6.2 IO & IRQ Overview
For a serial port to work properly it first must be given both an IO address and an IRQ. For old hardware (of mid 1990s), jumpers on a card or the a saved BIOS setting does it. For newer hardware the BIOS or Linux must set them at boot-time, and the new hardware doesn't remember how it was set once it's powered Enabling hardware it gives it both an IRQ and an IO address. Without an IO address, it can't be used. Without an IRQ it will need to use inefficient polling methods for which one must set the IRQ to 0 in the serial driver. In olden days IRQs and IO addresses were set by jumpers or switches on a serial port card. Today these are set by digital signals sent to the hardware by the BIOS or Linux. It all should get configured automatically (provided the BIOS has not been previously set up to disable it) so that you only need to read this if you're having problems or if you want to understand how it works.
The driver must of course know both the IO address and IRQ so that it can talk to the serial port chip. Modern serial port drivers (kernel 2.4) try to determine this by PnP methods so one doesn't normally need to tell the driver (by using "setserial"). A driver may also set an IO address or IRQ in the hardware. But unfortunately, there is some PCI serial port hardware that the driver doesn't recognize so you might need to enable the port yourself. See PCI: Enabling a disabled port
For the old ISA bus, the driver also probes likely serial port addresses to see if there are any serial ports there. This works for the case of jumpers and sometimes works for a ISA PnP port when the driver doesn't do ISA PnP (prior to kernel 2.4).
Locating the serial port by giving it an IRQ and IO address is low-level configuring. It's often automatically done by the serial driver but sometimes you have to do it yourself. What follows repeats what was said above but in more detail.
The low-level configuring consists of assigning an IO address, IRQ, and names (such as ttyS2 = tts/2). This IO-IRQ pair must be set in both the hardware and told to the serial driver. And the driver needs to call this pair a name (such as ttyS2). We could call this "io-irq" configuring for short. The "setserial" program is one way to tell the driver. The other way is for the driver to use PnP methods to detect/set the IO/IRQ and then remember what it did. For jumpers, there is no PnP but the driver might detect the port if the jumpers are set to the usual I0/IRQ. If you need to configure but don't understand certain details it's easy to get into trouble.
When Linux starts, an effort is made to detect and configure (low-level) the serial ports. Exactly what happens depends on your BIOS, hardware, Linux distribution, kernel version, etc. If the serial ports work OK, there may be no need for you to do any more low-level configuring.
If you're having problems with the serial ports, then you may need to do low-level configuring. If you have kernel 2.2 or lower, then you need to do it if you:
- Plan to use more than 2 ISA serial ports
- Are installing a new serial port (such as an internal modem)
- One or more of your serial ports have non-standard IRQs or IO addresses
Starting with kernel 2.2 you may be able to use more that 2 serial ports without doing any low-level configuring by sharing interrupts. All PCI ports should support this but for ISA, it only works for some hardware. It may be just as easy to give each port a unique interrupt if they is available. See Interrupt sharing and Kernels 2.2+
The low-level configuring (setting the IRQ and IO address) seems to cause people more trouble than the high-level stuff, although for many it's fully automatic and there is no configuring to be done. Until the port in enabled and the serial driver knows the correct IRQ and IO address, the port will not usually not work at all.
A port may be disabled, either by the BIOS or by failure of Linux to find and enable the port. For modern ports (provided the BIOS hasn't disabled them) manual PnP
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