Re: [Gumstix-users] Interrupt notifications

[Kevyn-Alexandre P. <ka...@ro...>]

Hi Dave, Markus,

Instead of signals, inotify communicates with applications via a single
file descriptor. This file descriptor is select-, poll-, epoll- and
read-able. Simple and fast.

Also,

On Mon, 2011-04-11 at 00:32 +0100, Dave Hylands wrote:
> Hi Markus,
> 
> On Fri, Apr 8, 2011 at 9:14 PM, Markus Svilans <msv...@ae...> wrote:
> > Kevyn,
> >
> > Can you share some code, showing how you would use inotify with a serial
> > port, to detect when data is received on the port? Why is that approach
> > better/simpler than using just plain select() or poll()?
> 
> When using select or poll, there may be some latency since your thread
> which does the select or poll may be off doing something else when the
> new data arrive son the serial port.
> 

> When you use inotify, I see 2 choices: 1 - you process the data in the
> signal handler - which reduces the latency,

The old version, called dnotify, interface to user space is signals. Not
inotify the new version.

>  or 2 - you just set a flag
> which causes the data to be processed later - which is essentially the
> same as select/poll.

A call to inotify_add_watch() adds a watch for the one or more events
given by the bitmask mask on the file path to the inotify instance
associated with the file descriptor fd.

>  inotify will still have slightly higher latency
> than using a separate thread, since signal handlers are only called
> when the user code does system calls.
> 

> Using a separate thread gives the best responsiveness, but has a
> slightly higher complexity to setup than using inotify or select/poll.
> 
> Using a separate thread should also require less kernel-to-user transitions.
> 

Blocking on a read() system call does not give nice responsiveness,
unless your application is multi thread, You will need to add that new
thread! 

Like Robert says: "Thankfully, the inotify file descriptor can be polled
or selected on, allowing inotify to be multiplexed along with other I/O
and optionally integrated into an application's mainloop."

You can found advantages / disadvantages on internet about select / poll
(Poll have been design to fix problem of select, but this one is
sometimes even use for a reason of compatibility). Epoll is there to
improve poll / select limitations, when hundreds of file descriptors
have to be polled.   

Concerning the latency, it depends on your needs. Inotify events are
queued asynchronously, in real time as the events happen. It's to you to
decide how you will read these event if latency is a real issue and not
early optimization.   

Best Regards,

Kevyn-Alexandre Paré