Chapter 2 Overview of Solaris Device Drivers

This chapter gives an overview of Solaris device drivers. It provides information on the following subjects:

• What Is a Device Driver?

• Types of Device Drivers

• Driver Module Entry Points

• Device Driver Design Considerations

What Is a Device Driver?

A device driver is a kernel module responsible for managing low-level I/O operations for a particular hardware device. Device drivers can also be software-only, emulating a device that exists only in software, such as a RAM disk or a pseudo-terminal.

A device driver contains all the device-specific code necessary to communicate with a device and provides a standard set of interfaces to the rest of the system. This interface protects the kernel from device specifics just as the system call interface protects application programs from platform specifics. Application programs and the rest of the kernel need little (if any) device-specific code to address the device. In this way, device drivers make the system more portable and easier to maintain.

Types of Device Drivers

There are several kinds of device drivers, each handling a different kind of I/O. Block device drivers manage devices with physically addressable storage media, such as disks. All other devices are considered character devices. Two types of character device drivers are standard character device drivers and STREAMS device drivers.

Block Device Drivers

Devices that support a file system are known as block devices. Drivers written for these devices are known as block device drivers. Block device drivers take a file system request, in the form of a buf(9S) structure, and issue the I/O operations to the disk to transfer the specified block. The main interface to the file system is the strategy(9E) routine. See Chapter 11, Drivers for Block Devices for more information.

Block device drivers can also provide a character driver interface that allows utility programs to bypass the file system and access the device directly. This device access is commonly referred to as the raw interface to a block device.

Character Device Drivers

Character device drivers normally perform I/O in a byte stream. Examples of devices using character drivers include tape drives and serial ports. Character device drivers can also provide additional interfaces not present in block drivers, such as I/O control (ioctl) commands, memory mapping, and device polling. See Chapter 10, Drivers for Character Devices for more information.

Byte-Stream I/O

The main task of any device driver is to perform I/O, and many character device drivers do what is called byte-stream or character I/O. The driver transfers data to and from the device without using a specific device address. This is in contrast to block device drivers, where part of the file system request identifies a specific location on the device.

The read(9E) and write(9E) entry points handle byte-stream I/O for standard character drivers. See I/O Request Handling for more information.

Memory Mapped Devices

For certain devices, such as frame buffers, application programs having direct access to device memory is more efficient than byte-stream I/O. Applications can map device memory into their address spaces using the mmap(2) system call. To support memory mapping, device drivers implement segmap(9E) and devmap(9E) entry points. For information on devmap(9E), see Chapter 12, Mapping Device and Kernel Memory. For information on segmap(9E), see Chapter 10, Drivers for Character Devices.

Drivers that define the devmap(9E) entry point usually do not define read(9E) and write(9E) entry points, as application programs perform I/O directly to the devices after calling mmap(2).

STREAMS Drivers

STREAMS is a separate programming model for writing a character driver. Devices that receive data asynchronously (such as terminal and network devices) are suited to a STREAMS implementation. STREAMS device drivers must provide the loading and autoconfiguration support described in Chapter 5, Driver Autoconfiguration. See the Streams Programming Guide for additional information on how to write STREAMS drivers.

Driver Module Entry Points

Each device driver defines a standard set of functions called entry points, which are listed in Intro(9E). These entry points are called by the Solaris kernel to load and unload the driver, autoconfigure devices, and provide the character, block, or STREAMS driver I/O services. Drivers for different types of devices have different sets of entry points according to the kinds of operations the devices perform. A driver for a memory-mapped character-oriented device, for example, supports a devmap(9E) entry point, while a block driver does not support this entry.

Figure 2–1 Device Driver Overview

Some operations are common to all drivers, such as the functions that are required for module loading (_init(9E), _info(9E), and _fini(9E)), and the required autoconfiguration entry points attach(9E), detach(9E), and getinfo(9E). Drivers also support the optional autoconfiguration entry point for probe(9E). Most leaf drivers have open(9E) and close(9E) entry points to control access to their devices.

Traditionally, all driver function and variable names have some prefix added to them. Usually this is the name of the driver, such as xxopen() for the open(9E) routine of driver xx. In subsequent examples, xx is used as the driver prefix.

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In the Solaris 9 operating environment, only the loadable module routines must be visible outside the driver object module. Other routines can have the storage class static.

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Loadable Module Entry Points

All drivers are required to implement the loadable module entry points _init(9E), _fini(9E), and _info(9E) to load, unload, and report information about the driver module.

Drivers should allocate and initialize any global resources in _init(9E) and release their resources in _fini(9E).

Autoconfiguration Entry Points

Drivers are required to implement the attach(9E), detach(9E), and getinfo(9E) entry points for device autoconfiguration. Drivers might need to implement probe(9E) if the driver supports devices that are not self identifying, such as SCSI target devices. See Chapter 5, Driver Autoconfiguration for more information on these routines.

