Merge lp:~gerboland/qtubuntu/adopt-more-eglconvenience2 into lp:qtubuntu

[Gerry Boland]

Convert UbuntuOpenGLContext to use QEGLPlatformContext, and allow clients to customize chosen GLConfig

Credit to Loïc Molinari for the initial implementation.

Several things come of this change:
1. Convert UbuntuOpenGLContext to use QEGLPlatformContext means we can use more built-in Qt code to configure and use the GL context, which should ensure that Qt-on-Mir applications behave more consistently with Qt-on-X apps.
2. Allow apps to configure the SurfaceFormat/GLConfig, as opposed to always overruling it to be ABGR8888. But choose ABGR8888 as the default if not specified by the app.
3. If app does not request alpha channel, but still gets a GLConfig with an alpha channel, communicate to Mir that the alpha channel is unused (aka XBGR8888), and thus compostor need not blend the surface.

[Gerry Boland (gerboland)]

To test, verify that no visual regressions are evident in the application.

Also, use QT_LOGGING_RULES=ubuntumirclient=true to check things like this:

ubuntumirclient: EGL configuration attributes:
 EGL_BUFFER_SIZE: 32
 EGL_ALPHA_SIZE: 8
 EGL_BLUE_SIZE: 8
 EGL_GREEN_SIZE: 8
 EGL_RED_SIZE: 8
 EGL_DEPTH_SIZE: 24
 EGL_STENCIL_SIZE: 8
which should be the case for all QML apps.

Then, test that the "Actual format" contain:

Actual format: QSurfaceFormat(version 2.0, options QFlags(), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples 0, swapBehavior 0, swapInterval 1, profile 0) with associated Mir pixel format: XBGR8888

for applications which do not need proper transparency (note the "Mir pixel format" being XBGR8888 - X=disabled).

But if it does need transparency (can use qmlscene --transparent to test this), it should report ABGR8888 instead.

Note that QML needs the alpha buffer to render correctly. But it is perfectly ok for the compositor to ignore the alpha channel when compositing an opaque surface.

[Unity8 CI Bot (unity8-ci-bot)]

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review: Approve (continuous-integration)

[Loïc Molinari (loic.molinari)]

Cool!

Looks Good.

I guess we can get rid of this long waiting one [1] then (I still need to validate that there's nothing missing from there, but it doesn't seem so).

I don't remember exactly the details of [2] (IIRC I saw that somewhere in the Qt code base) but what do you think of not forcing the alpha size to 8 if not specified by the user? I guess if the user doesn't set the alpha size explicitely, he doesn't want his surface to support translucent surface anyway. You could then use mFormat instead of mWindow->requestedFormat() there: "if (mWindow->requestedFormat().alphaBufferSize() < 0) {".

Also, as it's done in [2], what do you think of forcing undefined color channels to 8 independently from the others instead of setting all of them to to 8 only if all are undefined? It sounds like a more predictive use case to me.

The env var part could be useful too for us to check the behaviour of some platforms. But that's definitely not a priority.

[1] https://code.launchpad.net/~unity-team/qtubuntu/requested-surface-format-fix/+merge/274379
[2] http://bazaar.launchpad.net/~loic.molinari/qtubuntu/requested-surface-format-fix-default-to-8-bit-rgb-config-and-improved-mir-pixel-format-retrieval/revision/267

review: Needs Information

[Gerry Boland (gerboland)]

> I guess we can get rid of this long waiting one [1] then (I still need to
> validate that there's nothing missing from there, but it doesn't seem so).

Yep done.

> I don't remember exactly the details of [2] (IIRC I saw that somewhere in the
> Qt code base) but what do you think of not forcing the alpha size to 8 if not
> specified by the user? I guess if the user doesn't set the alpha size
> explicitely, he doesn't want his surface to support translucent surface
> anyway. You could then use mFormat instead of mWindow->requestedFormat()
> there: "if (mWindow->requestedFormat().alphaBufferSize() < 0) {".

I did this actually to start with. But unless I'm mistaken, the QML renderer requires an alpha channel in order to do blending correctly. This is why I enforced an alpha channel being requested. I too would like to clarify this, because what you suggest does make more sense.

> Also, as it's done in [2], what do you think of forcing undefined color
> channels to 8 independently from the others instead of setting all of them to
> to 8 only if all are undefined? It sounds like a more predictive use case to
> me.
Well I presumed that if a user sets any channel size, then I'll assume they know what they're doing and so leave them alone. If they ask for RGB565, I'm not going to turn on alpha channel for them.

> The env var part could be useful too for us to check the behaviour of some
> platforms. But that's definitely not a priority.
True, that is nice. I'll include that.

[Loïc Molinari (loic.molinari)]

Nice, that looks perfect now!

I'm not approving right now since I'd like to do one more round of review next week.

[Daniel van Vugt (vanvugt)]

Is that a typo?

+ case mir_pixel_format_rgb_565: return "RGB888";

review: Needs Fixing

[Gerry Boland (gerboland)]

> Is that a typo?
>
> + case mir_pixel_format_rgb_565: return "RGB888";

Quite right, nice catch, thanks

[Gerry Boland (gerboland)]

Hmm, I've broken phone apps (they only get a GLES1.1 context now), need to fix that.

