The
ppbus
system provides a uniform, modular and architecture-independent
system for the implementation of drivers to control various parallel devices,
and to utilize different parallel port chipsets.
DEVICE DRIVERS
In order to write new drivers or port existing drivers, the ppbus system
provides the following facilities:
architecture-independent macros or functions to access parallel ports
mechanism to allow various devices to share the same parallel port
a user interface named
ppi(4)
that allows parallel port access from outside the kernel without conflicting
with kernel-in drivers.
Developing new drivers
The ppbus system has been designed to support the development of standard
and non-standard software:
Driver Ta Description
vpo Ta VPI0 parallel to Adaptec AIC-7110 SCSI controller driver
It uses standard and non-standard parallel port accesses.
ppi Ta Parallel port interface for general I/O
pps Ta Pulse per second Timing Interface
lpbb Ta Philips official parallel port I2C bit-banging interface
Porting existing drivers
Another approach to the ppbus system is to port existing drivers.
Various drivers have already been ported:
Driver Ta Description
lpt Ta lpt printer driver
plip Ta lp parallel network interface driver
ppbus should let you port any other software even from other operating systems
that provide similar services.
PARALLEL PORT CHIPSETS
Parallel port chipset support is provided by
ppc(4).
The ppbus system provides functions and macros to allocate a new
parallel port bus, then initialize it and upper peripheral device drivers.
ppc makes chipset detection and initialization and then calls ppbus attach
functions to initialize the ppbus system.
PARALLEL PORT MODEL
The logical parallel port model chosen for the ppbus system is the PC's
parallel port model.
Consequently, for the i386 implementation of ppbus,
most of the services provided by ppc are macros for inb()
and outb() calls.
But, for an other architecture, accesses to one of our logical
registers (data, status, control...) may require more than one I/O access.
Description
The parallel port may operate in the following modes:
compatible mode, also called Centronics mode
bidirectional 8/4-bits mode, also called NIBBLE mode
byte mode, also called PS/2 mode
Extended Capability Port mode, ECP
Enhanced Parallel Port mode, EPP
mixed ECP+EPP or ECP+PS/2 modes
Compatible mode
This mode defines the protocol used by most PCs to transfer data to a printer.
In this mode, data is placed on the port's data lines, the printer status is
checked for no errors and that it is not busy, and then a data Strobe is
generated by the software to clock the data to the printer.
Many I/O controllers have implemented a mode that uses a FIFO buffer to
transfer data with the Compatibility mode protocol.
This mode is referred to as
"Fast Centronics" or "Parallel Port FIFO mode".
Bidirectional mode
The NIBBLE mode is the most common way to get reverse channel data from a
printer or peripheral.
Combined with the standard host to printer mode, it
provides a complete bidirectional channel.
In this mode, outputs are 8-bits long.
Inputs are accomplished by reading
4 of the 8 bits of the status register.
Byte mode
In this mode, the data register is used either for outputs and inputs.
Then,
any transfer is 8-bits long.
Extended Capability Port mode
The ECP protocol was proposed as an advanced mode for communication with
printer and scanner type peripherals.
Like the EPP protocol, ECP mode provides
for a high performance bidirectional communication path between the host
adapter and the peripheral.
ECP protocol features include:
Run_Length_Encoding (RLE) data compression for host adapters
FIFOs for both the forward and reverse channels
DMA as well as programmed I/O for the host register interface.
Enhanced Parallel Port mode
The EPP protocol was originally developed as a means to provide a high
performance parallel port link that would still be compatible with the
standard parallel port.
The EPP mode has two types of cycle: address and data.
What makes the
difference at hardware level is the strobe of the byte placed on the data
lines.
Data are strobed with nAutofeed, addresses are strobed with
nSelectin signals.
A particularity of the ISA implementation of the EPP protocol is that an
EPP cycle fits in an ISA cycle.
In this fashion, parallel port peripherals can
operate at close to the same performance levels as an equivalent ISA plug-in
card.
At software level, you may implement the protocol you wish, using data and
address cycles as you want.
This is for the IEEE1284 compatible part.
Then,
peripheral vendors may implement protocol handshake with the following
status lines: PError, nFault and Select.
Try to know how these lines toggle
with your peripheral, allowing the peripheral to request more data, stop the
transfer and so on.
At any time, the peripheral may interrupt the host with the nAck signal without
disturbing the current transfer.
Mixed modes
Some manufacturers, like SMC, have implemented chipsets that support mixed
modes.
With such chipsets, mode switching is available at any time by
accessing the extended control register.
