Enhanced Char Driver Operations

In Chapter 3, "Char Drivers", we built a complete device driver that the user can write to and read from. But a real device usually offers more functionality than synchronous read and write. Now that we're equipped with debugging tools should something go awry, we can safely go ahead and implement new operations.

What is normally needed, in addition to reading and writing the device, is the ability to perform various types of hardware control via the device driver. Control operations are usually supported via the ioctl method. The alternative is to look at the data flow being written to the device and use special sequences as control commands. This latter technique should be avoided because it requires reserving some characters for controlling purposes; thus, the data flow can't contain those characters. Moreover, this technique turns out to be more complex to handle than ioctl. Nonetheless, sometimes it's a useful approach to device control and is used by tty's and other devices. We'll describe it later in this chapter in "Device Control Without ioctl".

As we suggested in the previous chapter, the ioctl system call offers a device specific entry point for the driver to handle "commands.'' ioctl is device specific in that, unlike read and other methods, it allows applications to access features unique to the hardware being driven, such as configuring the device and entering or exiting operating modes. These control operations are usually not available through the read/write file abstraction. For example, everything you write to a serial port is used as communication data, and you cannot change the baud rate by writing to the device. That is what ioctl is for: controlling the I/O channel.

Another important feature of real devices (unlike scull) is that data being read or written is exchanged with other hardware, and some synchronization is needed. The concepts of blocking I/O and asynchronous notification fill the gap and are introduced in this chapter by means of a modified scull device. The driver uses interaction between different processes to create asynchronous events. As with the original scull, you don't need special hardware to test the driver's workings. We will definitely deal with real hardware, but not until Chapter 8, "Hardware Management".

ioctl

The ioctl function call in user space corresponds to the following prototype:

 int ioctl(int fd, int cmd, ...);

The prototype stands out in the list of Unix system calls because of the dots, which usually represent not a variable number of arguments. In a real system, however, a system call can't actually have a variable number of arguments. System calls must have a well-defined number of arguments because user programs can access them only through hardware "gates,'' as outlined in "User Space and Kernel Space" in Chapter 2, "Building and Running Modules". Therefore, the dots in the prototype represent not a variable number of arguments but a single optional argument, traditionally identified as char *argp. The dots are simply there to prevent type checking during compilation. The actual nature of the third argument depends on the specific control command being issued (the second argument). Some commands take no arguments, some take an integer value, and some take a pointer to other data. Using a pointer is the way to pass arbitrary data to the ioctl call; the device will then be able to exchange any amount of data with user space.

The ioctl driver method, on the other hand, receives its arguments according to this declaration:

 int (*ioctl) (struct inode *inode, struct file *filp,
         unsigned int cmd, unsigned long arg);

The inode and filp pointers are the values corresponding to the file descriptor fd passed on by the application and are the same parameters passed to the open method. The cmd argument is passed from the user unchanged, and the optional arg argument is passed in the form of an unsigned long, regardless of whether it was given by the user as an integer or a pointer. If the invoking program doesn't pass a third argument, the arg value received by the driver operation has no meaningful value.

Because type checking is disabled on the extra argument, the compiler can't warn you if an invalid argument is passed to ioctl, and the programmer won't notice the error until runtime. This lack of checking can be seen as a minor problem with the ioctl definition, but it is a necessary price for the general functionality that ioctlprovides.

As you might imagine, most ioctl implementations consist of a switch statement that selects the correct behavior according to the cmd argument. Different commands have different numeric values, which are usually given symbolic names to simplify coding. The symbolic name is assigned by a preprocessor definition. Custom drivers usually declare such symbols in their header files; scull.hdeclares them for scull. User programs must, of course, include that header file as well to have access to those symbols.

Choosing the ioctl Commands

Before writing the code for ioctl, you need to choose the numbers that correspond to commands. Unfortunately, the simple choice of using small numbers starting from 1 and going up doesn't work well.

The command numbers should be unique across the system in order to prevent errors caused by issuing the right command to the wrong device. Such a mismatch is not unlikely to happen, and a program might find itself trying to change the baud rate of a non-serial-port input stream, such as a FIFO or an audio device. If each ioctl number is unique, then the application will get an EINVAL error rather than succeeding in doing something unintended.

To help programmers create unique ioctl command codes, these codes have been split up into several bitfields. The first versions of Linux used 16-bit numbers: the top eight were the "magic'' number associated with the device, and the bottom eight were a sequential number, unique within the device. This happened because Linus was "clueless'' (his own word); a better division of bitfields was conceived only later. Unfortunately, quite a few drivers still use the old convention. They have to: changing the command codes would break no end of binary programs. In our sources, however, we will use the new command code convention exclusively.

To choose ioctl numbers for your driver according to the new convention, you should first check include/asm/ioctl.h and Documentation/ioctl-number.txt. The header defines the bitfields you will be using: type (magic number), ordinal number, direction of transfer, and size of argument. The ioctl-number.txt file lists the magic numbers used throughout the kernel, so you'll be able to choose your own magic number and avoid overlaps. The text file also lists the reasons why the convention should be used.

The old, and now deprecated, way of choosing an ioctl number was easy: authors chose a magic eight-bit number, such as "k'' (hex 0x6b), and added an ordinal number, like this:

 #define SCULL_IOCTL1 0x6b01
 #define SCULL_IOCTL2 0x6b02
 /* .... */

If both the application and the driver agreed on the numbers, you only needed to implement the switch statement in your driver. However, this way of defining ioctlnumbers, which had its foundations in Unix tradition, shouldn't be used any more. We've only shown the old way to give you a taste of what ioctl numbers look like.

