backends/drm: add support for drm plane size hints

On a lot of hardware, using bigger plane sizes than necessary means wasting power.
This is specifically problematic with the cursor plane, where we so far only had a
single fixed size hint through drm caps, even though the hardware often could use
a smaller cursor size.
This adds support for the per-plane SIZE_HINTS property, which allows us to pick a
smaller cursor size when the cursor image fits into it, and should save some power
that way.

src/backends/drm/drm_egl_layer.cpp

@@ -152,9 +152,9 @@ QHash<uint32_t, QList<uint64_t>> EglGbmLayer::supportedDrmFormats() const
     return m_pipeline->formats(m_type);
 }
 
-std::optional<QSize> EglGbmLayer::fixedSize() const
+QList<QSize> EglGbmLayer::recommendedSizes() const
 {
-    return m_type == DrmPlane::TypeIndex::Cursor ? std::make_optional(m_pipeline->gpu()->cursorSize()) : std::nullopt;
+    return m_pipeline->recommendedSizes(m_type);
 }
 
 const ColorPipeline &EglGbmLayer::colorPipeline() const

src/backends/drm/drm_egl_layer.h

@@ -36,7 +36,7 @@ public:
     void releaseBuffers() override;
     DrmDevice *scanoutDevice() const override;
     QHash<uint32_t, QList<uint64_t>> supportedDrmFormats() const override;
-    std::optional<QSize> fixedSize() const override;
+    QList<QSize> recommendedSizes() const override;
     const ColorPipeline &colorPipeline() const override;
 
 private:

src/backends/drm/drm_pipeline.cpp

@@ -509,6 +509,27 @@ bool DrmPipeline::pruneModifier()
     }
 }
 
+QList<QSize> DrmPipeline::recommendedSizes(DrmPlane::TypeIndex planeType) const
+{
+    switch (planeType) {
+    case DrmPlane::TypeIndex::Primary:
+        if (m_pending.crtc && m_pending.crtc->primaryPlane()) {
+            return m_pending.crtc->primaryPlane()->recommendedSizes();
+        } else {
+            return QList<QSize>{};
+        }
+    case DrmPlane::TypeIndex::Cursor:
+        if (m_pending.crtc && m_pending.crtc->cursorPlane()) {
+            return m_pending.crtc->cursorPlane()->recommendedSizes();
+        } else {
+            return QList<QSize>{gpu()->cursorSize()};
+        }
+    case DrmPlane::TypeIndex::Overlay:
+        return QList<QSize>{};
+    }
+    Q_UNREACHABLE();
+}
+
 bool DrmPipeline::needsModeset() const
 {
     return m_pending.needsModeset;

src/backends/drm/drm_pipeline.h

@@ -77,6 +77,7 @@ public:
 
     QHash<uint32_t, QList<uint64_t>> formats(DrmPlane::TypeIndex planeType) const;
     bool pruneModifier();
+    QList<QSize> recommendedSizes(DrmPlane::TypeIndex planeType) const;
 
     void setOutput(DrmOutput *output);
     DrmOutput *output() const;

src/backends/drm/drm_plane.cpp

@@ -18,6 +18,7 @@
 #include "drm_pointer.h"
 
 #include <drm_fourcc.h>
+#include <ranges>
 
 namespace KWin
 {
@@ -66,6 +67,7 @@ DrmPlane::DrmPlane(DrmGpu *gpu, uint32_t planeId)
     , vmHotspotX(this, QByteArrayLiteral("HOTSPOT_X"))
     , vmHotspotY(this, QByteArrayLiteral("HOTSPOT_Y"))
     , inFenceFd(this, QByteArrayLiteral("IN_FENCE_FD"))
+    , sizeHints(this, QByteArrayLiteral("SIZE_HINTS"))
 {
 }
 
@@ -97,6 +99,7 @@ bool DrmPlane::updateProperties()
     vmHotspotX.update(props);
     vmHotspotY.update(props);
     inFenceFd.update(props);
+    sizeHints.update(props);
 