[Gerry Boland (gerboland)]

Ok, should be good to go now.
Tested on
- intel i945
- Nexus 4 & 7
- Freiza
- BQ Aquaris e4.5
- Meizu MX4

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review: Approve (continuous-integration)

[Loïc Molinari (loic.molinari)]

I'm a bit skeptical about the Mesa string hack from r335 since it's forcing a 1.0 context for on all the drivers based on Mesa, even those supporting higher versions. That, I guess, prevents all the apps depending on OpenGL ES 2 or OpenGL ES 3 features to create a context with Qt.

Couldn't we 1st test with the version requested by the user, and then if it fails, do a 1.4 request if the driver is based on Mesa?

or, if not possible, maybe for now we can do the 1.4 request only if it's i915? (what are the EGL_VENDOR and EGL_VERSION strings on i915?)

[Gerry Boland (gerboland)]

I did test on my more modern Intel machine and found the context returned by Mesa was the highest it supported. But yeah, I'll try to use it as a fallback-only position

[Gerry Boland (gerboland)]

Ok, the above remove the SurfaceFormat filtering stuff, instead
1. using QSurfaceFormat::defaultFormat to set the use of RGB888 by default. If user changes that, it's their responsibility.
2. adding a policy where if the user-requested config failed to be chosen on Mesa, it will retry with GL1.4.

[Gerry Boland (gerboland)]

It introduces a slight amount of code duplication unfortunately, but it is either that or replace a chunk of the EGLConvenience stuff. I think this is the easiest option

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review: Needs Fixing (continuous-integration)

[Loïc Molinari (loic.molinari)]

All good!

review: Approve

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review: Needs Fixing (continuous-integration)

[Michał Sawicz (saviq)]

Text conflict in src/ubuntumirclient/glcontext.cpp
Text conflict in src/ubuntumirclient/integration.cpp
2 conflicts encountered.

review: Needs Fixing

[lp:~gerboland/qtubuntu/adopt-more-eglconvenience2]

341. By Gerry Boland on 2016-06-03

Merge trunk, fix conflicts, give hints to branch predictor

[Emanuele Antonio Faraone (emanueleant03)]

review: Approve

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review: Approve (continuous-integration)

[Emanuele Antonio Faraone (emanueleant03)]

news?

[Loïc Molinari (loic.molinari)]

Couldn't we set workaround_brokenFBOReadBack right after context creation in qtubuntu? That would avoid, at each makeCurrent() call, fetching data from different places in memory (the context, the private context and the bss for platforms with non-broken FBO readback) with the potential cache misses implied.

review: Needs Information

[Gerry Boland (gerboland)]

> Couldn't we set workaround_brokenFBOReadBack right after context creation in
> qtubuntu? That would avoid, at each makeCurrent() call, fetching data from
> different places in memory (the context, the private context and the bss for
> platforms with non-broken FBO readback) with the potential cache misses
> implied.

That's not code I actually wrote in this branch, it is code that landed in trunk. That duplicates what the Android QPA plugin does . We trusted Qt guys knew what they were doing :)

[Loïc Molinari (loic.molinari)]

OK, then I guess it's something worth checking later :)

review: Approve

[Unity8 CI Bot (unity8-ci-bot)]

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=== modified file 'README'
--- README	2015-12-09 13:30:15 +0000
+++ README	2016-06-03 09:55:53 +0000
@@ -23,13 +23,10 @@
     $ qmlscene -platform ubuntumirclient Foo.qml
     $ QT_QPA_PLATFORM=ubuntumirclient qmlscene Foo.qml
 
-  The QPA plugins expose the following environment variables:
-
-    QTUBUNTU_SWAPINTERVAL: Specifies the required swap interval as an
-                           integer. 1 by default.
-
-    QTUBUNTU_MULTISAMPLE: Enables multisampling with using 4 samples
-                          per fragment.
+  This QPA plugin exposes the following environment variables:
+
+    QT_QPA_EGLFS_SWAPINTERVAL: Specifies the required swap interval as an
+                               integer. 1 by default.
 
     QTUBUNTU_NO_THREADED_OPENGL: Disables QtQuick threaded OpenGL
                                  rendering.
@@ -51,6 +48,8 @@
   * ubuntumirclient.swapBuffers - Messages related to surface buffer swapping.
   * ubuntumirclient             - For all other messages.
 
+  The QT_QPA_EGLFS_DEBUG environment variable prints a little more information
+  from Qt's internals.
 
 4. Building
 -----------

=== modified file 'src/ubuntumirclient/glcontext.cpp'
--- src/ubuntumirclient/glcontext.cpp	2016-05-27 13:05:20 +0000
+++ src/ubuntumirclient/glcontext.cpp	2016-06-03 09:55:53 +0000
@@ -25,58 +25,12 @@
 
 namespace {
 
-void printOpenGLESConfig() {
-    static bool once = true;
-    if (once) {
-        const char* string = (const char*) glGetString(GL_VENDOR);
-        qCDebug(ubuntumirclient, "OpenGL ES vendor: %s", string);
-        string = (const char*) glGetString(GL_RENDERER);
-        qCDebug(ubuntumirclient, "OpenGL ES renderer: %s", string);
-        string = (const char*) glGetString(GL_VERSION);
-        qCDebug(ubuntumirclient, "OpenGL ES version: %s", string);
-        string = (const char*) glGetString(GL_SHADING_LANGUAGE_VERSION);
-        qCDebug(ubuntumirclient, "OpenGL ES Shading Language version: %s", string);
-        string = (const char*) glGetString(GL_EXTENSIONS);
-        qCDebug(ubuntumirclient, "OpenGL ES extensions: %s", string);
-        once = false;
-    }
-}
-
-void printEglConfig(EGLDisplay display, EGLConfig config) {
+void printEglConfig(EGLDisplay display, EGLConfig config)
+{
     Q_ASSERT(display != EGL_NO_DISPLAY);
     Q_ASSERT(config != nullptr);
 