IEEE1284-1994 Standard
Background
This standard is also named "IEEE Standard Signaling Method for a
Bidirectional Parallel Peripheral Interface for Personal Computers".
It
defines a signaling method for asynchronous, fully interlocked, bidirectional
parallel communications between hosts and printers or other peripherals.
It
also specifies a format for a peripheral identification string and a method of
returning this string to the host outside of the bidirectional data stream.
This standard is architecture independent and only specifies dialog handshake
at signal level.
One should refer to architecture specific documentation in
order to manipulate machine dependent registers, mapped memory or other
methods to control these signals.
The IEEE1284 protocol is fully oriented with all supported parallel port
modes.
The computer acts as master and the peripheral as slave.
Any transfer is defined as a finite state automaton.
It allows software to
properly manage the fully interlocked scheme of the signaling method.
The compatible mode is supported "as is" without any negotiation because it
is compatible.
Any other mode must be firstly negotiated by the host to check
it is supported by the peripheral, then to enter one of the forward idle
states.
At any time, the slave may want to send data to the host.
This is only
possible from forward idle states (nibble, byte, ecp...).
So, the
host must have previously negotiated to permit the peripheral to
request transfer.
Interrupt lines may be dedicated to the requesting signals
to prevent time consuming polling methods.
But peripheral requests are only a hint to the master host.
If the host
accepts the transfer, it must firstly negotiate the reverse mode and then
starts the transfer.
At any time during reverse transfer, the host may
terminate the transfer or the slave may drive wires to signal that no more
data is available.
Implementation
IEEE1284 Standard support has been implemented at the top of the ppbus system
as a set of procedures that perform high level functions like negotiation,
termination, transfer in any mode without bothering you with low level
characteristics of the standard.
IEEE1284 interacts with the ppbus system as little as possible.
That means
you still have to request the ppbus when you want to access it, the negotiate
function does not do it for you.
And of course, release it later.
ARCHITECTURE
adapter, ppbus and device layers
First, there is the
adapter
layer, the lowest of the ppbus system.
It provides
chipset abstraction throw a set of low level functions that maps the logical
model to the underlying hardware.
Secondly, there is the
ppbus
layer that provides functions to:
share the parallel port bus among the daisy-chain like connected devices
manage devices linked to ppbus
propose an arch-independent interface to access the hardware layer.
Finally, the
device
layer gathers the parallel peripheral device drivers.
Parallel modes management
We have to differentiate operating modes at various ppbus system layers.
Actually, ppbus and adapter operating modes on one hands and for each
one, current and available modes are separated.
With this level of abstraction a particular chipset may commute from any
native mode to any other mode emulated with extended modes without
disturbing upper layers.
For example, most chipsets support NIBBLE mode as
native and emulated with ECP and/or EPP.
This architecture should support IEEE1284-1994 modes.
FEATURES
The boot process
The boot process starts with the probe stage of the
ppc(4)
driver during ISA bus (PC architecture) initialization.
During attachment of
the ppc driver, a new ppbus structure is allocated, then probe and attachment
for this new bus node are called.
ppbus attachment tries to detect any PnP parallel peripheral (according to
"Plug and Play Parallel Port Devices"
draft from (c)1993-4 Microsoft Corporation)
then probes and attaches known device drivers.
During probe, device drivers are supposed to request the ppbus and try to
set their operating mode.
This mode will be saved in the context structure and
returned each time the driver requests the ppbus.
Bus allocation and interrupts
ppbus allocation is mandatory not to corrupt I/O of other devices.
Another
usage of ppbus allocation is to reserve the port and receive incoming
interrupts.
High level interrupt handlers are connected to the ppbus system thanks to the
newbus
BUS_SETUP_INTR ();
and
BUS_TEARDOWN_INTR ();
functions.
But, in order to attach a handler, drivers must
own the bus.
Consequently, a ppbus request is mandatory in order to call the above
functions (see existing drivers for more info).
Note that the interrupt handler
is automatically released when the ppbus is released.
Microsequences
Microsequences
is a general purpose mechanism to allow fast low-level
manipulation of the parallel port.
Microsequences may be used to do either
standard (in IEEE1284 modes) or non-standard transfers.
The philosophy of
microsequences is to avoid the overhead of the ppbus layer and do most of
the job at adapter level.
A microsequence is an array of opcodes and parameters.
Each opcode codes an
operation (opcodes are described in
microseq(9)).
Standard I/O operations are implemented at ppbus level whereas basic I/O
operations and microseq language are coded at adapter level for efficiency.
As an example, the
vpo(4)
driver uses microsequences to implement:
a modified version of the NIBBLE transfer mode
various I/O sequences to initialize, select and allocate the peripheral
