Re: Interacting with a input kernel driver from user space

[Nuno Santos <nsantos@edigma.com>]

On 11/14/2011 04:09 PM, David Herrmann wrote:
> On Mon, Nov 14, 2011 at 5:00 PM, Nuno Santos<nsantos@edigma.com>  wrote:
>> Hi David,
>>
>> Thanks for your reply.
>>
>> The best way probably is using sysfs. Register one sysfs attribute per
>> value and configure your callbacks.
>> There are several other subsystems that provide wrappers for them. If
>> you could be more specific about the user-space interaction or
>> configuration values, then we could also be more specific (hopefully
>> ;)).
>>
>> sysfs? Need to dig about it. Is there any typical example I can look in
>> kernel source? If you could point me one that would be great.
> Almost every driver uses it. Look for "DEVICE_ATTR" in the driver
> sources or drivers/input/input.c for instance.
Ok. I'll look for that.
>> What do I need to exchange between the kernel and the user space is between
>> a simple byte exchange and a whole structure of several bytes.
>>
>> Do you know the concept of IOCTLS in windows? Basicly that's what looking
>> after.
> No, sorry. I don't.
No problem.
>
>> I need to be able to communicate to and from the device from the an
>> application build in Qt. So, there must be something really generic that I
>> can call from the application environment. In windows I use window API to
>> call IOCTLS interaction.
> Why? I thought this thing is an input device? Why does an application
> have to modify a running device? Is this modification local to the
> application<->device interface or does it also affect all other
> running applications that use this device?
Yes, it is an multitouch overlay input device. However all the 
processing is done on the host side. The device delivers raw data into 
the system and all the tracking and touch information is processed on 
the kernel side. The control panel for this device shows the input data 
and permits some parameter change. In order to visualize that data I 
need to be able to get a complete structure from it. When I change a 
parameter it will reflect the change to the input being reported to all 
the applications that use that input device.
>
> If it is a configuration value to put the device into a different
> state or similar, then you can use a sysfs attribute. The user can
> change this with "echo<value>  >/sys/class/input/inputX/<attribute>"
For setting new simple values I see I can use this interface, but two 
questions arise. How can I send a structure with this interface? Can I 
fopen this sys file and send the whole structure thru this mechanism. 
What about receiving data from the device?

Regards,

Nuno