-    static const struct { const EGLint attrib; const char* name; } kAttribs[] = {
-        { EGL_BUFFER_SIZE, "EGL_BUFFER_SIZE" },
-        { EGL_ALPHA_SIZE, "EGL_ALPHA_SIZE" },
-        { EGL_BLUE_SIZE, "EGL_BLUE_SIZE" },
-        { EGL_GREEN_SIZE, "EGL_GREEN_SIZE" },
-        { EGL_RED_SIZE, "EGL_RED_SIZE" },
-        { EGL_DEPTH_SIZE, "EGL_DEPTH_SIZE" },
-        { EGL_STENCIL_SIZE, "EGL_STENCIL_SIZE" },
-        { EGL_CONFIG_CAVEAT, "EGL_CONFIG_CAVEAT" },
-        { EGL_CONFIG_ID, "EGL_CONFIG_ID" },
-        { EGL_LEVEL, "EGL_LEVEL" },
-        { EGL_MAX_PBUFFER_HEIGHT, "EGL_MAX_PBUFFER_HEIGHT" },
-        { EGL_MAX_PBUFFER_PIXELS, "EGL_MAX_PBUFFER_PIXELS" },
-        { EGL_MAX_PBUFFER_WIDTH, "EGL_MAX_PBUFFER_WIDTH" },
-        { EGL_NATIVE_RENDERABLE, "EGL_NATIVE_RENDERABLE" },
-        { EGL_NATIVE_VISUAL_ID, "EGL_NATIVE_VISUAL_ID" },
-        { EGL_NATIVE_VISUAL_TYPE, "EGL_NATIVE_VISUAL_TYPE" },
-        { EGL_SAMPLES, "EGL_SAMPLES" },
-        { EGL_SAMPLE_BUFFERS, "EGL_SAMPLE_BUFFERS" },
-        { EGL_SURFACE_TYPE, "EGL_SURFACE_TYPE" },
-        { EGL_TRANSPARENT_TYPE, "EGL_TRANSPARENT_TYPE" },
-        { EGL_TRANSPARENT_BLUE_VALUE, "EGL_TRANSPARENT_BLUE_VALUE" },
-        { EGL_TRANSPARENT_GREEN_VALUE, "EGL_TRANSPARENT_GREEN_VALUE" },
-        { EGL_TRANSPARENT_RED_VALUE, "EGL_TRANSPARENT_RED_VALUE" },
-        { EGL_BIND_TO_TEXTURE_RGB, "EGL_BIND_TO_TEXTURE_RGB" },
-        { EGL_BIND_TO_TEXTURE_RGBA, "EGL_BIND_TO_TEXTURE_RGBA" },
-        { EGL_MIN_SWAP_INTERVAL, "EGL_MIN_SWAP_INTERVAL" },
-        { EGL_MAX_SWAP_INTERVAL, "EGL_MAX_SWAP_INTERVAL" },
-        { -1, NULL }
-    };
-    const char* string = eglQueryString(display, EGL_VENDOR);
+    const char *string = eglQueryString(display, EGL_VENDOR);
     qCDebug(ubuntumirclient, "EGL vendor: %s", string);
 
     string = eglQueryString(display, EGL_VERSION);
@@ -85,93 +39,19 @@
     string = eglQueryString(display, EGL_EXTENSIONS);
     qCDebug(ubuntumirclient, "EGL extensions: %s", string);
 
-    qCDebug(ubuntumirclient, "EGL configuration attibutes:");
-    for (int index = 0; kAttribs[index].attrib != -1; index++) {
-        EGLint value;
-        if (eglGetConfigAttrib(display, config, kAttribs[index].attrib, &value))
-            qCDebug(ubuntumirclient, "  %s: %d", kAttribs[index].name, static_cast<int>(value));
-    }
-}
-
-QString eglErrorToString(EGLint errorNumber)
-{
-    #define EGL_ERROR_CASE(error) case error: return QString(#error);
-
-    switch (errorNumber) {
-        EGL_ERROR_CASE(EGL_SUCCESS)
-        EGL_ERROR_CASE(EGL_NOT_INITIALIZED)
-        EGL_ERROR_CASE(EGL_BAD_ACCESS)
-        EGL_ERROR_CASE(EGL_BAD_ALLOC)
-        EGL_ERROR_CASE(EGL_BAD_ATTRIBUTE)
-        EGL_ERROR_CASE(EGL_BAD_CONTEXT)
-        EGL_ERROR_CASE(EGL_BAD_CONFIG)
-        EGL_ERROR_CASE(EGL_BAD_CURRENT_SURFACE)
-        EGL_ERROR_CASE(EGL_BAD_DISPLAY)
-        EGL_ERROR_CASE(EGL_BAD_SURFACE)
-        EGL_ERROR_CASE(EGL_BAD_MATCH)
-        EGL_ERROR_CASE(EGL_BAD_PARAMETER)
-        EGL_ERROR_CASE(EGL_BAD_NATIVE_PIXMAP)
-        EGL_ERROR_CASE(EGL_BAD_NATIVE_WINDOW)
-        EGL_ERROR_CASE(EGL_CONTEXT_LOST)
-        default:
-            return QString("?");
-    }
-
-    #undef EGL_ERROR_CASE
-}
-
-EGLenum api_in_use()
-{
-    #ifdef QTUBUNTU_USE_OPENGL
-    return EGL_OPENGL_API;
-    #else
-    return EGL_OPENGL_ES_API;
-    #endif
-}
-
-const int kSwapInterval = 1;
-
-int qGetEnvIntValue(const char *varName, bool *ok)
-{
-    return qgetenv(varName).toInt(ok);
+    qCDebug(ubuntumirclient, "EGL configuration attributes:");
+    q_printEglConfig(display, config);
 }
 
