Adding a check for a crtc mismatch instead of just rectangle comparing

src/platforms/gbm-kms/server/kms/display_buffer.cpp

@@ -63,34 +63,25 @@ mgg::DisplayBuffer::DisplayBuffer(
 {
     listener->report_successful_setup_of_native_resources();
 
-    bool needs_crtc_set = false;
+    // If any of the outputs have a CRTC mismatch, we will want to set all of them
+    // so that they're all showing the same buffer.
+    bool has_crtc_mismatch = false;
     for (auto& output : outputs)
     {
-        try
-        {
-            geometry::Rectangle rectangle = output->get_rectangle();
-            if (rectangle != area)
-            {
-                needs_crtc_set = true;
-                break;
-            }
-        }
-        catch (std::invalid_argument&)
-        {
-            needs_crtc_set = true;
+        has_crtc_mismatch = output->has_crtc_mismatch();
+        if (has_crtc_mismatch)
             break;
-        }
     }
 
-    if (needs_crtc_set)
+    if (has_crtc_mismatch)
     {
         mir::log_info("Clearing screen due to differing encountered and target modes");
         // TODO: Pull a supported format out of KMS rather than assuming XRGB8888
         auto initial_fb = std::make_shared<mgg::CPUAddressableFB>(
-                std::move(drm_fd),
-                false,
-                DRMFormat{DRM_FORMAT_XRGB8888},
-                area.size);
+            std::move(drm_fd),
+            false,
+            DRMFormat{DRM_FORMAT_XRGB8888},
+            area.size);
 
         auto mapping = initial_fb->map_writeable();
         ::memset(mapping->data(), 24, mapping->len());

src/platforms/gbm-kms/server/kms/kms_output.h

@@ -63,7 +63,12 @@ class KMSOutput
     virtual int max_refresh_rate() const = 0;
 
     virtual bool set_crtc(FBHandle const& fb) = 0;
-    virtual auto get_rectangle() -> mir::geometry::Rectangle = 0;
+
+    /**
+     * Check if the pending call to set_crtc is compatible with the current state of the CRTC.
+     * @returns true if a set_crtc is required, otherwise false
+     */
+    virtual bool has_crtc_mismatch() = 0;
     virtual void clear_crtc() = 0;
     virtual bool schedule_page_flip(FBHandle const& fb) = 0;
     virtual void wait_for_page_flip() = 0;

src/platforms/gbm-kms/server/kms/real_kms_output.cpp

@@ -34,6 +34,22 @@ namespace mgk = mg::kms;
 namespace geom = mir::geometry;
 
 
+namespace
+{
+bool kms_modes_are_equal(drmModeModeInfo const& info1, drmModeModeInfo const& info2)
+{
+    return (info1.clock == info2.clock &&
+            info1.hdisplay == info2.hdisplay &&
+            info1.hsync_start == info2.hsync_start &&
+            info1.hsync_end == info2.hsync_end &&
+            info1.htotal == info2.htotal &&
+            info1.hskew == info2.hskew &&
+            info1.vdisplay == info2.vdisplay &&
+            info1.vsync_start == info2.vsync_start &&
+            info1.vsync_end == info2.vsync_end &&
+            info1.vtotal == info2.vtotal);
+}
+}
 
 mgg::RealKMSOutput::RealKMSOutput(
     int drm_fd,
@@ -140,7 +156,7 @@ bool mgg::RealKMSOutput::set_crtc(FBHandle const& fb)
         return false;
     }
 
-    auto ret = drmModeSetCrtc(drm_fd_, current_crtc->crtc_id,
+    auto ret = drmModeSetCrtc(drm_fd_, current_crtc->crtc_id,                             
                               fb, fb_offset.dx.as_int(), fb_offset.dy.as_int(),
                               &connector->connector_id, 1,
                               &connector->modes[mode_index]);
@@ -155,17 +171,15 @@ bool mgg::RealKMSOutput::set_crtc(FBHandle const& fb)
     return true;
 }
 
-auto mgg::RealKMSOutput::get_rectangle() -> mir::geometry::Rectangle
+bool mgg::RealKMSOutput::has_crtc_mismatch()
 {
     if (!ensure_crtc())
     {
         mir::log_error("Output %s has no associated CRTC to get ", mgk::connector_name(connector).c_str());
-        BOOST_THROW_EXCEPTION(std::invalid_argument("get_crtc: Output has no associated CRTC to get"));
+        return true;
     }
 
-    return mir::geometry::Rectangle(
-        mir::geometry::Point(current_crtc->x, current_crtc->y),
-        mir::geometry::Size(current_crtc->width, current_crtc->height));
+    return !kms_modes_are_equal(current_crtc->mode, connector->modes[mode_index]);
 }
 
 void mgg::RealKMSOutput::clear_crtc()
@@ -394,21 +408,6 @@ void mgg::RealKMSOutput::refresh_hardware_state()
 
 namespace
 {
-
-bool kms_modes_are_equal(drmModeModeInfo const& info1, drmModeModeInfo const& info2)
-{
-    return (info1.clock == info2.clock &&
-            info1.hdisplay == info2.hdisplay &&
-            info1.hsync_start == info2.hsync_start &&
-            info1.hsync_end == info2.hsync_end &&
-            info1.htotal == info2.htotal &&
-            info1.hskew == info2.hskew &&
-            info1.vdisplay == info2.vdisplay &&
-            info1.vsync_start == info2.vsync_start &&
-            info1.vsync_end == info2.vsync_end &&
-            info1.vtotal == info2.vtotal);
-}
-
 double calculate_vrefresh_hz(drmModeModeInfo const& mode)
 {
     if (mode.htotal == 0 || mode.vtotal == 0)

src/platforms/gbm-kms/server/kms/real_kms_output.h

@@ -49,7 +49,7 @@ class RealKMSOutput : public KMSOutput
     int max_refresh_rate() const override;
 
     bool set_crtc(FBHandle const& fb) override;
-    auto get_rectangle() -> mir::geometry::Rectangle override;
+    bool has_crtc_mismatch() override;
     void clear_crtc() override;
     bool schedule_page_flip(FBHandle const& fb) override;
     void wait_for_page_flip() override;

tests/unit-tests/platforms/gbm-kms/kms/mock_kms_output.h

@@ -39,7 +39,7 @@ struct MockKMSOutput : public graphics::gbm::KMSOutput
         return set_crtc_thunk(&fb);
     }
 
-    MOCK_METHOD0(get_rectangle, geometry::Rectangle());
+    MOCK_METHOD0(has_crtc_mismatch, bool());
     MOCK_METHOD1(set_crtc_thunk, bool(graphics::gbm::FBHandle const*));
     MOCK_METHOD0(clear_crtc, void());