解析Linux下的并口编程(英文版)

　　The printer is accessible through /dev/lp0; in the same way, the parallel port itself is accessible through /dev/parport0. The difference is in the level of control that you have over the wires in the parallel port cable.

　　With the printer driver, a user-space program (such as the printer spooler) can send bytes in "printer protocol". Briefly, this means that for each byte, the eight data lines are set up, then a "strobe" line tells the printer to look at the data lines, and the printer sets an "acknowledgement" line to say that it got the byte. The printer driver also allows the user-space program to read bytes in "nibble mode", which is a way of transferring data from the peripheral to the computer half a byte at a time (and so it's quite slow).

　　In contrast, the ppdev driver (accessed via /dev/parport0) allows you to:

　　examine status lines,

　　set control lines,

　　set/examine data lines (and control the direction of the data lines),

　　wait for an interrupt (triggered by one of the status lines),

　　find out how many new interrupts have occurred,

　　set up a response to an interrupt,

　　use IEEE 1284 negotiation (for telling peripheral which transfer mode, to use)

　　transfer data using a specified IEEE 1284 mode.

　　Programming interface

　　The ppdev interface is largely the same as that of other character special devices, in that it supports open, close, read, write, and ioctl. The constants for the ioctl commands are in include/linux/ppdev.h.

　　Starting and stopping: open and close

　　The device node /dev/parport0 represents any device that is connected to parport0, the first parallel port in the system. Each time the device node is opened, it represents (to the process doing the opening) a different device. It can be opened more than once, but only one instance can actually be in control of the parallel port at any time. A process that has opened /dev/parport0 shares the parallel port in the same way as any other device driver. A user-land driver may be sharing the parallel port with in-kernel device drivers as well as other user-land drivers.

　　Control: ioctl

　　Most of the control is done, naturally enough, via the ioctl call. Using ioctl, the user-land driver can control both the ppdev driver in the kernel and the physical parallel port itself. The ioctl call takes as parameters a file descriptor (the one returned from opening the device node), a command, and optionally (a pointer to) some data.

　　PPCLAIM

　　Claims access to the port. As a user-land device driver writer, you will need to do this before you are able to actually change the state of the parallel port in any way. Note that some operations only affect the ppdev driver and not the port, such as PPSETMODE; they can be performed while access to the port is not claimed.

　　PPEXCL

　　Instructs the kernel driver to forbid any sharing of the port with other drivers, i.e. it requests exclusivity. The PPEXCL command is only valid when the port is not already claimed for use, and it may mean that the next PPCLAIM ioctl will fail: some other driver may already have registered itself on that port.

　　Most device drivers don't need exclusive access to the port. It's only provided in case it is really needed, for example for devices where access to the port is required for extensive periods of time (many seconds).

　　Note that the PPEXCL ioctl doesn't actually claim the port there and then---action is deferred until the PPCLAIM ioctl is performed.

　　PPRELEASE

　　Releases the port. Releasing the port undoes the effect of claiming the port. It allows other device drivers to talk to their devices (assuming that there are any).

　　PPYIELD

　　Yields the port to another driver. This ioctl is a kind of short-hand for releasing the port and immediately reclaiming it. It gives other drivers a chance to talk to their devices, but afterwards claims the port back. An example of using this would be in a user-land printer driver: once a few characters have been written we could give the port to another device driver for a while, but if we still have characters to send to the printer we would want the port back as soon as possible.

　　It is important not to claim the parallel port for too long, as other device drivers will have no time to service their devices. If your device does not allow for parallel port sharing at all, it is better to claim the parallel port exclusively (see PPEXCL).

　　PPNEGOT

　　Performs IEEE 1284 negotiation into a particular mode. Briefly, negotiation is the method by which the host and the peripheral decide on a protocol to use when transferring data.

　　An IEEE 1284 compliant device will start out in compatibility mode, and then the host can negotiate to another mode (such as ECP).

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