 } // anonymous namespace
 
-UbuntuOpenGLContext::UbuntuOpenGLContext(const QSurfaceFormat &surfaceFormat, UbuntuOpenGLContext *share,
-                                         EGLDisplay display, EGLConfig config)
-    : mSurfaceFormat(surfaceFormat)
-    , mEglDisplay(display)
+UbuntuOpenGLContext::UbuntuOpenGLContext(const QSurfaceFormat &format, QPlatformOpenGLContext *share,
+                                         EGLDisplay display)
+    : QEGLPlatformContext(format, share, display, 0)
 {
-    // Create an OpenGL ES 2 context.
-    QVector<EGLint> attribs;
-    attribs.append(EGL_CONTEXT_CLIENT_VERSION);
-    attribs.append(2);
-    attribs.append(EGL_NONE);
-    ASSERT(eglBindAPI(api_in_use()) == EGL_TRUE);
-
     if (ubuntumirclient().isDebugEnabled()) {
-        printEglConfig(mEglDisplay, config);
+        printEglConfig(display, eglConfig());
     }
-
-    // Set vblank swap interval.
-    bool ok;
-    int swapInterval = qGetEnvIntValue("QTUBUNTU_SWAPINTERVAL", &ok);
-    if (!ok)
-        swapInterval = kSwapInterval;
-
-    qCDebug(ubuntumirclient, "Setting swap interval to %d", swapInterval);
-    eglSwapInterval(mEglDisplay, swapInterval);
-
-    mEglContext = eglCreateContext(mEglDisplay, config, share ? share->eglContext() : EGL_NO_CONTEXT,
-                                   attribs.constData());
-
-    Q_ASSERT(mEglContext != EGL_NO_CONTEXT);
-}
-
-UbuntuOpenGLContext::~UbuntuOpenGLContext()
-{
-    ASSERT(eglDestroyContext(mEglDisplay, mEglContext) == EGL_TRUE);
 }
 
 static bool needsFBOReadBackWorkaround()
@@ -179,7 +59,7 @@
     static bool set = false;
     static bool needsWorkaround = false;
 
-    if (!set) {
+    if (Q_UNLIKELY(!set)) {
         const char *rendererString = reinterpret_cast<const char *>(glGetString(GL_RENDERER));
         needsWorkaround = qstrncmp(rendererString, "Mali-400", 8) == 0
                           || qstrncmp(rendererString, "Mali-T7", 7) == 0
@@ -202,47 +82,32 @@
         }
         return offscreen->buffer()->bind();
     } else {
-        EGLSurface eglSurface = static_cast<UbuntuWindow*>(surface)->eglSurface();
-        ASSERT(eglBindAPI(api_in_use()) == EGL_TRUE);
-
-        EGLBoolean result = eglMakeCurrent(mEglDisplay, eglSurface, eglSurface, mEglContext);
-        if (result == EGL_FALSE) {
-            qCCritical(ubuntumirclient, "eglMakeCurrent() failed with %s",
-                    qPrintable(eglErrorToString(eglGetError())));
-            return false;
-        }
-
-        QOpenGLContextPrivate *ctx_d = QOpenGLContextPrivate::get(context());
-        if (!ctx_d->workaround_brokenFBOReadBack && needsFBOReadBackWorkaround()) {
-            ctx_d->workaround_brokenFBOReadBack = true;
-        }
-
-        if (ubuntumirclient().isDebugEnabled()) {
-            printOpenGLESConfig();
-        }
-        return true;
+        const bool ret = QEGLPlatformContext::makeCurrent(surface);
+
+        if (Q_LIKELY(ret)) {
+            QOpenGLContextPrivate *ctx_d = QOpenGLContextPrivate::get(context());
+            if (!ctx_d->workaround_brokenFBOReadBack && needsFBOReadBackWorkaround()) {
+                ctx_d->workaround_brokenFBOReadBack = true;
+            }
+        }
+
+        return ret;
     }
 }
 
