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.