diff --git a/hwaccess.c b/hwaccess.c
index 1901ee6..aede7e3 100644
--- a/hwaccess.c
+++ b/hwaccess.c
@@ -147,22 +147,22 @@
 	sync_primitive();
 }
 
-uint8_t mmio_readb(void *addr)
+uint8_t mmio_readb(const void *addr)
 {
-	return *(volatile uint8_t *) addr;
+	return *(volatile const uint8_t *) addr;
 }
 
-uint16_t mmio_readw(void *addr)
+uint16_t mmio_readw(const void *addr)
 {
-	return *(volatile uint16_t *) addr;
+	return *(volatile const uint16_t *) addr;
 }
 
-uint32_t mmio_readl(void *addr)
+uint32_t mmio_readl(const void *addr)
 {
-	return *(volatile uint32_t *) addr;
+	return *(volatile const uint32_t *) addr;
 }
 
-void mmio_readn(void *addr, uint8_t *buf, size_t len)
+void mmio_readn(const void *addr, uint8_t *buf, size_t len)
 {
 	memcpy(buf, addr, len);
 	return;
@@ -183,17 +183,17 @@
 	mmio_writel(cpu_to_le32(val), addr);
 }
 
-uint8_t mmio_le_readb(void *addr)
+uint8_t mmio_le_readb(const void *addr)
 {
 	return le_to_cpu8(mmio_readb(addr));
 }
 
-uint16_t mmio_le_readw(void *addr)
+uint16_t mmio_le_readw(const void *addr)
 {
 	return le_to_cpu16(mmio_readw(addr));
 }
 
-uint32_t mmio_le_readl(void *addr)
+uint32_t mmio_le_readl(const void *addr)
 {
 	return le_to_cpu32(mmio_readl(addr));
 }
diff --git a/programmer.h b/programmer.h
index 3853413..1a6216a 100644
--- a/programmer.h
+++ b/programmer.h
@@ -351,16 +351,16 @@
 void mmio_writeb(uint8_t val, void *addr);
 void mmio_writew(uint16_t val, void *addr);
 void mmio_writel(uint32_t val, void *addr);
-uint8_t mmio_readb(void *addr);
-uint16_t mmio_readw(void *addr);
-uint32_t mmio_readl(void *addr);
-void mmio_readn(void *addr, uint8_t *buf, size_t len);
+uint8_t mmio_readb(const void *addr);
+uint16_t mmio_readw(const void *addr);
+uint32_t mmio_readl(const void *addr);
+void mmio_readn(const void *addr, uint8_t *buf, size_t len);
 void mmio_le_writeb(uint8_t val, void *addr);
 void mmio_le_writew(uint16_t val, void *addr);
 void mmio_le_writel(uint32_t val, void *addr);
-uint8_t mmio_le_readb(void *addr);
-uint16_t mmio_le_readw(void *addr);
-uint32_t mmio_le_readl(void *addr);
+uint8_t mmio_le_readb(const void *addr);
+uint16_t mmio_le_readw(const void *addr);
+uint32_t mmio_le_readl(const void *addr);
 #define pci_mmio_writeb mmio_le_writeb
 #define pci_mmio_writew mmio_le_writew
 #define pci_mmio_writel mmio_le_writel