-void UbuntuOpenGLContext::doneCurrent()
+// Following method used internally in the base class QEGLPlatformContext to access
+// the egl surface of a QPlatformSurface/UbuntuWindow
+EGLSurface UbuntuOpenGLContext::eglSurfaceForPlatformSurface(QPlatformSurface *surface)
 {
-    ASSERT(eglBindAPI(api_in_use()) == EGL_TRUE);
-    ASSERT(eglMakeCurrent(mEglDisplay, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT) == EGL_TRUE);
+    auto ubuntuWindow = static_cast<UbuntuWindow *>(surface);
+    return ubuntuWindow->eglSurface();
 }
 
-void UbuntuOpenGLContext::swapBuffers(QPlatformSurface* surface)
+void UbuntuOpenGLContext::swapBuffers(QPlatformSurface *surface)
 {
-    UbuntuWindow *ubuntuWindow = static_cast<UbuntuWindow*>(surface);
-
-    EGLSurface eglSurface = ubuntuWindow->eglSurface();
-    ASSERT(eglBindAPI(api_in_use()) == EGL_TRUE);
-    ASSERT(eglSwapBuffers(mEglDisplay, eglSurface) == EGL_TRUE);
-
+    QEGLPlatformContext::swapBuffers(surface);
+
+    // notify window on swap completion
+    auto ubuntuWindow = static_cast<UbuntuWindow *>(surface);
     ubuntuWindow->onSwapBuffersDone();
 }
-
-void (*UbuntuOpenGLContext::getProcAddress(const QByteArray& procName)) ()
-{
-    ASSERT(eglBindAPI(api_in_use()) == EGL_TRUE);
-    return eglGetProcAddress(procName.constData());
-}

=== modified file 'src/ubuntumirclient/glcontext.h'
--- src/ubuntumirclient/glcontext.h	2016-04-28 14:09:54 +0000
+++ src/ubuntumirclient/glcontext.h	2016-06-03 09:55:53 +0000
@@ -18,30 +18,22 @@
 #define UBUNTU_OPENGL_CONTEXT_H
 
 #include <qpa/qplatformopenglcontext.h>
+#include <private/qeglplatformcontext_p.h>
 
 #include <EGL/egl.h>
 
-class UbuntuOpenGLContext : public QPlatformOpenGLContext
+class UbuntuOpenGLContext : public QEGLPlatformContext
 {
 public:
-    UbuntuOpenGLContext(const QSurfaceFormat &surfaceFormat, UbuntuOpenGLContext *share,
-                        EGLDisplay display, EGLConfig config);
-    virtual ~UbuntuOpenGLContext();
-
-    // QPlatformOpenGLContext methods.
-    QSurfaceFormat format() const override { return mSurfaceFormat; }
-    void swapBuffers(QPlatformSurface *surface) override;
-    bool makeCurrent(QPlatformSurface *surface) override;
-    void doneCurrent() override;
-    bool isValid() const override { return mEglContext != EGL_NO_CONTEXT; }
-    void (*getProcAddress(const QByteArray& procName)) () override;
-
-    EGLContext eglContext() const { return mEglContext; }
-
-private:
-    const QSurfaceFormat mSurfaceFormat;
-    EGLContext mEglContext;
-    EGLDisplay mEglDisplay;
+    UbuntuOpenGLContext(const QSurfaceFormat &format, QPlatformOpenGLContext *share,
+                        EGLDisplay display);
+
+    // QEGLPlatformContext methods.
+    void swapBuffers(QPlatformSurface *surface) final;
+    bool makeCurrent(QPlatformSurface *surface) final;
+
+protected:
+    EGLSurface eglSurfaceForPlatformSurface(QPlatformSurface *surface) final;
 };
 
 #endif // UBUNTU_OPENGL_CONTEXT_H

=== modified file 'src/ubuntumirclient/integration.cpp'
--- src/ubuntumirclient/integration.cpp	2016-04-28 14:44:18 +0000
+++ src/ubuntumirclient/integration.cpp	2016-06-03 09:55:53 +0000
@@ -29,7 +29,6 @@
 
 // Qt
 #include <QGuiApplication>
-#include <private/qguiapplication_p.h>
 #include <qpa/qplatformnativeinterface.h>
 #include <qpa/qplatforminputcontextfactory_p.h>
 #include <qpa/qplatforminputcontext.h>
@@ -90,31 +89,17 @@
 
     mMirConnection = u_application_instance_get_mir_connection(mInstance);
 
+    // Choose the default surface format suited to the Mir platform
+    QSurfaceFormat defaultFormat;
+    defaultFormat.setRedBufferSize(8);
+    defaultFormat.setGreenBufferSize(8);
+    defaultFormat.setBlueBufferSize(8);
+    QSurfaceFormat::setDefaultFormat(defaultFormat);
+
     // Initialize EGL.
-    ASSERT(eglBindAPI(EGL_OPENGL_ES_API) == EGL_TRUE);
-
     mEglNativeDisplay = mir_connection_get_egl_native_display(mMirConnection);
     ASSERT((mEglDisplay = eglGetDisplay(mEglNativeDisplay)) != EGL_NO_DISPLAY);
     ASSERT(eglInitialize(mEglDisplay, nullptr, nullptr) == EGL_TRUE);
-
-    // Configure EGL buffers format for all Windows.
-    mSurfaceFormat.setRedBufferSize(8);
-    mSurfaceFormat.setGreenBufferSize(8);
-    mSurfaceFormat.setBlueBufferSize(8);
-    mSurfaceFormat.setAlphaBufferSize(8);
-    mSurfaceFormat.setDepthBufferSize(24);
-    mSurfaceFormat.setStencilBufferSize(8);
-    if (!qEnvironmentVariableIsEmpty("QTUBUNTU_MULTISAMPLE")) {
-        mSurfaceFormat.setSamples(4);
-        qCDebug(ubuntumirclient, "setting MSAA to 4 samples");
-    }
-#ifdef QTUBUNTU_USE_OPENGL
-    mSurfaceFormat.setRenderableType(QSurfaceFormat::OpenGL);
-#else
-    mSurfaceFormat.setRenderableType(QSurfaceFormat::OpenGLES);
-#endif
-
-    mEglConfig = q_configFromGLFormat(mEglDisplay, mSurfaceFormat, true);
 }
 
