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Dev - device driver interface


struct Dev
    int dc;
    char*   name;

    void    (*reset)(void);        /* native only */
    void    (*init)(void);
    void    (*shutdown)(void);     /* native */
    Chan*   (*attach)(char *spec);
    Walkqid*  (*walk)(Chan *c, Chan *nc, char **name, int nname);
    int     (*stat)(Chan *c, uchar *db, int dbsize);
    Chan*   (*open)(Chan *c, int mode);
    void    (*create)(Chan *c, char *name, int mode, ulong perm);
    void    (*close)(Chan *c);
    long    (*read)(Chan *c, void *buf, long nbytes, vlong offset);
    Block*  (*bread)(Chan *c, long nbytes, ulong offset);
    long    (*write)(Chan *c, void*, long, vlong offset);
    long    (*bwrite)(Chan *c, Block *b, ulong offset);
    void    (*remove)(Chan *c);
    int     (*wstat)(Chan *c, uchar *db, int dbsize);
    void    (*power)(int on);        /* native only */
    int     (*config)(int on, char *spec, DevConf *cf); /* native */


Every device driver serves a unique name space that represents to the corresponding device(s). Applications act on the space using the operations of sys-bind(2), sys-open(2), sys-read(2), sys-stat(2), and other system calls. Within the kernel, the Dev structure defines the interface between the kernel and a device driver for all operations on that driver's name space.

Dev identifies the driver, and lists a set of C functions that are the driver's operations. Most are operations on the Chan type that is the kernel representation of a file or directory active in a name space. The kernel converts system calls acting on file descriptors into calls to a device's Dev operations acting on channel values. All channel values presented through the Dev interface are associated with the corresponding device driver: for channel c, c->type specifies that driver. Within the driver, the c->qid.path of a channel c identifies a file in the driver's name space, or even a client-specific instance of a file (eg, for multiplexors such as ip(3)). The interpretation of the path is completely determined by the driver.

A device driver in the source file devx.c exports an initialised instance of Dev xdevtab. For instance, devcons.c contains the global initialiser:

Dev consdevtab = {


The kernel accesses the driver only through its Dev structure, and consequently entry points such as consinit, consread, etc. can (and should) be declared static, and thus local to the file.

The following elements of Dev identify the driver:

The device's type, represented by a Unicode character (`rune') that must be unique amongst those in a given kernel (and ideally for a given platform). Its value is the value of Dir.dtype in the result of a sys-stat(2) applied to any file in the device.
The name that identifies the driver in a kernel configuration file and in /dev/drivers (see cons(3)).

All the other entries are functions. In many cases, the values given in a device's Dev will be the default operations provided by devattach(10.2).

Called once during system initialisation by the native kernel's main after initialising all supporting subsystems, including memory allocation, traps, screen, MMU (if used), but with interrupts disabled, and before any kernel process environment has been established. Typically used on some platforms to force some devices into a sane state before interrupts are enabled.
Called once during system initialisation in the context of the first kernel process, with interrupts enabled, before the virtual machine has been started.
Called once in native kernels during system shut down. Used on only a few platforms to force a device into a state that will allow it to function correctly during and after a soft reboot (eg, without doing a full system hardware reset).
Called on each new attach to the device (eg, a reference to #c by sys-bind(2)). Spec is the string following the device character and before a subsequent `/' in the bind request. It is the empty string for most devies. If the attach is successful, attach should return a Chan the refers to the root of the tree served by the device driver. Normally, it will suffice to return the value of devattach(10.2).
walk(c, nc, name, nname)
Walks existing channel c from its current position in the device tree to that specified by the path represented by name[0], ... name[nname-1]. The driver must interpret `..' as a walk from the current position one level up towards the root of the device tree. The result is represented by a dynamically-allocated Walkqid value, with contents as described in devattach(10.2). Most drivers simply pass parameters on to devwalk in devattach(10.2) and return its result.
stat(c, db, nbytes)
Fill db with stat(5) data describing the file referenced by c. Nbytes gives the size of db; if the data will not fit, return the value specified for convD2M in styx(10.2). Most drivers simply pass parameters on to devstat in devattach(10.2); a few fill a local copy of a Dir structure, and call convD2M to store the machine-independent representation in db.
open(c, mode)
Open the file represented by Chan c, in the given mode (see sys-open(2)), and if successful, return a Chan value representing the result (usually c). Many drivers simply apply devopen of devattach(10.2). Exclusive use drivers might check and increment a reference count.
create(c, name, mode, perm)
C should be a directory. Create a new file name in that directory, with permissions perm, opened with the given mode. If successful, make c refer to the newly created file. Most drivers return an error on all creation attempts, by specifying devcreate of devattach(10.2) in the Dev table.
Close channel c. This must be implemented by all drivers; there is no default, although the function often is a no-op. Exclusive use drivers might decrement a reference count.
read(c, buf, nbytes, offset)
Implement a sys-read(2) of nbytes of data from the given offset in file c, and if successful, place the data in buf, and return the number of bytes read, which must be no greater than nbytes. Devices sometimes ignore the offset. All device drivers must implement read; there is no default. Note that if c is a directory, the data has an array of stat(5) data listing the directory contents, in the format prescribed by read(5). Most drivers have devdirread of devattach(10.2) do the work when c is the root directory of the device's tree.
bread(c, nbytes, offset)
Implement a sys-read(2) of nbytes of data from the given offset in file c, and if successful return the data in a Block (see allocb(10.2) and qio(10.2)). Most drivers use the default devbread provided by devattach(10.2), and nearly all ignore the offset in any case. Drivers that manipulate Blocks internally, such as ip(3), ssl(3) and similar protocol devices, and drivers that are likely to provide data to those devices, will provide a devbread implementation so as to reduce the number of times the data is copied.
write(c, buf, nbytes, offset)
Implement a write of nbytes of data from buf to file c, which must not be a directory, starting at the given byte offset. Return the number of bytes actually written. There is no default, but drivers that do not implement writes to any of their files can simply call error(Eperm) to signal an error.
bwrite(c, b, offset)
Similar to the write entry point, but the data is contained in a Block b (see allocb(10.2)). B should be freed before return, whether the driver signals an error or not. Most drivers use the default devbwrite from devattach(10.2), which calls the driver's write entry point using the data in b. Drivers that manipulate Blocks internally, such as ip(3), ssl(3) and similar protocol devices, will provide a devbwrite implementation so as to avoid copying the data needlessly.
Remove the file referenced by c. Most drivers raise an error by using the default devremove from devattach(10.2).
wstat(c, db, dbsize)
Change the attributes of file c, using the stat(5) data in buffer db, which is dbsize bytes long. Usually a driver will use convM2D of styx(10.2) to convert the data to a Dir structure, then apply the rules of stat(5) to decide which attributes are to be changed (and whether the change is allowed). Most drivers simply return an error on all wstat requests by using the default devwstat from devattach(10.2).
Reserved for use in native kernels, to allow the kernel to power the device on and off for power-saving; on is non-zero if the device is being powered up, and zero if it is being powered down. The device driver should save the device state if necessary. Leave the Dev entry null for now.
config(on, spec, cf)
Reserved for use in native kernels to allow a device to be configured on and off dynamically. Leave the Dev entry null for now.

The elements reset, shutdown, power, and config are currently present only in the native kernels.


intro(2), intro(5), allocb(10.2), devattach(10.2), newchan(10.2), qio(10.2)

DEV(10.2 ) Rev:  Thu Feb 15 14:42:59 GMT 2007