harvey/sys/man/9/devattach

.TH DEVATTACH 9
.SH NAME
devattach, devclone, devdir, devgen, devwalk, devdirread, devstat, devopen, devbread, devbwrite, devcreate, devremove, devwstat, devreset, devinit, devshutdown, openmode \- common device driver support
.SH SYNOPSIS
.nf
.ta \w'\fLBlock* 'u +10n
.B
typedef int
.B
Devgen(Chan *c, char *name, Dirtab *tab, int ntab, int i, Dir *dp)
.PP
.B
Chan*	devattach(int tc, char *spec)
.PP
.B
Chan*	devclone(Chan *c)
.PP
.B
void	devdir(Chan *c, Qid qid, char *n, long length,
.B
		char *user, long perm, Dir *dp)
.PP
.B
int	devgen(Chan *c, char *name, Dirtab *tab, int ntab,
.B
		int i, Dir *dp)
.PP
.B
Walkqid* devwalk(Chan *c, Chan *nc, char **name, int nname,
.B
		Dirtab *tab, int ntab, Devgen *gen)
.PP
.B
void	devstat(Chan *c, uchar *db, int n, Dirtab *tab,
.B
		int ntab, Devgen *gen)
.PP
.B
long	devdirread(Chan *c, char *d, long n, Dirtab *tab,
.B
		int ntab, Devgen *gen)
.PP
.B
Chan*	devopen(Chan *c, int omode, Dirtab *tab,
.B
		int ntab, Devgen *gen)
.PP
.B
Block*	devbread(Chan *c, long n, ulong offset)
.PP
.B
long	devbwrite(Chan *c, Block *bp, ulong offset)
.PP
.B
void	devcreate(Chan*, char*, int, ulong)
.PP
.B
void	devremove(Chan*)
.PP
.B
void	devwstat(Chan*, uchar*, int)
.PP
.B
void	devreset(void)
.PP
.B
void	devinit(void)
.PP
.B
void	devshutdown(void)
.PP
.B
int	openmode(ulong mode)
.SH DESCRIPTION
Device drivers call these functions to carry out essential tasks and default actions.
They do most of the name space management
for a driver that serves a simple name space
(eg, data and control files),
leaving the driver to concentrate on the device-specific details
of the I/O requests.
More complex drivers also make good use of them at the leaves
of their name space, and to help manage the
.B Chan
structures correctly.
.PP
A device has an associated
.IR type ,
represented as a Unicode character (`rune') that identifies the device
inside and outside the kernel.
It appears as the value of the
.B type
field in the
.B Dir
resulting from a
.IR sys-stat (2)
of any file provided by the device.
A device is named outside the kernel using
a path name starting with
.B #
followed by the device character
(eg,
.B c
in
.B #c
for the console).
Any subsequent characters before
the next '/' or end of string is the `device specifier',
interpreted solely by the device itself.
.PP
.I Devattach
returns a new channel representing
the root of the file tree
corresponding to device type
.IR tc ,
with device specifier
.IR spec .
It is normally called by a driver's
.I attach
function (see
.IR dev (9)).
The
.B qid
for the new channel is
.BR "(Qid){0,0,QTDIR}" ,
suitable for a root directory for many devices, but
a device driver is free to change it (provided the
.B QTDIR
bit remains in the
.BR Qid.type ).
.PP
.I Devclone
returns a new channel that is a copy of
.IR c .
An attempt to clone an open channel causes a
.IR panic (9).
.PP
The
.L Dir
structure is shown below:
.IP
.EX
typedef
struct Dir
{
    /* system-modified data */
    ushort  type;   /* server type */
    uint    dev;    /* server subtype */
    /* file data */
    Qid     qid;    /* unique id from server */
    ulong   mode;   /* permissions */
    ulong   atime;  /* last read time */
    ulong   mtime;  /* last write time */
    vlong   length; /* file length */
    char    *name;  /* last element of path */
    char    *uid;   /* owner name */
    char    *gid;   /* group name */
    char    *muid;  /* last modifier name */
} Dir;
.EE
.PP
This
.B Dir
structure corresponds directly to the Limbo
.B Dir
adt described in
.IR sys-stat (2).
.PP
Given a channel and assorted other information, 
.I devdir
initialises a Dir structure at
.IR dp .
.I Devdir
supplies the following data itself:
.RS
.TF length
.TP
.B atime
last access time (set to current time)
.TP
.B mtime
last modification time (set to kernel creation date)
.TP
.B gid
group name (set to
.IR eve (9))
.TP
.B length
length in bytes (set to zero, which
is normal for most devices)
.RE
.PD
.PP
Note that
.I devdir
assigns the values of
.I name
and
.I user
directly to fields of
.BI * dp,
and consequently those values must remain valid until the last use of
.BI * dp.