 void UbuntuClientIntegration::initialize()
@@ -197,7 +182,7 @@
 
 QPlatformWindow* UbuntuClientIntegration::createPlatformWindow(QWindow* window)
 {
-    return new UbuntuWindow(window, mClipboard, mInput, mNativeInterface, mEglDisplay, mEglConfig, mMirConnection);
+    return new UbuntuWindow(window, mClipboard, mInput, mNativeInterface, mEglDisplay, mMirConnection);
 }
 
 bool UbuntuClientIntegration::hasCapability(QPlatformIntegration::Capability cap) const
@@ -246,8 +231,25 @@
 QPlatformOpenGLContext* UbuntuClientIntegration::createPlatformOpenGLContext(
         QOpenGLContext* context)
 {
-    return new UbuntuOpenGLContext(mSurfaceFormat, static_cast<UbuntuOpenGLContext*>(context->shareHandle()),
-                                   mEglDisplay, mEglConfig);
+    QSurfaceFormat format(context->format());
+
+    auto platformContext = new UbuntuOpenGLContext(format, context->shareHandle(), mEglDisplay);
+    if (!platformContext->isValid()) {
+        // Older Intel Atom-based devices only support OpenGL 1.4 compatibility profile but by default
+        // QML asks for at least OpenGL 2.0. The XCB GLX backend ignores this request and returns a
+        // 1.4 context, but the XCB EGL backend tries to honour it, and fails. The 1.4 context appears to
+        // have sufficient capabilities on MESA (i915) to render correctly however. So reduce the default
+        // requested OpenGL version to 1.0 to ensure EGL will give us a working context (lp:1549455).
+        static const bool isMesa = QString(eglQueryString(mEglDisplay, EGL_VENDOR)).contains(QStringLiteral("Mesa"));
+        if (isMesa) {
+            qCDebug(ubuntumirclient, "Attempting to choose OpenGL 1.4 context which may suit Mesa");
+            format.setMajorVersion(1);
+            format.setMinorVersion(4);
+            delete platformContext;
+            platformContext = new UbuntuOpenGLContext(format, context->shareHandle(), mEglDisplay);
+        }
+    }
+    return platformContext;
 }
 
 QStringList UbuntuClientIntegration::themeNames() const

=== modified file 'src/ubuntumirclient/integration.h'
--- src/ubuntumirclient/integration.h	2016-04-28 14:09:54 +0000
+++ src/ubuntumirclient/integration.h	2016-06-03 09:55:53 +0000
@@ -66,9 +66,7 @@
 
     // New methods.
     MirConnection *mirConnection() const { return mMirConnection; }
-    QSurfaceFormat surfaceFormat() const { return mSurfaceFormat; }
     EGLDisplay eglDisplay() const { return mEglDisplay; }
-    EGLConfig eglConfig() const { return mEglConfig; }
     EGLNativeDisplayType eglNativeDisplay() const { return mEglNativeDisplay; }
     UbuntuScreenObserver *screenObserver() const { return mScreenObserver.data(); }
 
@@ -99,9 +97,7 @@
 
     // EGL related
     EGLDisplay mEglDisplay{EGL_NO_DISPLAY};
-    EGLConfig mEglConfig{nullptr};
     EGLNativeDisplayType mEglNativeDisplay;
-    QSurfaceFormat mSurfaceFormat;
 };
 
 #endif // UBUNTU_CLIENT_INTEGRATION_H

=== modified file 'src/ubuntumirclient/window.cpp'
--- src/ubuntumirclient/window.cpp	2016-04-28 14:44:18 +0000
+++ src/ubuntumirclient/window.cpp	2016-06-03 09:55:53 +0000
@@ -29,6 +29,7 @@
 #include <QMutexLocker>
 #include <QSize>
 #include <QtMath>
+#include <private/qeglconvenience_p.h>
 
 // Platform API
 #include <ubuntu/application/instance.h>
@@ -101,6 +102,24 @@
     }
 }
 
