Advantech PCI-1736UP Specifikace stáhnout pdf (Strana 152)

Comedi

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All user-space Comedi instructions and commands are transmitted to kernel space through a traditional ioctl() system call. (See

lib/ioctl.c in Comedilib.) The user-space information command is encoded as a number in the ioctl() call, and decoded

in the kernel-space library. There, they are executed by their kernel-space counterparts. This is done in the comedi_fops.c

ﬁle in the Comedi sources: the comedi_unlocked_ioctl() function processes the results of the ioctl() system call,

interprets its contents, and then calls the corresponding kernel-space do_..._ioctl() function(s). For example, a Comedi

instruction is further processed by the do_insn_ioctl() function. (Which, in turn, uses parse_insn() for further detailed

processing.)

The data corresponding to instructions and commands is transmitted with the copy_from_user() function; acquisition data

captured by the interface card passes the kernel/user-space boundary with the help of a copy_to_user() function.

6.2 Generic functionality

The major include ﬁles of the kernel-space part of Comedi are:

• include/linux/comedidev.h: the header ﬁle for kernel-only structures (device, subdevice, async (i.e., buffer/event/in-

terrupt/callback functionality for asynchronous DAQ in a Comedi command), driver, lrange), variables, inline functions and

constants.

• include/linux/comedi_rt.h: all the real-time stuff, such as management of ISR in RTAI and RTLinux/Free, and

spinlocks for atomic sections.

• include/linux/comedilib.h: the header ﬁle for the kernel library of Comedi (kcomedilib module).

From all the relevant Comedi device driver code that is found in the comedi kernel module source directory, the generic

functionality is contained in two parts:

• A couple of C ﬁles contain the infrastructural support. From these C ﬁles, it’s especially the comedi_fops.c ﬁle that

implements what makes Comedi into what people want to use it for: a library that has solved 90% of the DAQ device driver

efforts, once and for all.

• For real-time applications, the subdirectory kcomedilib implements an interface in the kernel that is similar to the Comedi

interface accessible through the user-space Comedi library.

There are some differences in what is possible and/or needed in kernel-space and in user-space, so the functionalities offered in

kcomedilib are not an exact copy of the user-space library. For example, locking, interrupt handling, real-time execution,

callback handling, etc., are only available in kernel-space.

Most drivers don’t make use (yet) of these real-time functionalities.

6.2.1 Data structures

This Section explains the generic data structures that a device driver interacts with:

typedef struct comedi_lrange_struct comedi_lrange;

typedef struct comedi_subdevice_struct comedi_subdevice;

typedef struct comedi_device_struct comedi_device:

typedef struct comedi_async_struct comedi_async

typedef struct comedi_driver_struct comedi_driver;

They can be found in include/linux/comedidev.h. Most of the ﬁelds are ﬁlled in by the Comedi infrastructure, but

there are still quite a handful that your driver must provide or use. As for the user-level Comedi, each of the hierarchical layers

has its own data structures: range (comedi_lrange), subdevice, and device.

Note that these kernel-space data structures have similar names as their user-space equivalents, but they have a different (kernel-

side) view on the DAQ problem and a different meaning: they encode the interaction with the hardware, not with the user.

However, the comedi_insn and comedi_cmd data structures are shared between user-space and kernel-space: this should come

as no surprise, since these data structures contain all information that the user-space program must transfer to the kernel-space

driver for each acquisition.

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