The part of the interface most used by drivers is reading and writing memory-mapped registers on the device. Linux provides interfaces to read and write 8-bit, 16-bit, 32-bit and 64-bit quantities. Due to a historical accident, these are named byte, word, long and quad accesses. Both read and write accesses are supported; there is no prefetch support at this time.

The functions are named readb, readw, readl, readq, readb_relaxed, readw_relaxed, readl_relaxed, readq_relaxed, writeb, writew, writel and writeq.

Some devices (such as framebuffers) would like to use larger transfers than 8 bytes at a time. For these devices, the memcpy_toio, memcpy_fromio and memset_io functions are provided. Do not use memset or memcpy on IO addresses; they are not guaranteed to copy data in order.

The read and write functions are defined to be ordered. That is the compiler is not permitted to reorder the I/O sequence. When the ordering can be compiler optimised, you can use __readb and friends to indicate the relaxed ordering. Use this with care.

While the basic functions are defined to be synchronous with respect to each other and ordered with respect to each other the busses the devices sit on may themselves have asynchronicity. In particular many authors are burned by the fact that PCI bus writes are posted asynchronously. A driver author must issue a read from the same device to ensure that writes have occurred in the specific cases the author cares. This kind of property cannot be hidden from driver writers in the API. In some cases, the read used to flush the device may be expected to fail (if the card is resetting, for example). In that case, the read should be done from config space, which is guaranteed to soft-fail if the card doesn't respond.

The following is an example of flushing a write to a device when the driver would like to ensure the write's effects are visible prior to continuing execution.

static inline void
qla1280_disable_intrs(struct scsi_qla_host *ha)
{
	struct device_reg *reg;

	reg = ha->iobase;
	/* disable risc and host interrupts */
	WRT_REG_WORD(&reg->ictrl, 0);
	/*
	 * The following read will ensure that the above write
	 * has been received by the device before we return from this
	 * function.
	 */
	RD_REG_WORD(&reg->ictrl);
	ha->flags.ints_enabled = 0;
}

In addition to write posting, on some large multiprocessing systems (e.g. SGI Challenge, Origin and Altix machines) posted writes won't be strongly ordered coming from different CPUs. Thus it's important to properly protect parts of your driver that do memory-mapped writes with locks and use the mmiowb to make sure they arrive in the order intended. Issuing a regular readX will also ensure write ordering, but should only be used when the driver has to be sure that the write has actually arrived at the device (not that it's simply ordered with respect to other writes), since a full readX is a relatively expensive operation.

Generally, one should use mmiowb prior to releasing a spinlock that protects regions using writeb or similar functions that aren't surrounded by readb calls, which will ensure ordering and flushing. The following pseudocode illustrates what might occur if write ordering isn't guaranteed via mmiowb or one of the readX functions.

CPU A:  spin_lock_irqsave(&dev_lock, flags)
CPU A:  ...
CPU A:  writel(newval, ring_ptr);
CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
        ...
CPU B:  spin_lock_irqsave(&dev_lock, flags)
CPU B:  writel(newval2, ring_ptr);
CPU B:  ...
CPU B:  spin_unlock_irqrestore(&dev_lock, flags)

In the case above, newval2 could be written to ring_ptr before newval. Fixing it is easy though:

CPU A:  spin_lock_irqsave(&dev_lock, flags)
CPU A:  ...
CPU A:  writel(newval, ring_ptr);
CPU A:  mmiowb(); /* ensure no other writes beat us to the device */
CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
        ...
CPU B:  spin_lock_irqsave(&dev_lock, flags)
CPU B:  writel(newval2, ring_ptr);
CPU B:  ...
CPU B:  mmiowb();
CPU B:  spin_unlock_irqrestore(&dev_lock, flags)

See tg3.c for a real world example of how to use mmiowb

PCI ordering rules also guarantee that PIO read responses arrive after any outstanding DMA writes from that bus, since for some devices the result of a readb call may signal to the driver that a DMA transaction is complete. In many cases, however, the driver may want to indicate that the next readb call has no relation to any previous DMA writes performed by the device. The driver can use readb_relaxed for these cases, although only some platforms will honor the relaxed semantics. Using the relaxed read functions will provide significant performance benefits on platforms that support it. The qla2xxx driver provides examples of how to use readX_relaxed. In many cases, a majority of the driver's readX calls can safely be converted to readX_relaxed calls, since only a few will indicate or depend on DMA completion.