qmk_firmware/docs/flash_driver.md

FLASH Driver Configuration

The FLASH driver can be swapped out depending on the needs of the keyboard, or whether extra hardware is present.

Driver	Description
FLASH_DRIVER = spi	Supports writing to almost all NOR Flash chips. See the driver section below.

SPI FLASH Driver Configuration

Currently QMK supports almost all NOR Flash chips over SPI. As such, requires a working spi_master driver configuration. You can override the driver configuration via your config.h:

config.h override	Description	Default Value
#define EXTERNAL_FLASH_SPI_SLAVE_SELECT_PIN	SPI Slave select pin in order to inform that the FLASH is currently being addressed	none
#define EXTERNAL_FLASH_SPI_CLOCK_DIVISOR	Clock divisor used to divide the peripheral clock to derive the SPI frequency	8
#define EXTERNAL_FLASH_PAGE_SIZE	The Page size of the FLASH in bytes, as specified in the datasheet	256
#define EXTERNAL_FLASH_SECTOR_SIZE	The sector size of the FLASH in bytes, as specified in the datasheet	(4 * 1024)
#define EXTERNAL_FLASH_BLOCK_SIZE	The block size of the FLASH in bytes, as specified in the datasheet	(64 * 1024)
#define EXTERNAL_FLASH_SIZE	The total size of the FLASH in bytes, as specified in the datasheet	(512 * 1024)
#define EXTERNAL_FLASH_ADDRESS_SIZE	The Flash address size in bytes, as specified in datasheet	3

::: warning All the above default configurations are based on MX25L4006E NOR Flash. :::