     if (!type.isValid() || !srcX.isValid() || !srcY.isValid() || !srcW.isValid() || !srcH.isValid()
         || !crtcX.isValid() || !crtcY.isValid() || !crtcW.isValid() || !crtcH.isValid() || !fbId.isValid()) {
@@ -124,6 +127,22 @@ bool DrmPlane::updateProperties()
             m_supportedFormats.insert(DRM_FORMAT_XRGB8888, modifiers);
         }
     }
+    m_sizeHints.clear();
+    if (sizeHints.isValid() && sizeHints.immutableBlob()) {
+        // TODO switch to drm_plane_size_hint once we require libdrm 2.4.122
+        struct SizeHint
+        {
+            uint16_t width;
+            uint16_t height;
+        };
+        std::span<SizeHint> hints(reinterpret_cast<SizeHint *>(sizeHints.immutableBlob()->data), sizeHints.immutableBlob()->length / sizeof(SizeHint));
+        std::ranges::transform(hints, std::back_inserter(m_sizeHints), [](const SizeHint &hint) {
+            return QSize(hint.width, hint.height);
+        });
+    }
+    if (m_sizeHints.empty() && type.enumValue() == TypeIndex::Cursor) {
+        m_sizeHints = {gpu()->cursorSize()};
+    }
     return true;
 }
 
@@ -201,6 +220,11 @@ bool DrmPlane::supportsTransformation(OutputTransform transform) const
 {
     return rotation.isValid() && rotation.hasEnum(outputTransformToPlaneTransform(transform));
 }
+
+QList<QSize> DrmPlane::recommendedSizes() const
+{
+    return m_sizeHints;
+}
 }
 
 #include "moc_drm_plane.cpp"

src/backends/drm/drm_plane.h

@@ -43,6 +43,8 @@ public:
 
     void set(DrmAtomicCommit *commit, const QRect &src, const QRect &dst);
 
+    QList<QSize> recommendedSizes() const;
+
     enum class TypeIndex : uint64_t {
         Overlay = 0,
         Primary = 1,
@@ -94,12 +96,14 @@ public:
     DrmProperty vmHotspotX;
     DrmProperty vmHotspotY;
     DrmProperty inFenceFd;
+    DrmProperty sizeHints;
 
 private:
     std::shared_ptr<DrmFramebuffer> m_current;
 
     QHash<uint32_t, QList<uint64_t>> m_supportedFormats;
     uint32_t m_possibleCrtcs;
+    QList<QSize> m_sizeHints;
 };
 
 }

src/compositor_wayland.cpp

@@ -401,11 +401,19 @@ void WaylandCompositor::addOutput(Output *output)
             }
             QRectF nativeCursorRect = output->transform().map(scaledRect(outputLocalRect, output->scale()), output->pixelSize());
             QSize bufferSize(std::ceil(nativeCursorRect.width()), std::ceil(nativeCursorRect.height()));
-            if (const auto fixedSize = outputLayer->fixedSize()) {
+            const auto recommendedSizes = outputLayer->recommendedSizes();
-                if (fixedSize->width() < bufferSize.width() || fixedSize->height() < bufferSize.height()) {
+            if (!recommendedSizes.empty()) {
+                auto bigEnough = recommendedSizes | std::views::filter([bufferSize](const auto &size) {
+                    return size.width() >= bufferSize.width() && size.height() >= bufferSize.height();
+                });
+                const auto it = std::ranges::min_element(bigEnough, [](const auto &left, const auto &right) {
+                    return left.width() * left.height() < right.width() * right.height();
+                });
+                if (it == bigEnough.end()) {
+                    // no size found, this most likely won't work
                     return false;
                 }
-                bufferSize = *fixedSize;
+                bufferSize = *it;
                 nativeCursorRect = output->transform().map(QRectF(outputLocalRect.topLeft() * output->scale(), bufferSize), output->pixelSize());
             }
             outputLayer->setHotspot(output->transform().map(cursor->hotspot() * output->scale(), bufferSize));

src/core/outputlayer.cpp

@@ -37,9 +37,9 @@ void OutputLayer::setHotspot(const QPointF &hotspot)
     m_hotspot = hotspot;
 }
 
-std::optional<QSize> OutputLayer::fixedSize() const
+QList<QSize> OutputLayer::recommendedSizes() const
 {
-    return std::nullopt;
+    return {};
 }
 
 QRegion OutputLayer::repaints() const

src/core/outputlayer.h

@@ -42,10 +42,10 @@ public:
     void setHotspot(const QPointF &hotspot);
 
     /**
-     * For most drm drivers, the buffer used for the cursor has to have a fixed size.
+     * For some layers it can be beneficial to use specific sizes only.
-     * If such a fixed size is required by the backend, this function should return it
+     * This returns those specific sizes, if present
      */
-    virtual std::optional<QSize> fixedSize() const;
+    virtual QList<QSize> recommendedSizes() const;
 
     QRegion repaints() const;
     void resetRepaints();