platforms/drm: re-use buffers for testing

b38bb416 introduced flicker when restarting compositing because the
DrmGpu::findWorkingCombination method usually creates new buffers for
the commit, without rendering into them. Instead of that, re-use
existing buffers where possible

src/plugins/platforms/drm/abstract_egl_drm_backend.h

@@ -62,6 +62,8 @@ public:
 
     virtual QSharedPointer<DrmBuffer> renderTestFrame(DrmAbstractOutput *output) = 0;
 
+    virtual uint32_t drmFormat() const = 0;
+
     static AbstractEglDrmBackend *renderingBackend() {
         return static_cast<AbstractEglDrmBackend*>(primaryBackend());
     }

src/plugins/platforms/drm/drm_buffer.cpp

@@ -18,18 +18,21 @@
 // drm
 #include <xf86drm.h>
 #include <xf86drmMode.h>
+#include <drm_fourcc.h>
 
 namespace KWin
 {
 
-DrmBuffer:: DrmBuffer(DrmGpu *gpu)
+DrmBuffer:: DrmBuffer(DrmGpu *gpu, uint32_t format, uint64_t modifier)
     : m_gpu(gpu)
+    , m_format(format)
+    , m_modifier(modifier)
 {
 }
 
 // DrmDumbBuffer
 DrmDumbBuffer::DrmDumbBuffer(DrmGpu *gpu, const QSize &size)
-    : DrmBuffer(gpu)
+    : DrmBuffer(gpu, DRM_FORMAT_XRGB8888, DRM_FORMAT_MOD_LINEAR)
 {
     m_size = size;
     drm_mode_create_dumb createArgs;

src/plugins/platforms/drm/drm_buffer.h

@@ -20,7 +20,7 @@ class DrmGpu;
 class DrmBuffer
 {
 public:
-    DrmBuffer(DrmGpu *gpu);
+    DrmBuffer(DrmGpu *gpu, uint32_t format, uint64_t modifier);
     virtual ~DrmBuffer() = default;
 
     virtual bool needsModeChange(DrmBuffer *b) const {Q_UNUSED(b) return false;}
@@ -36,11 +36,19 @@ public:
     DrmGpu *gpu() const {
         return m_gpu;
     }
+    uint32_t format() const {
+        return m_format;
+    }
+    uint64_t modifier() const {
+        return m_modifier;
+    }
 
 protected:
     quint32 m_bufferId = 0;
     QSize m_size;
     DrmGpu *m_gpu;
+    uint32_t m_format;
+    uint64_t m_modifier;
 };
 
 class DrmDumbBuffer : public DrmBuffer

src/plugins/platforms/drm/drm_buffer_gbm.cpp

@@ -85,13 +85,13 @@ bool GbmBuffer::map(uint32_t flags)
 
 
 DrmGbmBuffer::DrmGbmBuffer(DrmGpu *gpu, GbmSurface *surface, gbm_bo *bo)
-    : DrmBuffer(gpu), GbmBuffer(surface, bo)
+    : DrmBuffer(gpu, gbm_bo_get_format(bo), gbm_bo_get_modifier(bo)), GbmBuffer(surface, bo)
 {
     initialize();
 }
 
 DrmGbmBuffer::DrmGbmBuffer(DrmGpu *gpu, gbm_bo *buffer, KWaylandServer::ClientBuffer *clientBuffer)
-    : DrmBuffer(gpu), GbmBuffer(buffer, clientBuffer)
+    : DrmBuffer(gpu, gbm_bo_get_format(buffer), gbm_bo_get_modifier(buffer)), GbmBuffer(buffer, clientBuffer)
 {
     initialize();
 }
@@ -108,7 +108,6 @@ DrmGbmBuffer::~DrmGbmBuffer()
 void DrmGbmBuffer::initialize()
 {
     m_size = QSize(gbm_bo_get_width(m_bo), gbm_bo_get_height(m_bo));
-    uint32_t format = gbm_bo_get_format(m_bo);
     uint32_t handles[4] = { };
     uint32_t strides[4] = { };
     uint32_t offsets[4] = { };
@@ -119,7 +118,7 @@ void DrmGbmBuffer::initialize()
             handles[i] = gbm_bo_get_handle_for_plane(m_bo, i).u32;
             strides[i] = gbm_bo_get_stride_for_plane(m_bo, i);
             offsets[i] = gbm_bo_get_offset(m_bo, i);
-            modifiers[i] = gbm_bo_get_modifier(m_bo);
+            modifiers[i] = m_modifier;
         }
     } else {
         handles[0] = gbm_bo_get_handle(m_bo).u32;
@@ -128,17 +127,17 @@ void DrmGbmBuffer::initialize()
     }
 
