plugins/qpa: handle modifiers more correctly

Instead of hardcoding ARGB8888 and using implicit modifiers, look through
the list of available formats and modifiers and pick a match that egl will
actually accept.

src/core/shmgraphicsbufferallocator.cpp

@@ -81,7 +81,7 @@ GraphicsBuffer *ShmGraphicsBufferAllocator::allocate(const GraphicsBufferOptions
     if (!options.software) {
         return nullptr;
     }
-    if (!options.modifiers.isEmpty()) {
+    if (!options.modifiers.isEmpty() && !options.modifiers.contains(DRM_FORMAT_MOD_LINEAR)) {
         return nullptr;
     }
 

src/plugins/qpa/backingstore.cpp

@@ -16,6 +16,7 @@
 #include "window.h"
 
 #include <QPainter>
+#include <libdrm/drm_fourcc.h>
 
 namespace KWin
 {
@@ -41,7 +42,11 @@ void BackingStore::resize(const QSize &size, const QRegion &staticContents)
 void BackingStore::beginPaint(const QRegion &region)
 {
     Window *platformWindow = static_cast<Window *>(window()->handle());
-    Swapchain *swapchain = platformWindow->swapchain();
+    Swapchain *swapchain = platformWindow->swapchain({{DRM_FORMAT_ARGB8888, {DRM_FORMAT_MOD_LINEAR}}});
+    if (!swapchain) {
+        qCCritical(KWIN_QPA, "Failed to ceate a swapchain for the backing store!");
+        return;
+    }
 
     const auto oldBuffer = m_buffer;
     m_buffer = swapchain->acquire();

src/plugins/qpa/eglplatformcontext.cpp

@@ -84,7 +84,7 @@ bool EGLPlatformContext::makeCurrent(QPlatformSurface *surface)
         QOpenGLContextPrivate::setCurrentContext(context());
 
         Window *window = static_cast<Window *>(surface);
-        Swapchain *swapchain = window->swapchain();
+        Swapchain *swapchain = window->swapchain(m_eglDisplay->supportedDrmFormats());
 
         GraphicsBuffer *buffer = swapchain->acquire();
         if (!buffer) {

src/plugins/qpa/swapchain.cpp

@@ -11,10 +11,11 @@ namespace KWin
 namespace QPA
 {
 
-Swapchain::Swapchain(GraphicsBufferAllocator *allocator, const GraphicsBufferOptions &options)
+Swapchain::Swapchain(GraphicsBufferAllocator *allocator, const GraphicsBufferOptions &options, GraphicsBuffer *initialBuffer)
     : m_allocator(allocator)
     , m_allocationOptions(options)
 {
+    m_buffers.push_back(initialBuffer);
 }
 
 Swapchain::~Swapchain()
@@ -46,5 +47,14 @@ GraphicsBuffer *Swapchain::acquire()
     return buffer;
 }
 
+uint32_t Swapchain::format() const
+{
+    return m_allocationOptions.format;
+}
+
+QVector<uint64_t> Swapchain::modifiers() const
+{
+    return m_allocationOptions.modifiers;
+}
 }
 }

src/plugins/qpa/swapchain.h

@@ -18,12 +18,14 @@ namespace QPA
 class Swapchain
 {
 public:
-    Swapchain(GraphicsBufferAllocator *allocator, const GraphicsBufferOptions &options);
+    Swapchain(GraphicsBufferAllocator *allocator, const GraphicsBufferOptions &options, GraphicsBuffer *initialBuffer);
     ~Swapchain();
 
     QSize size() const;
 
     GraphicsBuffer *acquire();
+    uint32_t format() const;
+    QVector<uint64_t> modifiers() const;
 
 private:
     GraphicsBufferAllocator *m_allocator;

src/plugins/qpa/window.cpp

@@ -7,6 +7,10 @@
 
     SPDX-License-Identifier: GPL-2.0-or-later
 */
+// this needs to be on top, epoxy has an error if you include it after GL/gl.h,
+// which Qt does include
+#include "utils/drm_format_helper.h"
+
 #include "window.h"
 #include "composite.h"
 #include "core/renderbackend.h"
@@ -38,10 +42,12 @@ Window::~Window()
     unmap();
 }
 
-Swapchain *Window::swapchain()
+Swapchain *Window::swapchain(const QHash<uint32_t, QVector<uint64_t>> &formats)
 {
     const QSize nativeSize = geometry().size() * devicePixelRatio();
-    if (!m_swapchain || m_swapchain->size() != nativeSize) {
+    if (!m_swapchain || m_swapchain->size() != nativeSize
+        || !formats.contains(m_swapchain->format())
+        || m_swapchain->modifiers() != formats[m_swapchain->format()]) {
         const bool software = window()->surfaceType() == QSurface::RasterSurface; // RasterGLSurface is unsupported by us
 
         GraphicsBufferAllocator *allocator;
@@ -52,11 +58,22 @@ Swapchain *Window::swapchain()
             allocator = Compositor::self()->backend()->graphicsBufferAllocator();
         }
 
-        m_swapchain = std::make_unique<Swapchain>(allocator, GraphicsBufferOptions{
-                                                                 .size = nativeSize,
-                                                                 .format = DRM_FORMAT_ARGB8888,
-                                                                 .software = software,
-                                                             });
+        for (auto it = formats.begin(); it != formats.end(); it++) {
+            if (auto info = formatInfo(it.key()); info && info->bitsPerColor == 8 && info->alphaBits == 8) {
+                const auto options = GraphicsBufferOptions{
+                    .size = nativeSize,
+                    .format = it.key(),
+                    .modifiers = it.value(),
+                    .software = software,
+                };
+                auto buffer = allocator->allocate(options);
+                if (!buffer) {
+                    continue;
+                }
+                m_swapchain = std::make_unique<Swapchain>(allocator, options, buffer);
+                break;
+            }
+        }
     }
     return m_swapchain.get();
 }

src/plugins/qpa/window.h

@@ -37,7 +37,7 @@ public:
     void requestActivateWindow() override;
 
     InternalWindow *internalWindow() const;
-    Swapchain *swapchain();
+    Swapchain *swapchain(const QHash<uint32_t, QVector<uint64_t>> &formats);
 
 private:
     void map();

src/utils/drm_format_helper.h

@@ -7,8 +7,8 @@
     SPDX-License-Identifier: GPL-2.0-or-later
 */
 #pragma once
-#include <drm_fourcc.h>
 #include <epoxy/gl.h>
+#include <libdrm/drm_fourcc.h>
 #include <optional>
 #include <stdint.h>