man/man4/fdc.4

.\"
.\" Copyright (c) 1994 Wilko Bulte
.\" Copyright (c) 2001 Joerg Wunsch
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.Dd October 10, 2023
.Dt FDC 4
.Os
.Sh NAME
.Nm fdc
.Nd "PC architecture floppy disk controller driver"
.Sh SYNOPSIS
.Cd device fdc
.Pp
In
.Pa /boot/device.hints :
.Cd hint.fdc.0.at="isa"
.Cd hint.fdc.0.port="0x3F0"
.Cd hint.fdc.0.irq="6"
.Cd hint.fdc.0.drq="2"
.Cd hint.fdc.0.flags="0x0"
.Cd hint.fd.0.at="fdc0"
.Cd hint.fd.0.drive="0"
.Cd hint.fd.0.flags="0x0"
.Cd hint.fd.1.at="fdc0"
.Cd hint.fd.1.drive="1"
.Cd hint.fd.1.flags="0x0"
.Sh DESCRIPTION
.Ss Device Usage
This driver provides access to floppy disk drives.
Floppy disks using
either FM (single-density) or MFM (double or high-density) recording
can be handled.
.Pp
Floppy disk controllers can connect up to four drives each.
The
.Nm
driver can currently handle up to two drives per controller (or four
drives on ACPI).
Upon
driver initialization, an attempt is made to find out the type of the
floppy controller in use.
The known controller types are either the
original NE765 or i8272 chips, or alternatively
.Em enhanced
controllers that are compatible with the NE72065 or i82077 chips.
These enhanced controllers (among other enhancements) implement a FIFO
for floppy data transfers that will automatically be enabled once an
enhanced chip has been detected.
This FIFO activation can be disabled
using the per-controller flags value of
.Ar 0x1 .
.Pp
By default, this driver creates a single device node
.Pa /dev/fd Ns Ar N
for each attached drive with number
.Ar N .
For historical reasons, device nodes that use a trailing UFS-style
partition letter (ranging from
.Sq a
through
.Sq h )
can also be accessed, which will be implemented as symbolic links to
the main device node.
.Pp
Accessing the main device node will attempt to autodetect the density
of the available medium for multi-density devices.
Thus it is
possible to use either a 720 KB medium or a 1440 KB medium in a
high-density 3.5 inch standard floppy drive.
Normally, this
autodetection will only happen once at the first call to
.Xr open 2
for the device after inserting the medium.
This assumes the drive
offers proper changeline support so media changes can be detected by
the driver.
To indicate a drive that does not have the changeline support,
this can be overridden using the per-drive device flags value of
.Ar 0x10
(causing each call to
.Xr open 2
to perform the autodetection).
.Pp
When trying to use a floppy device with special-density media, other
device nodes can be created, of the form
.Pa /dev/fd Ns Ar N . Ns Ar MMMM ,
where
.Ar N
is the drive number, and
.Ar MMMM
is a number between one and four digits describing the device density.
Up to 15 additional subdevices per drive can be created that way.
The
administrator is free to decide on a policy how to assign these
numbers.
The two common policies are to either implement subdevices
numbered 1 through 15, or to use a number that describes the medium
density in kilobytes.
Initially, each of those devices will be
configured to the maximal density that is possible for the drive type
(like 1200 KB for 5.25 inch HD drives or 1440 KB for 3.5 inch HD
drives).
The desired density to be used on that subdevice needs to be
configured using
.Xr fdcontrol 8 .
.Pp
Drive types are configured using the lower four bits of the per-drive
device flags.
The following values can be specified:
.Bl -tag -width 2n -offset indent
.It Ar 1
5.25 inch double-density device with 40 cylinders (360 KB native
capacity)
.It Ar 2
5.25 inch high-density device with 80 cylinders (1200 KB native
capacity)
.It Ar 3
3.5 inch double-density device with 80 cylinders (720 KB native
capacity)
.It Ar 4
3.5 inch high-density device with 80 cylinders (1440 KB native
capacity)
.It Ar 5
3.5 inch extra-density device with 80 cylinders (2880 KB native
capacity, usage currently restricted to at most 1440 KB media)
.It Ar 6
Same as type 5, available for compatibility with some BIOSes
.El
.Pp
On IA32 architectures, the drive type can be specified as 0 for the
drives.
