platforms/atomic-kms: Fix cursors across multiple monitors. (#3742)

`KMSOutput::has_cursor` was misleadingly named, and so we implemented it
incorrectly. It is *used* to tell whether or not a cursor image needs to
be set on that particular output before it is visible, *not* whether
that output *supports* a cursor.

Rename it to something hopefully more easily understandable, and then
fix the implementation.

Fixes: #3677

src/platforms/atomic-kms/server/kms/atomic_kms_output.cpp

@@ -477,7 +477,7 @@ bool mga::AtomicKMSOutput::page_flip(FBHandle const& fb)
     return true;
 }
 
-void mga::AtomicKMSOutput::set_cursor(gbm_bo* buffer)
+void mga::AtomicKMSOutput::set_cursor_image(gbm_bo* buffer)
 {
     if (auto conf = configuration.lock(); conf->current_crtc)
     {
@@ -490,6 +490,7 @@ void mga::AtomicKMSOutput::set_cursor(gbm_bo* buffer)
         {
             mir::log_warning("set_cursor: drmModeSetCursor failed (%s)", strerror(-result));
         }
+        cursor_image_set = true;
     }
 }
 
@@ -516,6 +517,8 @@ bool mga::AtomicKMSOutput::clear_cursor()
             mir::log_warning("clear_cursor: drmModeSetCursor failed (%s)", strerror(-result));
     }
 
+    cursor_image_set = false;
+
     return !result;
 }
 
@@ -920,15 +923,8 @@ int mga::AtomicKMSOutput::drm_fd() const
     return drm_fd_;
 }
 
-bool mga::AtomicKMSOutput::has_cursor() const
+bool mga::AtomicKMSOutput::has_cursor_image() const
 {
-    // According to this:
-    // https://github.com/canonical/mir/pull/3665#discussion_r1835985441. The
-    // only point of failure is in the `Cursor` constructor, which throws an
-    // exception, which is caught in `mga::create_hardware_cursor` and signals
-    // to the calling code to use a software cursor instead.
-    //
-    // tldr; If this method is called, we have a cursor.
-    return true;
+    return cursor_image_set;
 }
 

src/platforms/atomic-kms/server/kms/atomic_kms_output.h

@@ -54,10 +54,10 @@ class AtomicKMSOutput : public KMSOutput
     void clear_crtc() override;
     bool page_flip(FBHandle const& fb) override;
 
-    void set_cursor(gbm_bo* buffer) override;
+    void set_cursor_image(gbm_bo* buffer) override;
     void move_cursor(geometry::Point destination) override;
     bool clear_cursor() override;
-    bool has_cursor() const override;
+    bool has_cursor_image() const override;
 
     void set_power_mode(MirPowerMode mode) override;
     void set_gamma(GammaCurves const& gamma) override;
@@ -93,6 +93,7 @@ class AtomicKMSOutput : public KMSOutput
     mir::Synchronised<Configuration> configuration;
     drmModeCrtc saved_crtc;
     bool using_saved_crtc;
+    std::atomic<bool> cursor_image_set{false};
 };
 
 }

src/platforms/atomic-kms/server/kms/cursor.cpp

@@ -391,16 +391,16 @@ void mga::Cursor::place_cursor_at_locked(
             if (changed_orientation)
                 pad_and_write_image_data_locked(lg, buffer);
 
-            if (force_state || !output.has_cursor() || changed_orientation)
+            if (force_state || !output.has_cursor_image() || changed_orientation)
             {
-                output.set_cursor(buffer);
-                if (!output.has_cursor())
+                output.set_cursor_image(buffer);
+                if (!output.has_cursor_image())
                     set_on_all_outputs = false;
             }
         }
         else
         {
-            if (force_state || output.has_cursor())
+            if (force_state || output.has_cursor_image())
             {
                 output.clear_cursor();
             }

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

@@ -72,10 +72,10 @@ class KMSOutput
 
     virtual bool page_flip(FBHandle const& fb) = 0;
 
-    virtual void set_cursor(gbm_bo* buffer) = 0;
+    virtual void set_cursor_image(gbm_bo* buffer) = 0;
     virtual void move_cursor(geometry::Point destination) = 0;
     virtual bool clear_cursor() = 0;
-    virtual bool has_cursor() const = 0;
+    virtual bool has_cursor_image() const = 0;
 
     virtual void set_power_mode(MirPowerMode mode) = 0;
     virtual void set_gamma(GammaCurves const& gamma) = 0;