rhi: Add a note about size to newBuffer()

Also make sure the gl backend is consistent with the other three.

Change-Id: I2f6b783f5fa474c94ede460f5d7ac6fe8129a4f6
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

2 files changed, 11 insertions, 4 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index 7770cac870..7443c0a04f 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -4947,6 +4947,11 @@ QRhiShaderResourceBindings *QRhi::newShaderResourceBindings()
     backends. See \l{QRhiBuffer::UsageFlag}{UsageFlags} and
     \l{QRhi::NonDynamicUniformBuffers}{the feature flags}.
 
+    \note Backends may choose to allocate buffers bigger than \a size. This is
+    done transparently to applications, so there are no special restrictions on
+    the value of \a size. QRhiBuffer::size() will always report back the value
+    that was requested in \a size.
+
     \sa QRhiResource::release()
  */
 QRhiBuffer *QRhi::newBuffer(QRhiBuffer::Type type,
diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index a4ab1cf2ec..379801efbd 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -2847,13 +2847,15 @@ bool QGles2Buffer::build()
     QRHI_RES_RHI(QRhiGles2);
     QRHI_PROF;
 
+    const int nonZeroSize = m_size <= 0 ? 256 : m_size;
+
     if (m_usage.testFlag(QRhiBuffer::UniformBuffer)) {
         if (int(m_usage) != QRhiBuffer::UniformBuffer) {
             qWarning("Uniform buffer: multiple usages specified, this is not supported by the OpenGL backend");
             return false;
         }
-        ubuf.resize(m_size);
-        QRHI_PROF_F(newBuffer(this, m_size, 0, 1));
+        ubuf.resize(nonZeroSize);
+        QRHI_PROF_F(newBuffer(this, nonZeroSize, 0, 1));
         return true;
     }
 
@@ -2868,11 +2870,11 @@ bool QGles2Buffer::build()
 
     rhiD->f->glGenBuffers(1, &buffer);
     rhiD->f->glBindBuffer(targetForDataOps, buffer);
-    rhiD->f->glBufferData(targetForDataOps, m_size, nullptr, m_type == Dynamic ? GL_DYNAMIC_DRAW : GL_STATIC_DRAW);
+    rhiD->f->glBufferData(targetForDataOps, nonZeroSize, nullptr, m_type == Dynamic ? GL_DYNAMIC_DRAW : GL_STATIC_DRAW);
 
     usageState.access = AccessNone;
 
-    QRHI_PROF_F(newBuffer(this, m_size, 1, 0));
+    QRHI_PROF_F(newBuffer(this, nonZeroSize, 1, 0));
     rhiD->registerResource(this);
     return true;
 }