Character and Block Driver Entry Points

Drivers for character and block devices export a cb_ops(9S) structure, which defines the driver entry points for block device access and character device access. Both types of drivers are required to support open(9E) and close(9E). Block drivers are required to support strategy(9E), while character drivers can choose to implement whatever mix of read(9E), write(9E), ioctl(9E), mmap(9E), or devmap(9E) entry points as appropriate for the type of device. Character drivers can also support a polling interface through chpoll(9E), as well as asynchronous I/O through aread(9E) and awrite(9E).

For information on character driver entry points, see Chapter 10, Drivers for Character Devices. For information on block driver entry points, see Chapter 11, Drivers for Block Devices.

Power Management Entry Point

Drivers for hardware devices that provide Power Management functionality can support the optional power(9E) entry point. See Chapter 9, Power Management for details about this entry point.

Device Driver Design Considerations

Device driver must be compatible with the Solaris operating environment, both as a consumer and provider of services. This section discusses the following issues which should be considered in device driver design:

• DDI/DKI Facilities

• Driver Context

• Returning Errors

• Dynamic Memory Allocation

• Hotplugging

• Driver Layout

DDI/DKI Facilities

This section discusses design considerations necessary for using the DDI/DKI interfaces.

Device IDs

The Solaris DDI provides interfaces that allow drivers to provide a persistent, unique identifier for a device. The device ID can be used to identify or locate a device and is independent of the device's name or number (dev_t). Applications can use the functions defined in libdevid(3LIB) to read and manipulate the device IDs registered by the drivers.

Device Properties

The attributes of a device or device driver are specified by properties. A property is a name-value pair. The name is a string that identifies the property with an associated value. Properties can be defined by the FCode of a self-identifying device, by a hardware configuration file (see the driver.conf(4) man page), or by the driver itself using the ddi_prop_update(9F) family of routines.

Interrupt Handling

The Solaris 9 DDI/DKI addresses these aspects of device interrupt handling:

• Registering device interrupts with the system

• Removing device interrupts

• Dispatching interrupts to interrupt handlers

Device interrupt sources are contained in a property called interrupt, which is either provided by the PROM of a self-identifying device, in a hardware configuration file, or by the booting system on the IA platform.

Callback Functions

Certain DDI mechanisms provide a callback mechanism. DDI functions provide a mechanism for scheduling a callback when a condition is met. Conditions for which callback functions are used include:

• When a transfer has completed

• When a resource might have become available

• When a time-out period has expired

In some sense, callback functions are similar to entry points—interrupt handlers, for example. DDI functions that allow callbacks expect the callback function to perform certain tasks. In the case of DMA routines, a callback function must return a value indicating whether the callback function needs to be rescheduled in case of a failure.

Callback functions execute as a separate interrupt thread and must handle all the usual multithreading issues.

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A driver must cancel all scheduled callback functions before detaching a device.

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Software State Management

To assist device driver writers in allocating state structures, the Solaris 9 DDI/DKI provides a set of memory management routines called the software state management routines (also known as the soft state routines). These routines dynamically allocate, retrieve, and destroy memory items of a specified size, and hide the details of list management. An instance number is used to identify the desired memory item; this number can be (and usually is) the instance number assigned by the system.

Routines are provided to:

• Initialize a driver's soft state list

• Allocate space for an instance of a driver's soft state

• Retrieve a pointer to an instance of a driver's soft state

• Free the memory for an instance of a driver's soft state

• Finish using a driver's soft state list

See Loadable Driver Interfaces for an example of how to use these routines.

Programmed I/O Device Access

Programmed I/O device access is the act of reading and writing of device registers or device memory by the host CPU. The Solaris DDI provides interfaces for mapping a device's registers or memory by the kernel as well as interfaces for reading and writing to device memory from the driver. These interfaces are designed to enable drivers to be developed that are platform and bus independent, by automatically managing any difference in device and host endianness as well as enforcing any memory-store ordering requirements imposed by the device.

Direct Memory Access (DMA)

Solaris defines a high-level, architecture-independent model for supporting DMA-capable devices. The Solaris DDI is designed to shield drivers from platform-specific details, which enables a common driver to be developed that runs across multiple platforms and architectures.

Driver and Device Statistics

Solaris provides a rich set of interfaces for maintaining and exporting kernel-level statistics, or kstats. Drivers are free to use these interfaces to export driver and device statistics that can be used by user applications to observe the internal state of the driver. See the kstat_create(9F) and kstat(3KSTAT) man pages for additional information.

Driver Context

The driver context determines which kernel routines the driver is permitted to call. There are four contexts in which driver code executes:

• User context – A driver entry point has user context if it was directly invoked because of a user thread. For example, the read(9E) entry point of the driver, invoked by a read(2) system call, has user context.

