feat(graphics): drove virtio-gpu's hardware cursor

kernel/drivers/graphics/gfxfb.h

@@ -258,18 +258,19 @@ class display {
 
     /**
      * @brief Place the hotspot of an output's cursor at (`x`, `y`) in the
-     * output's pixels, even off the output. A move before any picture only
-     * records the position, and the cursor stays hidden. The call may wait
-     * only for the display's own lock and for the hardware to take the
-     * position, within GFXFB_PRESENT_TIMEOUT_NS of giving it, never for a
-     * vertical blank or for its driver's task.
+     * output's pixels, even off the output. A move while the cursor is hidden,
+     * before any picture or after a hide, only records the position, and the
+     * cursor stays hidden. The call may wait only for the display's own lock
+     * and for the hardware to take the position, within
+     * GFXFB_PRESENT_TIMEOUT_NS of giving it, never for a vertical blank or for
+     * its driver's task.
      * @param output Index of the output, below the count.
      * @param x Column of the hotspot, negative left of the output.
      * @param y Row of the hotspot, negative above the output.
-     * @return OK once the hardware was given the position or, before any
-     * picture, once the display recorded it. Otherwise the first that holds of
-     * ERR_UNSUPPORTED on an output without a hardware cursor, ERR_DEVICE once
-     * the hardware has failed, ERR_DARK when the output is not lit, and
+     * @return OK once the hardware was given the position or, while the cursor
+     * is hidden, once the display recorded it. Otherwise the first that holds
+     * of ERR_UNSUPPORTED on an output without a hardware cursor, ERR_DEVICE
+     * once the hardware has failed, ERR_DARK when the output is not lit, and
      * ERR_REFUSED when the hardware refused the move.
      */
     virtual int32_t move_cursor(uint32_t output, int32_t x, int32_t y) = 0;

kernel/drivers/graphics/gfxfb_node.h

@@ -207,10 +207,10 @@ class boot_display : public display {
  * failing with `fs::ERR_BUSY` while another client controls the buffers or
  * reserves control, then fails with `fs::ERR_IO` once the display has failed,
  * and otherwise returns the display's result. The cursor calls for one output
- * reach the display one at a time, never waiting for a present there. A
- * closing client that controls the buffers hides each output's cursor before
- * its control ends, unless the display has failed, and an output whose hide
- * the display refuses keeps showing its cursor.
+ * reach the display one at a time, and gfxfb never makes one wait for a
+ * present there. A closing client that controls the buffers hides each
+ * output's cursor before its control ends, unless the display has failed, and
+ * an output whose hide the display refuses keeps showing its cursor.
  */
 class gfxfb_node : public fs::node {
 public:
@@ -323,7 +323,7 @@ class gfxfb_node : public fs::node {
     uint32_t    m_controller_calls = 0;  // admitted calls of `m_controller` that have not ended
 
     sync::mutex    m_present_locks[GFXFB_OUTPUTS_MAX]; // so presents for one output reach the display one at a time
-    sync::mutex    m_cursor_locks[GFXFB_OUTPUTS_MAX];  // likewise for cursor calls, which never wait for a present
+    sync::mutex    m_cursor_locks[GFXFB_OUTPUTS_MAX];  // likewise for cursor calls, which never wait for a present lock
     sync::spinlock m_outputs_lock;                     // over the members below, taken with interrupts disabled
     output_records m_outputs[GFXFB_OUTPUTS_MAX];       // written unlocked only by `show()`, before the node is opened
     bool           m_display_failed = false;           // set for good by a display call's ERR_DEVICE or a failed end