eglfs: Add env.var. to disable the dedicated drmHandleEvent thread

Setting QT_QPA_EGLFS_KMS_NO_EVENT_READER_THREAD=1 makes it operate
like it did before 5.12.7: just calling drmhandleEvent (guarded by
a mutex) on the current (main or render, depending on the QQ render
loop) thread.

This should not be needed and is discouraged (will certainly cause
deadlocks in multiscreen setups + QQ threaded render loop on certain
embedded systems), but it seems necessary to provide a way to revert
back to the old way of functioning as there are reports about
problems with screen cloning when the dedicated event reading thread
is used.

Task-number: QTBUG-91882
Change-Id: I4cddcd09149dcab9e135467b6ef0e047a2a0ecff
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
(cherry picked from commit 820775166132b073a941f2389fba81db49619688)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/eglfs/deviceintegration/eglfs_kms/qeglfskmsgbmdevice.cpp

@@ -47,7 +47,13 @@ bool QEglFSKmsGbmDevice::open()
 
     setFd(fd);
 
+    if (usesEventReader()) {
+        qCDebug(qLcEglfsKmsDebug, "Using dedicated drm event reading thread");
         m_eventReader.create(this);
+    } else {
+        qCDebug(qLcEglfsKmsDebug, "Not using dedicated drm event reading thread; "
+                "threaded multi-screen setups may experience problems");
+    }
 
     return true;
 }
@@ -56,6 +62,7 @@ void QEglFSKmsGbmDevice::close()
 {
     // Note: screens are gone at this stage.
 
+    if (usesEventReader())
         m_eventReader.destroy();
 
     if (m_gbm_device) {
@@ -138,4 +145,10 @@ void QEglFSKmsGbmDevice::registerScreen(QPlatformScreen *screen,
         m_globalCursor->reevaluateVisibilityForScreens();
 }
 
+bool QEglFSKmsGbmDevice::usesEventReader() const
+{
+    static const bool eventReaderThreadDisabled = qEnvironmentVariableIntValue("QT_QPA_EGLFS_KMS_NO_EVENT_READER_THREAD");
+    return !eventReaderThreadDisabled;
+}
+
 QT_END_NAMESPACE

src/plugins/platforms/eglfs/deviceintegration/eglfs_kms/qeglfskmsgbmdevice_p.h

@@ -19,6 +19,7 @@
 
 #include "qeglfskmsgbmcursor_p.h"
 #include <private/qeglfskmsdevice_p.h>
+#include <private/qeglfskmseventreader_p.h>
 
 #include <gbm.h>
 
@@ -51,11 +52,14 @@ public:
                         const QPoint &virtualPos,
                         const QList<QPlatformScreen *> &virtualSiblings) override;
 
+    bool usesEventReader() const;
+    QEglFSKmsEventReader *eventReader() { return &m_eventReader; }
+
 private:
     Q_DISABLE_COPY(QEglFSKmsGbmDevice)
 
     gbm_device *m_gbm_device;
-
+    QEglFSKmsEventReader m_eventReader;
     QEglFSKmsGbmCursor *m_globalCursor;
 };
 

src/plugins/platforms/eglfs/deviceintegration/eglfs_kms/qeglfskmsgbmscreen.cpp

@@ -20,6 +20,8 @@ QT_BEGIN_NAMESPACE
 
 Q_DECLARE_LOGGING_CATEGORY(qLcEglfsKmsDebug)
 
+QMutex QEglFSKmsGbmScreen::m_nonThreadedFlipMutex;
+
 static inline uint32_t drmFormatToGbmFormat(uint32_t drmFormat)
 {
     Q_ASSERT(DRM_FORMAT_XRGB8888 == GBM_FORMAT_XRGB8888);
@@ -227,6 +229,18 @@ void QEglFSKmsGbmScreen::ensureModeSet(uint32_t fb)
     }
 }
 
+void QEglFSKmsGbmScreen::nonThreadedPageFlipHandler(int fd,
+                                                    unsigned int sequence,
+                                                    unsigned int tv_sec,
+                                                    unsigned int tv_usec,
+                                                    void *user_data)
+{
+    Q_UNUSED(fd);
+    QEglFSKmsGbmScreen *screen = static_cast<QEglFSKmsGbmScreen *>(user_data);
+    screen->flipFinished();
+    screen->pageFlipped(sequence, tv_sec, tv_usec);
+}
+
 void QEglFSKmsGbmScreen::waitForFlip()
 {
     if (m_headless || m_cloneSource)
@@ -236,12 +250,24 @@ void QEglFSKmsGbmScreen::waitForFlip()
     if (!m_gbm_bo_next)
         return;
 
+    QEglFSKmsGbmDevice *dev = static_cast<QEglFSKmsGbmDevice *>(device());
+    if (dev->usesEventReader()) {
         m_flipMutex.lock();
-    device()->eventReader()->startWaitFlip(this, &m_flipMutex, &m_flipCond);
+        dev->eventReader()->startWaitFlip(this, &m_flipMutex, &m_flipCond);
         m_flipCond.wait(&m_flipMutex);
         m_flipMutex.unlock();
-
         flipFinished();
+    } else {
+        QMutexLocker lock(&m_nonThreadedFlipMutex);
+        while (m_gbm_bo_next) {
+            drmEventContext drmEvent;
+            memset(&drmEvent, 0, sizeof(drmEvent));
+            drmEvent.version = 2;
+            drmEvent.vblank_handler = nullptr;
+            drmEvent.page_flip_handler = nonThreadedPageFlipHandler;
+            drmHandleEvent(device()->fd(), &drmEvent);
+        }
+    }
 
 #if QT_CONFIG(drm_atomic)
     device()->threadLocalAtomicReset();

src/plugins/platforms/eglfs/deviceintegration/eglfs_kms/qeglfskmsgbmscreen_p.h

@@ -52,6 +52,11 @@ protected:
     void flipFinished();
     void ensureModeSet(uint32_t fb);
     void cloneDestFlipFinished(QEglFSKmsGbmScreen *cloneDestScreen);
+    static void nonThreadedPageFlipHandler(int fd,
+                                           unsigned int sequence,
+                                           unsigned int tv_sec,
+                                           unsigned int tv_usec,
+                                           void *user_data);
 
     gbm_surface *m_gbm_surface;
 
@@ -61,6 +66,7 @@ protected:
 
     QMutex m_flipMutex;
     QWaitCondition m_flipCond;
+    static QMutex m_nonThreadedFlipMutex;
 
     QScopedPointer<QEglFSKmsGbmCursor> m_cursor;
 

src/plugins/platforms/eglfs/deviceintegration/eglfs_kms_support/qeglfskmsdevice_p.h

@@ -17,7 +17,6 @@
 //
 
 #include "private/qeglfsglobal_p.h"
-#include "qeglfskmseventreader_p.h"
 #include <QtKmsSupport/private/qkmsdevice_p.h>
 
 QT_BEGIN_NAMESPACE
@@ -31,11 +30,6 @@ public:
                         bool isPrimary,
                         const QPoint &virtualPos,
                         const QList<QPlatformScreen *> &virtualSiblings) override;
-
-    QEglFSKmsEventReader *eventReader() { return &m_eventReader; }
-
-protected:
-    QEglFSKmsEventReader m_eventReader;
 };
 
 QT_END_NAMESPACE