(Sometimes that requires the use of an auxiliary buffer, such as
.BR up->genbuf .)
If channel
.I c
corresponds to a file descriptor on which Styx is served,
.I devdir
sets both the flag bit
.B QTMOUNT
in
.IB dp ->qid.type
and the flag bit
.B DMMOUNT
in
.IB dp ->mode
(see
.I export
in
.IR sys-dial (2)
and
.I mount
in
.IR sys-bind (2)).
.PP
A simple name space can be represented in a driver by an array of
.B Dirtab
structures.
The array is typically static when the names and permissions
are static, but can be dynamically allocated and initialised if required.
The structure of
.B Dirtab
is shown below:
.IP
.EX
typedef
struct Dirtab
{
        char    name[KNAMELEN];
        Qid     qid;
        vlong   length;
        long    perm;
} Dirtab;
.EE
.PP
The name
.RB ` . '
.I must
appear as the first entry in a
.B Dirtab
if the default
.I devgen
function is used.
On the other hand, the name
.RB ` .. '
must never appear in a
.B Dirtab
table.
Drivers that support a directory hierarchy must walk up the hierarchy towards
the root when their
.I walk
function receives
.RB ` .. '
as a file name component.
The name
.RB ` . '
is never seen by a driver.
.PP
The
.IR devdirread ,
.IR devopen ,
.IR devstat ,
and
.IR devwalk
functions all take a
.I gen
function argument,
of type
.BR Devgen ,
which they invoke to retrieve the items in
a
.B Chan
that represents a directory.
.I Gen
takes a channel
.I c
(a directory),
a file
.I name
(which is nil except during
.IR devwalk ),
an array of
.B Dirtab
structures
.I tab
of length
.IR ntab ,
and a table index
.IR i .
The functions calling
.I gen
expect it to place the
.IR i 'th
entry in the directory into
.IR \f5*\fPdp .
It should return 1
if the call was successful,
-1 if
.I i
is beyond the index of the last directory entry,
or 0 if there is no entry at
.IR i ,
but there are entries beyond it.
When
.I i
has the special value
.B DEVDOTDOT
then
.I gen
should set
.IR \f5*\fPdp
to reflect the parent of
.IR c ;
if
.I c
is a one-level device directory, then `..' is equivalent to `.'.
Custom implementations of
.I gen
often ignore
.IR devtab ,
and instead return their own dynamically generated
set of directory entries from some other source.
Exceptionally, during
.I devwalk
a non-nil
.I name
is provided: it is the name being looked up, and a device-specific
.I gen
can short-circuit the search by returning -1 if the name does not exist,
or filling in
.IR \f5*\fPdp
and returning 1 if it does exist.
.PP
The function
.I devgen
is compatible with
.BR Devgen ;
it returns the
.IR i 'th
entry in
.IR devtab ,
and can be used to provide a simple, static
set of directory entries.
.PP
.I Devwalk
walks channel
.I c
to the file in the device named by the path encoded in
.IR name ,
which is an array of strings of length
.IR nname .
It provides the interface to
.IR walk (5)
within the kernel, and that specification must be well understood to appreciate
all the nuances of its interface.
Fortunately, in nearly all device drivers, a device's
.I walk
function typically passes its parameters on to
.I devwalk
(adding the device's own
.B Dirtab
array as the the value of
.IR tab ),
and simply returning the result of
.IR devwalk .
.PP
.I Devwalk
walks
.I c
using the given set of names, and if the walk is successful, the
channel
.I nc
will refer to the result of the walk
(specifically,
.IB nc ->qid
is set to the Qid for the file).
If
.I nc
is nil,
.I devwalk
will allocate a new channel itself, that is initially a clone of
.IR c .
As in
.IR walk (5),
.I devwalk
can return a partial result,
represented by
a dynamically allocated value of the following structure:
.IP
.EX
struct Walkqid
{
    Chan  *clone;
    int   nqid;
    Qid   qid[1];	/* actually nname in length */
};
.EE
.PP
The value must be freed after use.
For each element of
.I name ,
.I devwalk
passes
the
.I tab
parameter to
.I gen
together with the currently-sought element of
.IR name .
If the first element is not found,
.I devwalk
returns nil; otherwise, it returns a
.B Walkqid
value in which
.B nqid
elements of the array
.B qid
are set to the qids (see
.IR intro (5))
of each valid element of
.IR name .
If all
.I nname
elements were successfully traversed, then
.B nqid
will have the value
.IR nname ,
and
.B clone
will refer to the result of the walk,
which is either
.I nc
if given, or
the new channel allocated by
.IR devwalk .
Otherwise, at least one element succeeded and
.B nqid
is less than
.I nname
and
.B clone
is nil.
On an error or incomplete walk,
the error string is set to the error that stopped the walk (eg,
.B Enonexist
or
.BR Enotdir ).
