1 Hardware driver interface
2 =========================
4 [ this is a little outdated -ds ]
5 [ this is a lot outdated -fmh ]
6 [ this is a little bit less outdated, hopefully -rs ]
7 [ the 'adding new drivers' section has been updated -ija ]
24 This comedi hardware driver writing HOWTO is written to help you write a
25 driver for your particular choice of hardware. You should be familiar with
26 how comedi works from the user's point of view, i.e., from a process that is
27 utilizing the comedi driver.
29 This guide does not explain the details of things like reading and writing
30 I/O ports, Linux kernel internals, or interrupt processing. These issues are
31 covered in other documents, e.g.
33 - The IO-Port Programming Mini HOWTO:
34 http://www.linuxdoc.org/HOWTO/mini/IO-Port-Programming.html
36 - Linux Kernel Module Programming Guide
37 http://www.linuxdoc.org/LDP/lkmpg/mpg.html
39 - Kernel Hacker's Guide
40 http://www.linuxdoc.org/LDP/khg/HyperNews/get/khg.html
42 - Linux Device Drivers, 2nd Edition
43 http://www.xml.com/ldd/chapter/book/index.html
52 Currently hardware drivers need to be part of the source tree and be
53 compiled with the rest of the comedi driver into the module comedi.o. Later
54 versions will hopefully support separate compiling/separate modules, etc.
56 The source for the comedi module is located in the 'comedi/' directory,
57 including the device independent part. The source files of the hardware
58 drivers for the different boards are located in 'comedi/drivers/', the
59 kernel space library (which is used for accessing comedi from realtime
60 programs) lives in 'comedi/kcomedilib/'.
62 In the drivers' directory there is a striped-down example ('skel.c') that
63 may be a good starting point for new hardware drivers.
70 The best way to write a new driver is to take one of the existing ones (e.g.
71 the 'skel' driver) and modify it to your own needs. For integrating new
72 drivers in the comedi source tree the following things have to be done:
74 - Put your new driver into 'comedi/drivers/mydriver.c'.
75 - Edit 'comedi/drivers/Makefile.am' and update 'module_PROGRAMS',
76 'pci_modules2', 'pcmcia_modules', or 'usb_modules', depending on the type
77 of driver. (There is also a 'pci_modules1' used by a few helper modules.)
78 Add a 'mydriver_ko_sources' macro set to your 'mydriver.c' file (look at
79 the other examples). If your driver has its own include files, add them
80 to the EXTRA_DIST macro.
81 - Edit 'comedi/drivers/Kbuild' and add a 'obj-m += mydriver.o',
82 'obj-$(COMEDI_CONFIG_PCI_MODULES) += mydriver.o',
83 'obj-$(COMEDI_CONFIG_PCMCIA_MODULES) += mydriver.o', or
84 'obj-$(COMEDI_CONFIG_USB_MODULES) += mydriver.o' line, depending on the
87 Now run 'autoreconf' to update the configure script, re-run './configure'
88 with any necessary command-line options to update the makefiles, and run
89 'make' to rebuild comedi and be happy.
91 If you want to have your driver included in the comedi distribution
92 (you _definitely_ want to :) ) send it to the Comedi mailing list for
93 review and integration. (See the top-level README for details of the
101 Implementation details for the following things can be found in the skel
102 driver. Each driver has to register two functions which are called when you
103 configure and deconfigure your board:
108 In the 'attach' function all properties of a device and its subdevices are
109 defined. As part of this pointers to the low level instructions being
110 supported by the subdevice have to be set (see next section) which define
111 the basic functionality.
117 Instructions (insns) are comedi's low level functins for accessing all kinds
118 of channels, like analog or digital IOs.
120 Drivers for digital IOs should implement the following functions:
123 Drivers set this if they have a function that supports reading and writing
124 multiple bits in a digital I/O subdevice at the same time. Most (if not
125 all) of the drivers use this interface instead of insn_read and insn_write
129 Implements INSN_CONFIG instructions. Currently used for configuring the
130 direction of digital I/O lines, although will eventually be used for
131 generic configuration of drivers that is outside the scope of the
132 currently defined Comedi interface.
134 Drivers for analog IOs should implement these function:
137 Analog inputs have to implement insn_read.
140 The same with insn_write.
143 [THIS SEEMS TO BE OBSOLETE??? -rs] -------------------------------------------
145 Inside the initialization function, you should perform the following tasks:
147 o Announce that the hardware driver has begun initialization by a
148 printk("comedi%d: driver: ",minor);
150 o Check and request the I/O port region, IRQ, DMA, and other hardware
151 resources. It is convenient here if you verify the existence of the
152 hardware and the correctness of the other information given.
153 Sometimes, unfortunately, this cannot be done.
155 o Fill in the comedi_device structure.
157 o Allocate your private data structure and subdevices.
159 o Set up each subdevice.
161 o Return 0, indicating success. If there were any errors along
162 the way, you should return the appropriate error number. If
163 an error is returned, the _detach function is called. The
164 _detach function should check any resources that may have been
165 allocated and release them as necessary. The comedi core frees
166 dev->subdevices and dev->private, so this does not need to be
172 A few things to strive for:
174 o Your hardware driver should be functional appropriate to
175 the resources allocated. I.e., if the driver is fully
176 functional when configured with an IRQ and DMA, it should
177 still function moderately well with just an IRQ, or still
178 do minor tasks without IRQ or DMA. Does your driver really
179 require an IRQ to do digital I/O? Maybe someone will want
180 to use your driver *just* to do digital I/O and has no
181 interrupts available.
183 o Drivers are to have absolutely *NO* global variables, mainly
184 because the existence of global variables immediately negates
185 any possibility of using the driver for two devices. The
186 pointer dev->private should be used to point to a structure
187 containing any additional variables needed by a driver/device
190 o Drivers should report errors and warnings via a printk line
191 that starts with "comedi%d: driver_name:" where %d is the
192 minor number of the device.