diff --git a/bitbang_spi.c b/bitbang_spi.c
index ac50e67..6d14961 100644
--- a/bitbang_spi.c
+++ b/bitbang_spi.c
@@ -101,7 +101,8 @@
 		return ERROR_FLASHROM_BUG;
 	}
 
-	mst.data = master;
+	/* Cast away `const`, but local code must ensure it's still treated as such. */
+	mst.data = (struct bitbang_spi_master *)master;
 	register_spi_master(&mst);
 
 	/* Only mess with the bus if we're sure nobody else uses it. */
diff --git a/programmer.h b/programmer.h
index f1915df..1955094 100644
--- a/programmer.h
+++ b/programmer.h
@@ -608,7 +608,7 @@
 	int (*read)(struct flashctx *flash, uint8_t *buf, unsigned int start, unsigned int len);
 	int (*write_256)(struct flashctx *flash, const uint8_t *buf, unsigned int start, unsigned int len);
 	int (*write_aai)(struct flashctx *flash, const uint8_t *buf, unsigned int start, unsigned int len);
-	const void *data;
+	void *data;
 };
 
 int default_spi_send_command(const struct flashctx *flash, unsigned int writecnt, unsigned int readcnt,
@@ -690,7 +690,7 @@
 	int (*read) (struct flashctx *flash, uint8_t *buf, unsigned int start, unsigned int len);
 	int (*write) (struct flashctx *flash, const uint8_t *buf, unsigned int start, unsigned int len);
 	int (*erase) (struct flashctx *flash, unsigned int blockaddr, unsigned int blocklen);
-	const void *data;
+	void *data;
 };
 int register_opaque_master(const struct opaque_master *mst);
 
@@ -714,7 +714,7 @@
 	uint16_t (*chip_readw) (const struct flashctx *flash, const chipaddr addr);
 	uint32_t (*chip_readl) (const struct flashctx *flash, const chipaddr addr);
 	void (*chip_readn) (const struct flashctx *flash, uint8_t *buf, const chipaddr addr, size_t len);
-	const void *data;
+	void *data;
 };
 int register_par_master(const struct par_master *mst, const enum chipbustype buses);
 struct registered_master {