rhi: gl: Switch GetBufferSubData emulation to MapBufferRange

Use only APIs that are in GLES 3.0.

glMapBuffer() is an old OES extension, stop bothering with that.
Not the least because ANGLE claims supporting it and then fails the
map. (not that we care much about ANGLE, but, for instance, the qrhi
autotest is run with ANGLE configurations as well in the CI, so have
to still take care of it for the duration of Qt 5.x)

Change-Id: I29140402cedffe0430f920ee0c061673257c3aa1
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

3 files changed, 19 insertions, 13 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index 8ec8ae9208..bea8890234 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -568,11 +568,10 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
     supports QRhiGraphicsPipeline::TriangleFan.
 
     \value ReadBackNonUniformBuffer Indicates that
-    {QRhiResourceUpdateBatch::readBackBuffer()}{reading buffer contents} is
+    \l{QRhiResourceUpdateBatch::readBackBuffer()}{reading buffer contents} is
     supported for QRhiBuffer instances with a usage different than
-    UniformBuffer. While this is supported in the majority of cases, it can be
-    unsupported, for example, on OpenGL ES 2.0 implementations without the
-    MapBuffer extension.
+    UniformBuffer. While this is supported in the majority of cases, it will be
+    unsupported with OpenGL ES older than 3.0.
 */
 
 /*!
diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index a6094183fb..fa99ee5e25 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -276,6 +276,10 @@ QT_BEGIN_NAMESPACE
 #define GL_POINT_SPRITE                   0x8861
 #endif
 
+#ifndef GL_MAP_READ_BIT
+#define GL_MAP_READ_BIT                   0x0001
+#endif
+
 Q_DECLARE_LOGGING_CATEGORY(lcOpenGLProgramDiskCache)
 
 /*!
@@ -492,7 +496,9 @@ bool QRhiGles2::create(QRhi::Flags flags)
     else
         caps.textureCompareMode = true;
 
-    caps.mapBuffer = f->hasOpenGLExtension(QOpenGLExtensions::MapBuffer);
+    // proper as in ES 3.0 (glMapBufferRange), not the old glMapBuffer
+    // extension(s) (which is not in ES 3.0...messy)
+    caps.properMapBuffer = f->hasOpenGLExtension(QOpenGLExtensions::MapBufferRange);
 
     if (!caps.gles) {
         f->glEnable(GL_VERTEX_PROGRAM_POINT_SIZE);
@@ -737,7 +743,7 @@ bool QRhiGles2::isFeatureSupported(QRhi::Feature feature) const
     case QRhi::TriangleFanTopology:
         return true;
     case QRhi::ReadBackNonUniformBuffer:
-        return !caps.gles || caps.mapBuffer;
+        return !caps.gles || caps.properMapBuffer;
     default:
         Q_UNREACHABLE();
         return false;
@@ -2107,13 +2113,14 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
             QRhiBufferReadbackResult *result = cmd.args.getBufferSubData.result;
             f->glBindBuffer(cmd.args.getBufferSubData.target, cmd.args.getBufferSubData.buffer);
             if (caps.gles) {
-                if (caps.mapBuffer) {
-                    void *p = f->glMapBuffer(cmd.args.getBufferSubData.target, GL_READ_ONLY);
+                if (caps.properMapBuffer) {
+                    void *p = f->glMapBufferRange(cmd.args.getBufferSubData.target,
+                                                  cmd.args.getBufferSubData.offset,
+                                                  cmd.args.getBufferSubData.size,
+                                                  GL_MAP_READ_BIT);
                     if (p) {
                         result->data.resize(cmd.args.getBufferSubData.size);
-                        memcpy(result->data.data(),
-                               reinterpret_cast<char *>(p) + cmd.args.getBufferSubData.offset,
-                               size_t(cmd.args.getBufferSubData.size));
+                        memcpy(result->data.data(), p, size_t(cmd.args.getBufferSubData.size));
                         f->glUnmapBuffer(cmd.args.getBufferSubData.target);
                     }
                 }
diff --git a/src/gui/rhi/qrhigles2_p_p.h b/src/gui/rhi/qrhigles2_p_p.h
index a4a9c15ce7..29cccb0c9c 100644
--- a/src/gui/rhi/qrhigles2_p_p.h
+++ b/src/gui/rhi/qrhigles2_p_p.h
@@ -753,7 +753,7 @@ public:
               baseVertex(false),
               compute(false),
               textureCompareMode(false),
-              mapBuffer(false)
+              properMapBuffer(false)
         { }
         int ctxMajor;
         int ctxMinor;
@@ -784,7 +784,7 @@ public:
         uint baseVertex : 1;
         uint compute : 1;
         uint textureCompareMode : 1;
-        uint mapBuffer : 1;
+        uint properMapBuffer : 1;
     } caps;
     QGles2SwapChain *currentSwapChain = nullptr;
     QVector<GLint> supportedCompressedFormats;