rhi: Add a flag to indicate preferring a software adapter

...if there is one and the concept is applicable in the first place.

Change-Id: Iab202c1c1cdd229f4910159de4cae7ce30805ea9
Reviewed-by: Christian Strømme <christian.stromme@qt.io>

4 files changed, 60 insertions, 4 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index fbdc90bd1b..585dc8a8fa 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -439,6 +439,18 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
     visible in external GPU debugging tools will not be available and functions
     like QRhiCommandBuffer::debugMarkBegin() will become a no-op. Avoid
     enabling in production builds as it may involve a performance penalty.
+
+    \value PreferSoftwareRenderer Indicates that backends should prefer
+    choosing an adapter or physical device that renders in software on the CPU.
+    For example, with Direct3D there is typically a "Basic Render Driver"
+    adapter available with \c{DXGI_ADAPTER_FLAG_SOFTWARE}. Setting this flag
+    requests the backend to choose that adapter over any other, as long as no
+    specific adapter was forced by other backend-specific means. With Vulkan
+    this maps to preferring physical devices with
+    \c{VK_PHYSICAL_DEVICE_TYPE_CPU}. When not available, or when it is not
+    possible to decide if an adapter/device is software-based, this flag is
+    ignored. It may also be ignored with graphics APIs that have no concept and
+    means of enumerating adapters/devices.
  */
 
 /*!
diff --git a/src/gui/rhi/qrhi_p.h b/src/gui/rhi/qrhi_p.h
index 51e70af18e..6f0b8e0b04 100644
--- a/src/gui/rhi/qrhi_p.h
+++ b/src/gui/rhi/qrhi_p.h
@@ -1294,7 +1294,8 @@ public:
 
     enum Flag {
         EnableProfiling = 1 << 0,
-        EnableDebugMarkers = 1 << 1
+        EnableDebugMarkers = 1 << 1,
+        PreferSoftwareRenderer = 1 << 2
     };
     Q_DECLARE_FLAGS(Flags, Flag)
 
diff --git a/src/gui/rhi/qrhid3d11.cpp b/src/gui/rhi/qrhid3d11.cpp
index 2350691dc0..0977a3042d 100644
--- a/src/gui/rhi/qrhid3d11.cpp
+++ b/src/gui/rhi/qrhid3d11.cpp
@@ -119,6 +119,11 @@ QT_BEGIN_NAMESPACE
     \c{ID3D11Texture2D *}.
  */
 
