APPLE_SPI_IF_FIFO
u32 fifo = reg_read(spi, APPLE_SPI_IF_FIFO) & reg_read(spi, APPLE_SPI_IE_FIFO);
fifo = reg_read(spi, APPLE_SPI_IF_FIFO);
reg_write(spi, APPLE_SPI_IF_FIFO, ~0);
fifo_flags = reg_read(spi, APPLE_SPI_IF_FIFO);