+const char *mirPixelFormatToStr(MirPixelFormat pixelFormat)
+{
+    switch (pixelFormat) {
+    case mir_pixel_format_invalid:   return "invalid";
+    case mir_pixel_format_abgr_8888: return "ABGR8888";
+    case mir_pixel_format_xbgr_8888: return "XBGR8888";
+    case mir_pixel_format_argb_8888: return "ARGB8888";
+    case mir_pixel_format_xrgb_8888: return "XRGB8888";
+    case mir_pixel_format_bgr_888:   return "BGR888";
+    case mir_pixel_format_rgb_888:   return "RGB888";
+    case mir_pixel_format_rgb_565:   return "RGB565";
+    case mir_pixel_format_rgba_5551: return "RGBA5551";
+    case mir_pixel_format_rgba_4444: return "RGBA4444";
+    case mir_pixel_formats:
+    default:                         return "???";
+    }
+}
+
 MirSurfaceState qtWindowStateToMirSurfaceState(Qt::WindowState state)
 {
     switch (state) {
@@ -124,14 +143,6 @@
     return id++;
 }
 
-MirPixelFormat defaultPixelFormatFor(MirConnection *connection)
-{
-    MirPixelFormat format;
-    unsigned int nformats;
-    mir_connection_get_available_surface_formats(connection, &format, 1, &nformats);
-    return format;
-}
-
 UAUiWindowRole roleFor(QWindow *window)
 {
     QVariant roleVariant = window->property("role");
@@ -151,12 +162,11 @@
     return parent ? static_cast<UbuntuWindow *>(parent->handle()) : nullptr;
 }
 
-Spec makeSurfaceSpec(QWindow *window, UbuntuInput *input, MirConnection *connection)
+Spec makeSurfaceSpec(QWindow *window, UbuntuInput *input, MirPixelFormat pixelFormat, MirConnection *connection)
 {
     const auto geom = window->geometry();
     const int width = geom.width() > 0 ? geom.width() : 1;
     const int height = geom.height() > 0 ? geom.height() : 1;
-    const auto pixelFormat = defaultPixelFormatFor(connection);
 
     if (U_ON_SCREEN_KEYBOARD_ROLE == roleFor(window)) {
         qCDebug(ubuntumirclient, "makeSurfaceSpec(window=%p) - creating input method surface (width=%d, height=%d", window, width, height);
@@ -217,11 +227,11 @@
     }
 }
 
-MirSurface *createMirSurface(QWindow *window, int mirOutputId, UbuntuInput *input,
+MirSurface *createMirSurface(QWindow *window, int mirOutputId, UbuntuInput *input, MirPixelFormat pixelFormat,
                              MirConnection *connection, mir_surface_event_callback inputCallback,
                              void* inputContext)
 {
-    auto spec = makeSurfaceSpec(window, input, connection);
+    auto spec = makeSurfaceSpec(window, input, pixelFormat, connection);
 
     // Install event handler as early as possible
     mir_surface_spec_set_event_handler(spec.get(), inputCallback, inputContext);
@@ -244,6 +254,18 @@
     return surface;
 }
 
+MirPixelFormat disableAlphaBufferIfPossible(MirPixelFormat pixelFormat)
+{
+    switch(pixelFormat) {
+    case mir_pixel_format_abgr_8888:
+        return mir_pixel_format_xbgr_8888;
+    case mir_pixel_format_argb_8888:
+        return mir_pixel_format_xrgb_8888;
+    default: // can do nothing, leave it alone
+        return pixelFormat;
+    }
+}
+
 // FIXME - in order to work around https://bugs.launchpad.net/mir/+bug/1346633
 // we need to guess the panel height (3GU)
 int panelHeight()
@@ -266,8 +288,7 @@
 class UbuntuSurface
 {
 public:
-    UbuntuSurface(UbuntuWindow *platformWindow, EGLDisplay display, EGLConfig config, int mirOutputId,
-                  UbuntuInput *input, MirConnection *connection)
+    UbuntuSurface(UbuntuWindow *platformWindow, EGLDisplay display, UbuntuInput *input, MirConnection *connection)
         : mWindow(platformWindow->window())
         , mPlatformWindow(platformWindow)
         , mInput(input)
@@ -275,9 +296,44 @@
         , mEglDisplay(display)
         , mNeedsRepaint(false)
         , mParented(mWindow->transientParent() || mWindow->parent())
+        , mFormat(mWindow->requestedFormat())
         , mShellChrome(mWindow->flags() & LowChromeWindowHint ? mir_shell_chrome_low : mir_shell_chrome_normal)
     {
-        mMirSurface = createMirSurface(mWindow, mirOutputId, input, connection, surfaceEventCallback, this);
+        // Have Qt choose most suitable EGLConfig for the requested surface format, and update format to reflect it
+        EGLConfig config = q_configFromGLFormat(display, mFormat, true);
+        if (config == 0) {
+            // Older Intel Atom-based devices only support OpenGL 1.4 compatibility profile but by default
+            // QML asks for at least OpenGL 2.0. The XCB GLX backend ignores this request and returns a
+            // 1.4 context, but the XCB EGL backend tries to honour it, and fails. The 1.4 context appears to
+            // have sufficient capabilities on MESA (i915) to render correctly however. So reduce the default
+            // requested OpenGL version to 1.0 to ensure EGL will give us a working context (lp:1549455).
+            static const bool isMesa = QString(eglQueryString(display, EGL_VENDOR)).contains(QStringLiteral("Mesa"));
+            if (isMesa) {
+                qCDebug(ubuntumirclient, "Attempting to choose OpenGL 1.4 context which may suit Mesa");
+                mFormat.setMajorVersion(1);
+                mFormat.setMinorVersion(4);
+                config = q_configFromGLFormat(display, mFormat, true);
+            }
+        }
+        if (config == 0) {
+            qCritical() << "Qt failed to choose a suitable EGLConfig to suit the surface format" << mFormat;
+        }
+
+        mFormat = q_glFormatFromConfig(display, config, mFormat);
+
+        // Have Mir decide the pixel format most suited to the chosen EGLConfig. This is the only way
+        // Mir will know what EGLConfig has been chosen - it cannot deduce it from the buffers.
+        auto pixelFormat = mir_connection_get_egl_pixel_format(connection, display, config);
+        // But the chosen EGLConfig might have an alpha buffer enabled, even if not requested by the client.
+        // If that's the case, try to edit the chosen pixel format in order to disable the alpha buffer.
+        // This is an optimisation for the compositor, as it can avoid blending this surface.
+        if (mWindow->requestedFormat().alphaBufferSize() < 0) {
+            pixelFormat = disableAlphaBufferIfPossible(pixelFormat);
+        }
+
+        const auto outputId = static_cast<UbuntuScreen *>(mWindow->screen()->handle())->mirOutputId();
+
+        mMirSurface = createMirSurface(mWindow, outputId, input, pixelFormat, connection, surfaceEventCallback, this);
         mEglSurface = eglCreateWindowSurface(mEglDisplay, config, nativeWindowFor(mMirSurface), nullptr);
 
