Next
Previous
Contents
The USB (Universal Serial Bus) is a high speed bus on an external
cable that plugs into a PC. The external bus cable has its own
communication protocols and doesn't use any IRQs, I/0 addresses (or
other bus-resources). Communication is by packets, something like the
Internet, only there are time-slice allocations which prevent any one
device from hogging the bus if other devices need it. There are free
time slots which allow every device to send a short message to the bus
controller without any need for IRQs on the bus.
However, the USB bus controller inside the PC does have an IRQ and an
address on the PCI bus (or ISA) which are used for communication
between the CPU and all devices on the USB. Thus there's no resource
allocations needed for the individual devices on the USB. One could
also think of this as all devices on the USB sharing the one interrupt
and address. If a device is on the USB it needs a driver that
understands the USB.
But each device on the USB does have an IDs, just like cards do on the PCI
bus. So Linux likely maintains a table of these IDs so that device
drivers can check them to find their device. The USB also support
"hot plug". To find out what is on the USB bus, you could use a
general hardware detection tool like "discover" or "hwinfo".
"Hot plug" is where you plug something into a PC (usually via a
cable connection) and it is instantly detected. If required, it is
configured with bus-resources. The driver for it is also started,
perhaps by loading a module for it. For this to work the hardware
used must be designed for hot plugging. One can hot plug certain PCI
cards (Cardbus), USB devices, and IEEE 1394 devices (Firewire).
When a new device is detected, it's registers are read so as to get
the ID number of the device. Then to find a driver, Linux must
maintain a table mapping ID numbers to drivers. It wasn't until
kernel 2.4 that such a table existed since Linux once shunned
centralized PnP. It's named: MODULE_DEVICE_TABLE.
"Hot Swap" is where you remove an old device and then plug in a new
device to replace the old one. You have thus "swapped" devices. Now
in addition to being able to detect that a new device has been plugged
in, the removal of the old device needs to be detected too.
External devices that connect to the serial port via a cable (such
as external modems) can also be called Plug-and-Play. Since only the
serial port itself needs bus-resources (an IRQ and I/O address) there are
no bus-resources to allocate to such plug-in devices. In this case,
PnP is used only to identify the modem (read it's model code number).
This could be important if the modem is a software modem (linmodem)
and requires a special driver. There is a special PnP specification
for such external serial devices (something connected to the serial
port).
Linux doesn't support this yet ?? For a hardware modem, the ordinary
serial driver will work OK so there's little need for using the
special serial PnP to find a driver. You still need to tell the
communications program what port (such as /dev/ttyS1) the modem is on.
With PnP you wouldn't need to even do this. With the advent of
software modems that have Linux drivers (linmodems), it would be nice
to have the appropriate driver install itself automatically via PnP.
Next
Previous
Contents
| 
Free eBook: 