In that case, the CMOS configuration memory will be
consulted to obtain the value for that drive.
The ACPI probe automatically determines these values via the _FDE and
_FDI methods, but this can be overridden by specifying a drive type hint.
.Pp
Normally, each configured drive will be probed at initialization
time, using a short seek sequence.
This is intended to find out about
drives that have been configured but are actually missing or
otherwise not responding.
(The ACPI probe method does not perform this seek.)
In some environments (like laptops with
detachable drives), it might be desirable to bypass this drive probe,
and pretend a drive to be there so the driver autoconfiguration will
work even if the drive is currently not present.
For that purpose, a
per-drive device flags value of
.Ar 0x20
needs to be specified.
.Ss Programming Interface
In addition to the normal read and write functionality, the
.Nm
driver offers a number of configurable options using
.Xr ioctl 2 .
In order to access any of this functionality, programmers need to
include the header file
.In sys/fdcio.h
into their programs.
The call to
.Xr open 2
can be performed in two possible ways.
When opening the device
without the
.Dv O_NONBLOCK
flag set, the device is opened in a normal way, which would cause the
main device nodes to perform automatic media density selection, and which
will yield a file descriptor that is fully available for any I/O operation
or any of the following
.Xr ioctl 2
commands.
.Pp
When opening the device with
.Dv O_NONBLOCK
set, automatic media density selection will be bypassed, and the device
remains in a half-opened state.
No actual I/O operations are possible, but
many of the
.Xr ioctl 2
commands described below can be performed.
This mode is intended for
access to the device without the requirement to have an accessible
media present, like for status inquiries to the drive, or in order to
format a medium.
.Dv O_NONBLOCK
needs to be cleared before I/O operations are possible on the descriptor,
which requires a prior specification of the density using the
.Dv FD_STYPE
command (see below).
Operations that are not allowed on the half-opened
descriptor will cause an error value of
.Er EAGAIN .
.Pp
The following
.Xr ioctl 2
commands are currently available:
.Bl -tag -width ".Dv FD_READID"
.It Dv FD_FORM
Used to format a floppy disk medium.
Third argument is a pointer to a
.Vt "struct fd_formb"
specifying which track to format, and which parameters to fill into
the ID fields of the floppy disk medium.
.It Dv FD_GTYPE
Returns the current density definition record for the selected device.
Third argument is a pointer to
.Vt "struct fd_type" .
.It Dv FD_STYPE
Adjusts the density definition of the selected device.
Third argument
is a pointer to
.Vt "struct fd_type" .
For the fixed-density subdevices (1 through 15 per drive), this
operation is restricted to a process with superuser privileges.
For
the auto-selecting subdevice 0, the operation is temporarily allowed
to any process, but this setting will be lost again upon the next
autoselection.
This can be used when formatting a new medium (which
will require to open the device using
.Dv O_NONBLOCK ,
and thus to later adjust the density using
.Dv FD_STYPE ) .
.It Dv FD_GOPTS
Obtain the current drive options.
Third argument is a pointer to
.Vt int ,
containing a bitwise union of the following possible flag values:
.Bl -tag -width ".Dv FDOPT_NOERRLOG"
.It Dv FDOPT_NORETRY
Do not automatically retry operations upon failure.
.It Dv FDOPT_NOERRLOG
Do not cause
.Dq "hard error"
kernel logs for failed I/O operations.
.It Dv FDOPT_NOERROR
Do not indicate I/O errors when returning from
.Xr read 2
or
.Xr write 2
system calls.
The caller is assumed to use
.Dv FD_GSTAT
calls in order to inquire about the success of each operation.