+// help mingw with its ancient sdk headers
+#ifndef DXGI_ADAPTER_FLAG_SOFTWARE
+#define DXGI_ADAPTER_FLAG_SOFTWARE 2
+#endif
+
 QRhiD3D11::QRhiD3D11(QRhiD3D11InitParams *params, QRhiD3D11NativeHandles *importDevice)
     : ofr(this),
       deviceCurse(this)
@@ -227,11 +232,29 @@ bool QRhiD3D11::create(QRhi::Flags flags)
         int requestedAdapterIndex = -1;
         if (qEnvironmentVariableIsSet("QT_D3D_ADAPTER_INDEX"))
             requestedAdapterIndex = qEnvironmentVariableIntValue("QT_D3D_ADAPTER_INDEX");
+
+        if (requestedAdapterIndex < 0 && flags.testFlag(QRhi::PreferSoftwareRenderer)) {
+            for (int adapterIndex = 0; dxgiFactory->EnumAdapters1(UINT(adapterIndex), &adapter) != DXGI_ERROR_NOT_FOUND; ++adapterIndex) {
+                DXGI_ADAPTER_DESC1 desc;
+                adapter->GetDesc1(&desc);
+                adapter->Release();
+                if (desc.Flags & DXGI_ADAPTER_FLAG_SOFTWARE) {
+                    requestedAdapterIndex = adapterIndex;
+                    break;
+                }
+            }
+        }
+
         for (int adapterIndex = 0; dxgiFactory->EnumAdapters1(UINT(adapterIndex), &adapter) != DXGI_ERROR_NOT_FOUND; ++adapterIndex) {
             DXGI_ADAPTER_DESC1 desc;
             adapter->GetDesc1(&desc);
             const QString name = QString::fromUtf16(reinterpret_cast<char16_t *>(desc.Description));
-            qCDebug(QRHI_LOG_INFO, "Adapter %d: '%s' (flags 0x%x)", adapterIndex, qPrintable(name), desc.Flags);
+            qCDebug(QRHI_LOG_INFO, "Adapter %d: '%s' (vendor 0x%X device 0x%X flags 0x%X)",
+                    adapterIndex,
+                    qPrintable(name),
+                    desc.VendorId,
+                    desc.DeviceId,
+                    desc.Flags);
             if (!adapterToUse && (requestedAdapterIndex < 0 || requestedAdapterIndex == adapterIndex)) {
                 adapterToUse = adapter;
                 qCDebug(QRHI_LOG_INFO, "  using this adapter");
diff --git a/src/gui/rhi/qrhivulkan.cpp b/src/gui/rhi/qrhivulkan.cpp
index 1ed3ffc206..64fea37292 100644
--- a/src/gui/rhi/qrhivulkan.cpp
+++ b/src/gui/rhi/qrhivulkan.cpp
@@ -388,22 +388,42 @@ bool QRhiVulkan::create(QRhi::Flags flags)
             qWarning("Failed to enumerate physical devices: %d", err);
             return false;
         }
+
         int physDevIndex = -1;
         int requestedPhysDevIndex = -1;
         if (qEnvironmentVariableIsSet("QT_VK_PHYSICAL_DEVICE_INDEX"))
             requestedPhysDevIndex = qEnvironmentVariableIntValue("QT_VK_PHYSICAL_DEVICE_INDEX");
+
+        if (requestedPhysDevIndex < 0 && flags.testFlag(QRhi::PreferSoftwareRenderer)) {
+            for (int i = 0; i < int(physDevCount); ++i) {
+                f->vkGetPhysicalDeviceProperties(physDevs[i], &physDevProperties);
+                if (physDevProperties.deviceType == VK_PHYSICAL_DEVICE_TYPE_CPU) {
+                    requestedPhysDevIndex = i;
+                    break;
+                }
+            }
+        }
+
         for (int i = 0; i < int(physDevCount); ++i) {
             f->vkGetPhysicalDeviceProperties(physDevs[i], &physDevProperties);
-            qCDebug(QRHI_LOG_INFO, "Physical device %d: '%s' %d.%d.%d", i,
+            qCDebug(QRHI_LOG_INFO, "Physical device %d: '%s' %d.%d.%d (api %d.%d.%d vendor 0x%X device 0x%X type %d)",
+                    i,
                     physDevProperties.deviceName,
                     VK_VERSION_MAJOR(physDevProperties.driverVersion),
                     VK_VERSION_MINOR(physDevProperties.driverVersion),
-                    VK_VERSION_PATCH(physDevProperties.driverVersion));
+                    VK_VERSION_PATCH(physDevProperties.driverVersion),
+                    VK_VERSION_MAJOR(physDevProperties.apiVersion),
+                    VK_VERSION_MINOR(physDevProperties.apiVersion),
+                    VK_VERSION_PATCH(physDevProperties.apiVersion),
+                    physDevProperties.vendorID,
+                    physDevProperties.deviceID,
+                    physDevProperties.deviceType);
             if (physDevIndex < 0 && (requestedPhysDevIndex < 0 || requestedPhysDevIndex == int(i))) {
                 physDevIndex = i;
                 qCDebug(QRHI_LOG_INFO, "    using this physical device");
             }
         }
+
         if (physDevIndex < 0) {
             qWarning("No matching physical device");
             return false;