.PP
.I Devstat
fills the array of bytes
.I db
with data in the format produced by
.IR stat (5)
that describes the file
referenced by channel
.IR c ,
which must have a corresponding entry
returned by
.IR gen
(ie, an entry with matching
.BR Qid.path ).
If
.I c
is a communications channel connecting a Styx server to a current mount point,
the
.B DMMOUNT
bit is set in the resulting
.BR Dir.mode ,
and
.B QTMOUNT
is set in
.BR Dir.qid.type .
As in
.IR stat (5),
the length of the data written to
.I db
varies; if more than
.I n
bytes are needed,
.I devstat
raises the
.IR error (9)
.BR Ebadarg .
Otherwise, it returns the number of bytes in
.I db
actually used.
.PP
If an entry with the desired qid is not found in the table, but
.I c
corresponds to a directory
(ie,
.B QTDIR
is set in
.IR c\f5->qid.type\fP ),
it is taken to be a
.I stat
of a notional directory containing the files listed in
.IR tab .
.I Dirstat
then builds the corresponding Dir structure:
its
.B Dir.name
is taken from
.IR c\f5->path->elem\fP ;
the length is
.BI DIRLEN*nelem(tab) ;
and
.B Dir.perm
is 0555 (read-execute for all).
.PP
.I Devdirread
calls
.I gen
to obtain successive
.B Dir
structures representing entries in the open directory
.IR c .
These are converted to standard format (see
.I convD2M
in
.IR fcall (2))
and placed in the buffer
.IR b .
It returns the number of bytes in the result.
At most
.I n
bytes will be returned, in multiples of
.BR DIRLEN .
Because the kernel maintains the current offset in
.IR c ,
successive calls to
.I devdirread
return successive directory components.
.PP
.I Devopen
is called to check and complete a request to open channel
.I c
for I/O according to
.IR omode
(the open mode of
.IR sys-open (2)).
It calls
.I gen
to obtain successive directory entries
which it searches
for a Qid matching that of
.IR c ,
and ensures that the current user has permission to open
.I c
with the given mode,
.IR omode ,
and that the mode itself is valid
(see
.I openmode
below).
Permission is checked against the permission in the
matching entry.
If no matching Qid is found, it is assumed
that the notional parent directory of the files represented in
.I tab
is to be opened.
Such a directory is deemed to have mode
0555, allowing access by any user.
A directory can only be opened for reading
.RB ( OREAD ).
.I Devopen
returns the channel
.I c
on success.
Last, it sets the bit
.B COPEN
in
.B Chan.flag
to mark
.I c
as open.
This convention can always be relied upon by the driver's
.I close
function to tell if an open succeeded.
On the otherhand,
if the open request was unsuccessful,
.I devopen
raises an appropriate
.IR error (9)
and does not return.
.PP
.I Devbread
returns a
.B Block
(see
.IR allocb (9))
containing up to
.I n
bytes read,
using
.BI "devtab[" c "->type]->read" ,
from
.I c
starting at the given
.IR offset .
The read pointer in the returned
.B Block
points to the start of the data;
the write pointer points to the next available byte.
.PP
.I Devbwrite
writes the data in
.B Block
.I bp
to the file
.I c
at the given
.IR offset ,
using the write function
.BI "devtab[" c "->type]->write" .
It then frees the block list
.I bp
before
returning the number of bytes written.
.PP
Most built-in devices do not allow
.IR create ,
.IR remove
or
.I wstat
on their files.
.IR Devcreate ,
.I devremove
and
.I devwstat
are stubs that raise an
.IR error (9),
.BR Eperm .
They can be named directly in a device driver's device
switch (the
.B Dev
structure in
.BR /sys/src/9/port/portdat.h :
see
.IR dev (9)).
.PP
.IR Devreset ,
.I devinit
and
.I devshutdown
are also stubs;
they do nothing.
A device driver puts them in its
.B Dev
structure when it need take no action on device reset, initialisation, or shut down.
.PP
.I Openmode
is used by a driver that does not use
.IR devopen ,
to check the open mode it receives in its open
routine.
.I Openmode
returns mode
.IR o ,
the mode parameter to
.IR sys-open (2)
or
.IR sys-create ,
shorn of
.BR OTRUNC
and similar options,
and reduced to one of
.BR OREAD ,
.BR OWRITE
or
.BR ORDWR .
In particular,
.B OEXEC
becomes
.B OREAD
within the kernel.
.I Openmode
raises an
.IR error (9)
.B Ebadarg
instead of returning, if
.I o
is an invalid mode (eg, reserved bits set).
.SH SOURCE
.B /sys/src/9/port/dev.c
.SH SEE ALSO
.IR allocb (9),
.IR eve (9),
.IR qio (9)