages sequentially in the first block and alerts when bytes read back incorrectly. This is useful for checking QSPI bus speeds, wire length, interference, etc.
One thing to note that I don't believe is clearly explained in the data sheet: writing must be sequential. These flash chips are broken into blocks, and each block is broken down into pages. Within a block, pages must be written sequentially from lowest address to highest address. If you attempt to write a page out of order, it will silently corrupt the data in that page. Random reads are fine, but random writes are not.