The new way to define numbers uses four bitfields, which have the following meanings. Any new symbols we introduce in the following list are defined in <linux/ioctl.h>.

type

The magic number. Just choose one number (after consulting ioctl-number.txt) and use it throughout the driver. This field is eight bits wide (_IOC_TYPEBITS).

 

number

The ordinal (sequential) number. It's eight bits (_IOC_NRBITS) wide.

 

direction

The direction of data transfer, if the particular command involves a data transfer. The possible values are _IOC_NONE (no data transfer), _IOC_READ, _IOC_WRITE, and _IOC_READ | _IOC_WRITE (data is transferred both ways). Data transfer is seen from the application's point of view; _IOC_READ means reading fromthe device, so the driver must write to user space. Note that the field is a bit mask, so _IOC_READ and _IOC_WRITE can be extracted using a logical AND operation.

 

size

The size of user data involved. The width of this field is architecture dependent and currently ranges from 8 to 14 bits. You can find its value for your specific architecture in the macro _IOC_SIZEBITS. If you intend your driver to be portable, however, you can only count on a size up to 255. It's not mandatory that you use the size field. If you need larger data structures, you can just ignore it. We'll see soon how this field is used.

 

The header file <asm/ioctl.h>, which is included by <linux/ioctl.h>, defines macros that help set up the command numbers as follows: _IO(type,nr), _IOR(type,nr,dataitem), _IOW(type,nr,dataitem), and _IOWR(type,nr,dataitem). Each macro corresponds to one of the possible values for the direction of the transfer. The type and number fields are passed as arguments, and the size field is derived by applying sizeof to the dataitem argument. The header also defines macros to decode the numbers: _IOC_DIR(nr), _IOC_TYPE(nr), _IOC_NR(nr), and _IOC_SIZE(nr). We won't go into any more detail about these macros because the header file is clear, and sample code is shown later in this section.

Here is how some ioctl commands are defined in scull. In particular, these commands set and get the driver's configurable parameters.

 
/* Use 'k' as magic number */
#define SCULL_IOC_MAGIC 'k'

#define SCULL_IOCRESET _IO(SCULL_IOC_MAGIC, 0)

/*
 * S means "Set" through a ptr
 * T means "Tell" directly with the argument value
 * G means "Get": reply by setting through a pointer
 * Q means "Query": response is on the return value
 * X means "eXchange": G and S atomically
 * H means "sHift": T and Q atomically
 */
#define SCULL_IOCSQUANTUM _IOW(SCULL_IOC_MAGIC, 1, scull_quantum)
#define SCULL_IOCSQSET  _IOW(SCULL_IOC_MAGIC, 2, scull_qset)
#define SCULL_IOCTQUANTUM _IO(SCULL_IOC_MAGIC,  3)
#define SCULL_IOCTQSET  _IO(SCULL_IOC_MAGIC,  4)
#define SCULL_IOCGQUANTUM _IOR(SCULL_IOC_MAGIC, 5, scull_quantum)
#define SCULL_IOCGQSET  _IOR(SCULL_IOC_MAGIC, 6, scull_qset)
#define SCULL_IOCQQUANTUM _IO(SCULL_IOC_MAGIC,  7)
#define SCULL_IOCQQSET  _IO(SCULL_IOC_MAGIC,  8)
#define SCULL_IOCXQUANTUM _IOWR(SCULL_IOC_MAGIC, 9, scull_quantum)
#define SCULL_IOCXQSET  _IOWR(SCULL_IOC_MAGIC,10, scull_qset)
#define SCULL_IOCHQUANTUM _IO(SCULL_IOC_MAGIC, 11)
#define SCULL_IOCHQSET  _IO(SCULL_IOC_MAGIC, 12)
#define SCULL_IOCHARDRESET _IO(SCULL_IOC_MAGIC, 15) /* debugging tool */

#define SCULL_IOC_MAXNR 15

The last command, HARDRESET, is used to reset the module's usage count to 0 so that the module can be unloaded should something go wrong with the counter. The actual source file also defines all the commands between IOCHQSET and HARDRESET, although they're not shown here.

We chose to implement both ways of passing integer arguments -- by pointer and by explicit value, although by an established convention ioctl should exchange values by pointer. Similarly, both ways are used to return an integer number: by pointer or by setting the return value. This works as long as the return value is a positive integer; on return from any system call, a positive value is preserved (as we saw for read and write), while a negative value is considered an error and is used to set errno in user space.

The "exchange'' and "shift'' operations are not particularly useful for scull. We implemented "exchange'' to show how the driver can combine separate operations into a single atomic one, and "shift'' to pair "tell'' and "query.'' There are times when atomic[24] test-and-set operations like these are needed, in particular, when applications need to set or release locks.

[24]A fragment of program code is said to be atomic when it will always be executed as though it were a single instruction, without the possibility of the processor being interrupted and something happening in between (such as somebody else's code running).

The explicit ordinal number of the command has no specific meaning. It is used only to tell the commands apart. Actually, you could even use the same ordinal number for a read command and a write command, since the actual ioctl number is different in the "direction'' bits, but there is no reason why you would want to do so. We chose not to use the ordinal number of the command anywhere but in the declaration, so we didn't assign a symbolic value to it. That's why explicit numbers appear in the definition given previously. The example shows one way to use the command numbers, but you are free to do it differently.

The value of the ioctl cmd argument is not currently used by the kernel, and it's quite unlikely it will be in the future. Therefore, you could, if you were feeling lazy, avoid the complex declarations shown earlier and explicitly declare a set of scalar numbers. On the other hand, if you did, you wouldn't benefit from using the bitfields. The header <linux/kd.h> is an example of this old-fashioned approach, using 16-bit scalar values to define the ioctl commands. That source file relied on scalar numbers because it used the technology then available, not out of laziness. Changing it now would be a gratuitous incompatibility.