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Linux Device Drivers, Second Edition by Alessandro Rubini, Jonathan Corbet

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Chapter 8. Hardware Management

Although playing with scull and similar toys is a good introduction to the software interface of a Linux device driver, implementing a real device requires hardware. The driver is the abstraction layer between software concepts and hardware circuitry; as such, it needs to talk with both of them. Up to now, we have examined the internals of software concepts; this chapter completes the picture by showing you how a driver can access I/O ports and I/O memory while being portable across Linux platforms.

This chapter continues in the tradition of staying as independent of specific hardware as possible. However, where specific examples are needed, we use simple digital I/O ports (like the standard PC parallel port) to show how the I/O instructions work, and normal frame-buffer video memory to show memory-mapped I/O.

We chose simple digital I/O because it is the easiest form of input/output port. Also, the Centronics parallel port implements raw I/O and is available in most computers: data bits written to the device appear on the output pins, and voltage levels on the input pins are directly accessible by the processor. In practice, you have to connect LEDs to the port to actually see the results of a digital I/O operation, but the underlying hardware is extremely easy to use.

I/O Ports and I/O Memory

Every peripheral device is controlled by writing and reading its registers. Most of the time a device has several registers, and they are accessed at consecutive addresses, either in the memory address space or in the I/O address space.

At the hardware level, there is no conceptual difference between memory regions and I/O regions: both of them are accessed by asserting electrical signals on the address bus and control bus (i.e., the read and write signals)[31] and by reading from or writing to the data bus.

While some CPU manufacturers implement a single address space in their chips, some others decided that peripheral devices are different from memory and therefore deserve a separate address space. Some processors (most notably the x86 family) have separate read and write electrical lines for I/O ports, and special CPU instructions to access ports.

Because peripheral devices are built to fit a peripheral bus, and the most popular I/O buses are modeled on the personal computer, even processors that do not have a separate address space for I/O ports must fake reading and writing I/O ports when accessing some peripheral devices, usually by means of external chipsets or extra circuitry in the CPU core. The latter solution is only common within tiny processors meant for embedded use.

For the same reason, Linux implements the concept of I/O ports on all computer platforms it runs on, even on platforms where the CPU implements a single address space. The implementation of port access sometimes depends on the specific make and model of the host computer (because different models use different chipsets to map bus transactions into memory address space).

Even if the peripheral bus has a separate address space for I/O ports, not all devices map their registers to I/O ports. While use of I/O ports is common for ISA peripheral boards, most PCI devices map registers into a memory address region. This I/O memory approach is generally preferred because it doesn’t require use of special-purpose processor instructions; CPU cores access memory much more efficiently, and the compiler has much more freedom in register allocation and addressing-mode selection when accessing memory.

I/O Registers and Conventional Memory

Despite the strong similarity between hardware registers and memory, a programmer accessing I/O registers must be careful to avoid being tricked by CPU (or compiler) optimizations that can modify the expected I/O behavior.

The main difference between I/O registers and RAM is that I/O operations have side effects, while memory operations have none: the only effect of a memory write is storing a value to a location, and a memory read returns the last value written there. Because memory access speed is so critical to CPU performance, the no-side-effects case has been optimized in several ways: values are cached and read/write instructions are reordered.

The compiler can cache data values into CPU registers without writing them to memory, and even if it stores them, both write and read operations can operate on cache memory without ever reaching physical RAM. Reordering can also happen both at compiler level and at hardware level: often a sequence of instructions can be executed more quickly if it is run in an order different from that which appears in the program text, for example, to prevent interlocks in the RISC pipeline. On CISC processors, operations that take a significant amount of time can be executed concurrently with other, quicker ones.

These optimizations are transparent and benign when applied to conventional memory (at least on uniprocessor systems), but they can be fatal to correct I/O operations because they interfere with those “side effects” that are the main reason why a driver accesses I/O registers. The processor cannot anticipate a situation in which some other process (running on a separate processor, or something happening inside an I/O controller) depends on the order of memory access. A driver must therefore ensure that no caching is performed and no read or write reordering takes place when accessing registers: the compiler or the CPU may just try to outsmart you and reorder the operations you request; the result can be strange errors that are very difficult to debug.

The problem with hardware caching is the easiest to face: the underlying hardware is already configured (either automatically or by Linux initialization code) to disable any hardware cache when accessing I/O regions (whether they are memory or port regions).

The solution to compiler optimization and hardware reordering is to place a memory barrier between operations that must be visible to the hardware (or to another processor) in a particular order. Linux provides four macros to cover all possible ordering needs.

#include <linux/kernel.h> , void barrier(void)

This function tells the compiler to insert a memory barrier, but has no effect on the hardware. Compiled code will store to memory all values that are currently modified and resident in CPU registers, and will reread them later when they are needed.

#include <asm/system.h> , void rmb(void); , void wmb(void); , void mb(void);

These functions insert hardware memory barriers in the compiled instruction flow; their actual instantiation is platform dependent. An rmb (read memory barrier) guarantees that any reads appearing before the barrier are completed prior to the execution of any subsequent read. wmb guarantees ordering in write operations, and the mb instruction guarantees both. Each of these functions is a superset of barrier.

A typical usage of memory barriers in a device driver may have this sort of form:

writel(dev->registers.addr, io_destination_address);
writel(dev->registers.size, io_size);
writel(dev->registers.operation, DEV_READ);
writel(dev->registers.control, DEV_GO);

In this case, it is important to be sure that all of the device registers controlling a particular operation have been properly set prior to telling it to begin. The memory barrier will enforce the completion of the writes in the necessary order.

Because memory barriers affect performance, they should only be used where really needed. The different types of barriers can also have different performance characteristics, so it is worthwhile to use the most specific type possible. For example, on the x86 architecture, wmb() currently does nothing, since writes outside the processor are not reordered. Reads are reordered, however, so mb() will be slower than wmb().

It is worth noting that most of the other kernel primitives dealing with synchronization, such as spinlock and atomic_t operations, also function as memory barriers.

Some architectures allow the efficient combination of an assignment and a memory barrier. Version 2.4 of the kernel provides a few macros that perform this combination; in the default case they are defined as follows:

#define set_mb(var, value)  do {var = value; mb();}  while 0
#define set_wmb(var, value) do {var = value; wmb();} while 0
#define set_rmb(var, value) do {var = value; rmb();} while 0

Where appropriate, <asm/system.h> defines these macros to use architecture-specific instructions that accomplish the task more quickly.

The header file sysdep.h defines macros described in this section for the platforms and the kernel versions that lack them.

[31] Not all computer platforms use a read and a write signal; some have different means to address external circuits. The difference is irrelevant at software level, however, and we’ll assume all have read and write to simplify the discussion.

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