As Linux desktop implementations become more sophisticated, they
increasingly need to know about what is going on in the kernel. The
desktop code would like to be able to respond properly to events like
"disc inserted," "disk full," "processor overheating," "printer on fire,"
and so on. So far, much of this functionality has been implemented by
polling devices and /proc
files and looking for changes. That
solution is, to say the least, inelegant.
As a way of improving things, Robert Love has posted a patch (since updated) adding a kernel event notification.
This patch, initially by Arjan van de Ven, uses the netlink mechanism to
broadcast events out to interested user-space processes. The intent is for
the events to be further redistributed using D-BUS, but other uses are possible.
Within the kernel, events are created with a call to
int send_kevent(enum kevent type,
const char *object,
const char *signal,
const char *fmt, ...);
The type argument gives the broad class of the event; current
options are KEVENT_GENERAL, KEVENT_STORAGE,
KEVENT_POWER, KEVENT_FS, and KEVENT_HOTPLUG.
The object is a unique string giving the source of the event; it
is derived from the location of the source file in the kernel tree. The
signal says what is actually happening, and the rest of the
arguments are a printk()-style format string and arguments giving
further information. The patch only adds one set of calls, for noting CPU
temperature transitions; they look like:
"Cpu: %d\n", cpu);
The patch as a whole is not particularly controversial, but there are some
concerns about the "object" namespace. Some developers would like to see
the mechanism more closely tied into the device model, so that the object
as represented here is related to an object in the sysfs hierarchy. Some
have asked whether this mechanism should replace the current hotplug
interface; that is not the intent, however. There has also been a call for
some wrappers to ensure that, for example, device drivers all generate the
same sort of event for the same kind of situation.
This is all detail work; chances are that the event code will find its way
into the mainline in some form. Then there is the little issue of making
the desktop actually respond to these events in a useful way. But that, of
course, is just a user-space problem.
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