Checking compatibility of custom framebuffer with Qt application

[ENzero]

I am new to Qt so please bear with me. I have created a custom framebuffer driver for embedded linux (angstrom) and I have been required to test it against a Qt application.

I have implemented the write, fillrect, copyarea function. When I ran my hello world app with linuxfb as the platform, I get:

QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::drawPoints: Painter not active

It did not display anything to the LCD but I can't tell which part went wrong.

Ultimately, how do I tell which function Qt is calling to interface with the framebuffer driver? So that I can tweak my driver to be compatible. Any suggestions will greatly be appreciated.

I am only using qt's hello world example project.

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

main.qml

import QtQuick 2.11
import QtQuick.Window 2.11

Window {
    visible: true
    width: 80
    height: 162
    title: qsTr("Hello World")
}

[raven-worx]

@ENzero
lets first check if your used Qt painting code is correct.
In case you do the painting (using QPainter) yourself post the code.

[ENzero]

Hi, thank you for replying.

In this case, I am just using the default hello world application of the qt. I have edited the question to include the source code.

[raven-worx]

@ENzero
QPainter::begin: Paint device returned engine == 0, type: 3 basically means that there is no valid paint device to paint on, as you have probably already guessed :)
But i guess this rather has to be debugged in Qt*s FB platform plugin

[raven-worx]

@ENzero
try

export QT_LOGGING_RULES=qt.qpa.*=true

and run your application. check if it outputs anything useful which might help with the issue

[ENzero]

I tried to export that and looked at framebuffer calls. Its seems that qt linuxfb just calls mmap and grabs a pointer to the buffer memory. It then just edits all the pixels using that pointer. Thanks for the suggestion! I was able to track that my framebuffer driver was wrong and was able to change it.