• Kernel context – A driver function has kernel context if it was invoked by some other part of the kernel. In a block device driver, the strategy(9E) entry point can be called by the pageout daemon to write pages to the device. Because the page daemon has no relation to the current user thread, strategy(9E) has kernel context in this case.

• Interrupt context – Interrupt context is a more restrictive form of kernel context. Driver interrupt routines operate in interrupt context and have an interrupt level associated with them. Callback routines also operate in an interrupt context. See Chapter 7, Interrupt Handlers for more information.

• High-level interrupt context – High-level interrupt context is a more restricted form of interrupt context. If ddi_intr_hilevel(9F) indicates that an interrupt is high level, the driver interrupt handler will run in high-level interrupt context. See Chapter 7, Interrupt Handlers for more information.

The manual pages in section 9F document the allowable contexts for each function. For example, in kernel context the driver must not call copyin(9F).

Returning Errors

Device drivers do not usually print messages, except for unexpected errors such as data corruption. Instead, the driver entry points should return error codes so that the application can determine how to handle the error. If the driver must print a message, it should use cmn_err(9F) to do so. This is similar to the C function printf(3C), which prints to the console, to the message buffer, or both.

The format string specifier interpreted by cmn_err(9F) is similar to the printf(3C) format string, with the addition of the format %b, which prints bit fields. Callers to cmn_err(9F) also specify the level, which indicates the label to be printed. The first character of the format string is treated specially. See the cmn_err(9F) man page for more details.

The level CE_PANIC has the side effect of crashing the system. This level should be used only if the system is in such an unstable state that to continue would cause more problems. It can also be used to get a system core dump when debugging. It should not be used in production device drivers.

Dynamic Memory Allocation

Device drivers must be prepared to simultaneously handle all attached devices that they claim to drive. There should be no driver limit on the number of devices that the driver handles, and all per-device information must be dynamically allocated.

void *kmem_alloc(size_t size, int flag);

The standard kernel memory allocation routine is kmem_alloc(9F). It is similar to the C library routine malloc(3C), with the addition of the flag argument. The flag argument can be either KM_SLEEP or KM_NOSLEEP, indicating whether the caller is willing to block if the requested size is not available. If KM_NOSLEEP is set, and memory is not available, kmem_alloc(9F) returns NULL.

kmem_zalloc(9F) is similar to kmem_alloc(9F), but also clears the contents of the allocated memory.

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Kernel memory is a limited resource, not pageable, and competes with user applications and the rest of the kernel for physical memory. Drivers that allocate a large amount of kernel memory can cause system performance to degrade.

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void kmem_free(void *cp, size_t size);

Memory allocated by kmem_alloc(9F) or by kmem_zalloc(9F) is returned to the system with kmem_free(9F). This is similar to the C library routine free(3C), with the addition of the size argument. Drivers must keep track of the size of each object they allocate in order to call kmem_free(9F) later.

Hotplugging

In general, this manual does not highlight hotplugging information; following the rules and suggestions for writing device drivers given in this book should enable any driver to handle hotplugging. In particular, you should ensure that autoconfiguration works (see Chapter 5, Driver Autoconfiguration) and always include a working detach(9E) routine in any driver. Writers of drivers with power management issues should also follow the information given in Chapter 9, Power Management. SCSI HBA drivers may need to add a cb_ops structure to their dev_ops structure (see Chapter 15, SCSI Host Bus Adapter Drivers) to take advantage of hotplugging capabilities.

Previous versions of the Solaris operating system required hotpluggable drivers to include a DT_HOTPLUG property, but such a property is no longer required. (Driver writers are free, however, to include it and have routines make use of it as they see fit.)

For more information, visit http://soldc.sun.com/developer/support/driver/notes/scsi-hotplug.html, which contains links to hotplugging whitepapers.

Driver Layout

Driver code is usually divided into the following files:

• Headers (.h files)

• Source files (.c files)

• Optional configuration files (driver.conf file)

Header Files

Header files define data structures specific to the device (such as a structure representing the device registers), data structures defined by the driver for maintaining state information, defined constants (such as those representing the bits of the device registers), and macros (such as those defining the static mapping between the minor device number and the instance number).

Some of this information, such as the state structure, may only be needed by the device driver. This information should go in private headers. These header files are included only by the device driver itself.

Any information that an application might require, such as the I/O control commands, should be in public header files. These are included by the driver and any applications that need information about the device.

There is no standard for naming private and public files. One possible convention is to name the private header file xximpl.h and the public header file xxio.h.

Source Files

A.c file for a device driver contains the data declarations and the code for the entry points of the driver. It contains the #include statements the driver needs, declares extern references, declares local data, sets up the cb_ops and dev_ops structures, declares and initializes the module configuration section, makes any other necessary declarations, and defines the driver entry points.

Configuration Files

Information for configuration files used with device drivers can be found in driver.conf(4), sbus(4), pci(4), and isa(4).