         // Window manager can give us a final size different from what we asked for
@@ -299,8 +355,11 @@
         platformWindow->QPlatformWindow::setGeometry(geom);
         QWindowSystemInterface::handleGeometryChange(mWindow, geom);
 
-        qCDebug(ubuntumirclient, "created surface at (%d, %d) with size (%d, %d), title '%s', role: '%d'\n",
-             geom.x(), geom.y(), geom.width(), geom.height(), mWindow->title().toUtf8().constData(), roleFor(mWindow));
+        qCDebug(ubuntumirclient) << "Created surface with geometry:" << geom << "title:" << mWindow->title()
+                                 << "role:" << roleFor(mWindow)
+                                 << "\nRequested format:" << mWindow->requestedFormat()
+                                 << "\nActual format:" << mFormat
+                                 << "with associated Mir pixel format:" << mirPixelFormatToStr(pixelFormat);
     }
 
     ~UbuntuSurface()
@@ -335,6 +394,8 @@
     void setSurfaceParent(MirSurface*);
     bool hasParent() const { return mParented; }
 
+    QSurfaceFormat format() const { return mFormat; }
+
 private:
     static void surfaceEventCallback(MirSurface* surface, const MirEvent *event, void* context);
     void postEvent(const MirEvent *event);
@@ -351,6 +412,7 @@
     bool mNeedsRepaint;
     bool mParented;
     QSize mBufferSize;
+    QSurfaceFormat mFormat;
 
     QMutex mTargetSizeMutex;
     QSize mTargetSize;
@@ -509,8 +571,7 @@
 }
 
 UbuntuWindow::UbuntuWindow(QWindow *w, const QSharedPointer<UbuntuClipboard> &clipboard,
-                           UbuntuInput *input, UbuntuNativeInterface *native, EGLDisplay eglDisplay,
-                           EGLConfig eglConfig, MirConnection *mirConnection)
+                           UbuntuInput *input, UbuntuNativeInterface *native, EGLDisplay eglDisplay, MirConnection *mirConnection)
     : QObject(nullptr)
     , QPlatformWindow(w)
     , mId(makeId())
@@ -520,8 +581,7 @@
     , mWindowVisible(false)
     , mWindowExposed(true)
     , mNativeInterface(native)
-    , mSurface(new UbuntuSurface{this, eglDisplay, eglConfig,
-               static_cast<UbuntuScreen*>(w->screen()->handle())->mirOutputId(), input, mirConnection})
+    , mSurface(new UbuntuSurface{this, eglDisplay, input, mirConnection})
     , mScale(1.0)
     , mFormFactor(mir_form_factor_unknown)
 {
@@ -709,6 +769,11 @@
     return mWindowVisible && mWindowExposed;
 }
 
+QSurfaceFormat UbuntuWindow::format() const
+{
+    return mSurface->format();
+}
+
 void* UbuntuWindow::eglSurface() const
 {
     return mSurface->eglSurface();

=== modified file 'src/ubuntumirclient/window.h'
--- src/ubuntumirclient/window.h	2016-04-28 14:44:18 +0000
+++ src/ubuntumirclient/window.h	2016-06-03 09:55:53 +0000
@@ -40,7 +40,7 @@
     Q_OBJECT
 public:
     UbuntuWindow(QWindow *w, const QSharedPointer<UbuntuClipboard> &clipboard,
-                 UbuntuInput *input, UbuntuNativeInterface* native, EGLDisplay eglDisplay, EGLConfig eglConfig,
+                 UbuntuInput *input, UbuntuNativeInterface* native, EGLDisplay eglDisplay,
                  MirConnection *mirConnection);
     virtual ~UbuntuWindow();
 
@@ -54,6 +54,8 @@
     void propagateSizeHints() override;
     bool isExposed() const override;
 
+    QSurfaceFormat format() const override;
+
     // Additional Window properties exposed by NativeInterface
     MirFormFactor formFactor() const { return mFormFactor; }
     float scale() const { return mScale; }