This
is intended to allow even erroneous data from bad blocks to be
retrieved using normal I/O operations.
.It Dv FDOPT_AUTOSEL
Device performs automatic density selection.
Unlike the above flags,
this one is read-only.
.El
.It Dv FD_SOPTS
Set device options, see above for their meaning.
Third argument is a
pointer to
.Vt int .
Drive options will always be cleared when closing the descriptor.
.It Dv FD_CLRERR
Clear the internal low-level error counter.
Normally, controller-level
I/O errors are only logged up to
.Dv FDC_ERRMAX
errors (currently defined to 100).
This command resets the counter.
Requires superuser privileges.
.It Dv FD_READID
Read one sector ID field from the floppy disk medium.
Third argument is
a pointer to
.Vt "struct fdc_readid" ,
where the read data will be returned.
Can be used to analyze a floppy
disk medium.
.It Dv FD_GSTAT
Return the recent floppy disk controller status, if available.
Third
argument is a pointer to
.Vt "struct fdc_status" ,
where the status registers (ST0, ST1, ST2, C, H, R, and N) are being
returned.
.Er EINVAL
will be caused if no recent status is available.
.It Dv FD_GDTYPE
Returns the floppy disk drive type.
Third argument is a pointer to
.Vt "enum fd_drivetype" .
This type is the same as being used in the per-drive configuration
flags, or in the CMOS configuration data or ACPI namespace on IA32 systems.
.El
.Sh SYSCTL VARIABLES
.Bl -tag -width "debug.fdc.debugflags"
.It Dv debug.fdc.debugflags
Selectively enable debugging by setting one or more flags.
.Bl -tag -width "0x40"
.It Dv 0x01
Dump device registers on reset.
.It Dv 0x02
When an IO operation completes, print the number of retries
when that number is greater than zero.
.It Dv 0x04
Print when the number of retries exceeds
.Dv debug.fdc.retries
.Pq Dv EIO .
Print when the option
.Dv FDOPT_NOERROR
is set and an error would have returned from a write operation.
.It Dv 0x08
Print detailed IO command information.
.It Dv 0x10
Print status registers.
.It Dv 0x20
Print detailed status registers when interrupts complete.
Print the source code line number close to the source of a
non-zero return from a thread worker operation.
.It Dv 0x40
Print when the disk appears to be lost.
Print cylinder, head, sector, and sector shift information
after a request to read an ID field.
Notify whether a disk probe resulted in finding a disk.
When detecting the density of media present, indicate whether
the autosensing was successful, and if so, the size of the
medium in kilobytes.
Print detailed type information when setting the drive type.
.It Dv 0x80
Print when an unknown IOCTL is used.
.El
.It Dv debug.fdc.fifo
For enhanced controllers, allows a non-default FIFO
threshold setting. The default is 8 bytes.
.It Dv debug.fdc.retries
Maximum number of retries to attempt. The default is 10.
.It Dv debug.fdc.spec1
Specification byte one (step-rate + head unload).
The default step rate is 6 ms. The default head unload
time is 240 ms.
.It Dv debug.fdc.spec2
Specification byte two (head load time + no-dma).
The default head load time is 16 ms, and no-dma is 0
.Pq disabled .
.It Dv debug.fdc.settle
Head settling time in
.Sy settle
/ hz seconds. The default value is set during device attach.
.El
.Sh FILES
.Bl -tag -width ".Pa /dev/fd*" -compact
.It Pa /dev/fd*
floppy disk device nodes
.El
.Sh SEE ALSO
.Xr fdread 1 ,
.Xr fdwrite 1 ,
.Xr ioctl 2 ,
.Xr open 2 ,
.Xr read 2 ,
.Xr write 2 ,
.Xr fdcontrol 8 ,
.Xr fdformat 8
.Sh AUTHORS
.An -nosplit
This man page was initially written by
.An Wilko Bulte ,
and later vastly rewritten by
.An J\(:org Wunsch .