IO Memory (Slideshow)

I/O Memory

Device registers mapped into memory

  • Access is transparent to software

  • Just like ordinary memory

  • … but the device listens

  • ⟶ side effects

Implications

  • Performance optimization are made at every level

    • Compiler may reorder memory access

    • CPU may reorder memory access

  • ⟶ May twist order of access that’s expected by device

I/O Memory: Reservation

Memory “regions”

  • Reserved by drivers (physical address, length)

  • Protection against accidental overlapping access

  • Shows up in /proc/iomem

  • No effect otherwise

    • Access works without

    • But: no reason not to use it

#include <linux/ioport.h>

struct resource *resource = request_mem_region(
   0x20200000, 180, "my-weird-driver");
release_mem_region(0x20200000, 180);

Making I/O Memory Accessible

I/O memory …

  • Not directly accessible (as is physical memory in general)

  • Not managed by struct page (⟶ later)

  • I/O Memory Mapping must be created

#include <asm/io.h>

void *base = ioremap(0x20200000, 180);
iounmap(base);

Accessing I/O Memory

Set of access functions that insert the right compiler and memory barriers …

  • Reading

    • unsigned int ioread8(void *addr);

    • unsigned int ioread16(void *addr);

    • unsigned int ioread32(void *addr);

  • Writing

    • void iowrite8(u8 value, void *addr);

    • void iowrite16(u16 value, void *addr);

    • void iowrite32(u32 value void *addr);

… and a lot more