     if (modifiers[0] != DRM_FORMAT_MOD_INVALID && m_gpu->addFB2ModifiersSupported()) {
-        if (drmModeAddFB2WithModifiers(m_gpu->fd(), m_size.width(), m_size.height(), format, handles, strides, offsets, modifiers, &m_bufferId, DRM_MODE_FB_MODIFIERS)) {
+        if (drmModeAddFB2WithModifiers(m_gpu->fd(), m_size.width(), m_size.height(), m_format, handles, strides, offsets, modifiers, &m_bufferId, DRM_MODE_FB_MODIFIERS)) {
             gbm_format_name_desc name;
-            gbm_format_get_name(format, &name);
+            gbm_format_get_name(m_format, &name);
             qCCritical(KWIN_DRM) << "drmModeAddFB2WithModifiers on GPU" << m_gpu->devNode() << "failed for a buffer with format" << name.name << "and modifier" << modifiers[0] << strerror(errno);
         }
     } else {
-        if (drmModeAddFB2(m_gpu->fd(), m_size.width(), m_size.height(), format, handles, strides, offsets, &m_bufferId, 0)) {
+        if (drmModeAddFB2(m_gpu->fd(), m_size.width(), m_size.height(), m_format, handles, strides, offsets, &m_bufferId, 0)) {
             // fallback
             if (drmModeAddFB(m_gpu->fd(), m_size.width(), m_size.height(), 24, 32, strides[0], handles[0], &m_bufferId) != 0) {
                 gbm_format_name_desc name;
-                gbm_format_get_name(format, &name);
+                gbm_format_get_name(m_format, &name);
                 qCCritical(KWIN_DRM) << "drmModeAddFB2 and drmModeAddFB both failed on GPU" << m_gpu->devNode() << "for a buffer with format" << name.name << "and modifier" << modifiers[0] << strerror(errno);
             }
         }

src/plugins/platforms/drm/drm_pipeline.cpp

@@ -275,10 +275,29 @@ bool DrmPipeline::checkTestBuffer()
     if (!isActive()) {
         return true;
     }
-    QSharedPointer<DrmBuffer> buffer;
-#if HAVE_GBM
     auto backend = m_gpu->eglBackend();
-    if (backend && m_output) {
+    QSharedPointer<DrmBuffer> buffer;
+    // try to re-use buffers if possible.
+    const auto &checkBuffer = [this, backend, &buffer](const QSharedPointer<DrmBuffer> &buf){
+        if (buf->format() == backend->drmFormat()
+            && (!m_gpu->atomicModeSetting() || supportedModifiers(buf->format()).contains(buf->modifier()))
+            && buf->size() == sourceSize()) {
+            buffer = buf;
+        }
+    };
+    if (m_primaryPlane && m_primaryPlane->next()) {
+        checkBuffer(m_primaryPlane->next());
+    } else if (m_primaryPlane && m_primaryPlane->current()) {
+        checkBuffer(m_primaryPlane->current());
+    } else if (m_crtc->next()) {
+        checkBuffer(m_crtc->next());
+    } else if (m_crtc->current()) {
+        checkBuffer(m_crtc->current());
+    }
+    // if we don't have a fitting buffer already, get or create one
+    if (buffer) {
+#if HAVE_GBM
+    } else if (backend && m_output) {
         buffer = backend->renderTestFrame(m_output);
     } else if (backend && m_gpu->gbmDevice()) {
         gbm_bo *bo = gbm_bo_create(m_gpu->gbmDevice(), sourceSize().width(), sourceSize().height(), GBM_FORMAT_XRGB8888, GBM_BO_USE_SCANOUT | GBM_BO_USE_RENDERING);
@@ -286,12 +305,10 @@ bool DrmPipeline::checkTestBuffer()
             return false;
         }
         buffer = QSharedPointer<DrmGbmBuffer>::create(m_gpu, bo, nullptr);
+#endif
     } else {
         buffer = QSharedPointer<DrmDumbBuffer>::create(m_gpu, sourceSize());
     }
-#else
-    buffer = QSharedPointer<DrmDumbBuffer>::create(m_gpu, sourceSize());
-#endif
     if (buffer && buffer->bufferId()) {
         m_oldTestBuffer = m_primaryBuffer;
         m_primaryBuffer = buffer;

src/plugins/platforms/drm/egl_gbm_backend.cpp

@@ -734,4 +734,9 @@ bool EglGbmBackend::hasOutput(AbstractOutput *output) const
     return m_outputs.contains(output);
 }
 
+uint32_t EglGbmBackend::drmFormat() const
+{
+    return m_gbmFormat;
+}
+
 }

src/plugins/platforms/drm/egl_gbm_backend.h

@@ -65,6 +65,7 @@ public:
     bool directScanoutAllowed(AbstractOutput *output) const override;
 
     QSharedPointer<DrmBuffer> renderTestFrame(DrmAbstractOutput *output) override;
+    uint32_t drmFormat() const override;
 
 protected:
     void cleanupSurfaces() override;

src/plugins/platforms/drm/egl_stream_backend.cpp

@@ -32,6 +32,7 @@
 #include <KWaylandServer/clientbuffer.h>
 #include <KWaylandServer/display.h>
 #include <KWaylandServer/eglstream_controller_interface.h>
+#include <drm_fourcc.h>
 
 namespace KWin
 {
@@ -576,6 +577,11 @@ bool EglStreamBackend::hasOutput(AbstractOutput *output) const
     return m_outputs.contains(output);
 }
 
+uint32_t EglStreamBackend::drmFormat() const
+{
+    return DRM_FORMAT_XRGB8888;
+}
+
 /************************************************
  * EglTexture
  ************************************************/

src/plugins/platforms/drm/egl_stream_backend.h

@@ -44,6 +44,7 @@ public:
     void removeOutput(DrmAbstractOutput *output) override;
 
     QSharedPointer<DrmBuffer> renderTestFrame(DrmAbstractOutput *output) override;
+    uint32_t drmFormat() const override;
 
 protected:
     void cleanupSurfaces() override;