Reading devices under /dev/.

jigarp

Hello,

I'm using qt for cross compile with qt 5.7
How to read device mounted under /dev/. in linux from qt.?

Thank you,

jsulm

@jigarp What device is that?

jigarp

It is like RPMSG device used for communication between multiple core for linux kernel.
I'm using linux kernel 4.9.59 .

jsulm

@jigarp OK, I don't know about it. I don't think it is really a Qt related topic. Is there any documentation for this device? Any examples in C/C++?

jigarp

@jsulm
This link provides information for linux drivers
https://www.kernel.org/doc/Documentation/rpmsg.txt

This will provide interface for Ti processors
http://processors.wiki.ti.com/index.php/PRU-ICSS_Remoteproc_and_RPMsg
http://processors.wiki.ti.com/index.php/PRU-ICSS_Rpmsg_Driver

It is a kind of charcter device.
I'm able to write it using QFile
don't know how to read.

Sample for linux userspace commands

echo Hello > /dev/rpmsg*
cat /dev/rpmsg*
C^

Can you provide details for how to read and write Character Device.

Thank you

jsulm

@jigarp You read like any other file

jigarp

In this way i'm trying to write data to rpmsg

QFile armIpcWriteMsg("/dev/rpmsg_pru30");
QTextStream outstream(&armIpcWriteMsg);
qDebug() << "Writing PRU Data";
if(armIpcWriteMsg.open(QIODevice::WriteOnly))
{
    outstream << "Hello";
armIpcWriteMsg.close();
}

In this way i'm trying to read.

QFile armIpcReadMsg("/dev/rpmsg_pru30");
if(armIpcReadMsg.exists())
{
    if(armIpcReadMsg.open(QIODevice::ReadOnly | QIODevice::Unbuffered))
    {
        char buff[20];

        armIpcReadMsg.read(buff,1);
        qDebug() << "PRU2ARM:" << buff;
        armIpcReadMsg.close();
    }
}

Is it the right way?

jsulm

@jigarp Should be OK. Did you try it out?

jigarp

I've tried it.
I'm able to write successfully.

But while trying to read file it waits until data became available.
I wan't to check for data availability before reading that file.

I've tried QFileinfo by monitoring its size but it is always zero.
Is there any way to check for ready read.

jsulm

@jigarp I don't think there is anything in Qt for that.
I see two possibilities:

Use a second thread where you call read()
Use C API call select() (see man select)

Cleiton Bueno

I agree!

Create a separate thread to handle the device, use (select () or poll ()) and signals to notify other classes or get to the GUI.

SGaist

Hi,

QFileSystemWatcher might be of interest.

jigarp

@Cleiton-Bueno @jsulm
In my case i'm reading this file in a thread but there are some more work involved so it is necessory to check whether data available or not.I can not wait until data available.
Is there any way to check data available like size or any other.
File.size() always responds with 0

Create a separate thread to handle the device, use (select () or poll ()) and signals to notify other classes or get to the GUI.

How to use Can you provide any samples???

jsulm

@jigarp Take a look at what @SGaist suggested

jigarp

@jsulm @SGaist
Not Working..

jsulm

@jigarp said in Reading devices under /dev/.:

Not Working..

Well, with such information it is impossible to say why it is not working.
Can you show what you tried and what the result was?