diff --git a/firmware/ice40-riscv/common/spi.c b/firmware/ice40-riscv/common/spi.c
index 9072d27..0a4414d 100644
--- a/firmware/ice40-riscv/common/spi.c
+++ b/firmware/ice40-riscv/common/spi.c
@@ -225,7 +225,7 @@
 }
 
 void
-flash_page_program(void *src, uint32_t addr, unsigned len)
+flash_page_program(const void *src, uint32_t addr, unsigned len)
 {
 	uint8_t cmd[4] = { FLASH_CMD_PAGE_PROGRAM, ((addr >> 16) & 0xff), ((addr >> 8) & 0xff), (addr & 0xff)  };
 	struct spi_xfer_chunk xfer[2] = {
diff --git a/firmware/ice40-riscv/common/spi.h b/firmware/ice40-riscv/common/spi.h
index 3a95358..aab5256 100644
--- a/firmware/ice40-riscv/common/spi.h
+++ b/firmware/ice40-riscv/common/spi.h
@@ -32,5 +32,5 @@
 uint8_t flash_read_sr(void);
 void flash_write_sr(uint8_t sr);
 void flash_read(void *dst, uint32_t addr, unsigned len);
-void flash_page_program(void *src, uint32_t addr, unsigned len);
+void flash_page_program(const void *src, uint32_t addr, unsigned len);
 void flash_sector_erase(uint32_t addr);