Proposal on WSDISPLAYIO damage reporting IOCTL

["Jiaxun Yang" <jiaxun.yang%flygoat.com@localhost>]

Hi all,

I'm new to NetBSD development and have been working on improving display support
for emulated platforms. This has resulted in a new driver for the Bochs DISPI
device (PR kern/59410) and enhancements to the not-yet-committed viogpu driver
(PR kern/58681).

While implementing X11 userspace support for viogpu, I identified a need for a
unified WSDISPLAYIO mechanism to report framebuffer damage areas to drivers.
Unlike conventional dumb framebuffers that continuously scan out VRAM content,
devices like viogpu (non-VGA compliant variant) require explicit notification
of damaged areas. The udl(4) driver presents a similar case, where only damaged
portions of the framebuffer should be transmitted over USB for efficiency.

Other Unix systems handled this differently:
- Linux FBDEV created device-specific IOCTLs for udlfb, though plans for a unified
  approach were superseded by DRM adoption
- OpenBSD implemented a device-specific IOCTL for udl(4) (which was commented out when
  ported to NetBSD)


I propose the following unified IOCTL interface:

```
--- a/sys/dev/wscons/wsconsio.h
+++ b/sys/dev/wscons/wsconsio.h
@@ -688,6 +688,7 @@ struct wsdisplayio_fbinfo {
 /* fbi_flags */
 #define WSFB_VRAM_IS_RAM       0x0001  /* hint for wsfb - don't shadow */
 #define WSFB_VRAM_IS_SPLIT     0x0002  /* workaround for wildcat... */
+#define WSFB_VRAM_NEEDS_DAMAGE 0x0004  /* framebuffer needs damage reporting */
 
 #define WSDISPLAYIO_GET_FBINFO _IOWR('W', 104, struct wsdisplayio_fbinfo)
 
@@ -747,4 +748,20 @@ struct wsdisplay_getfont {
 
 #define WSDISPLAYIO_GFONT      _IOWR('W', 108, struct wsdisplay_getfont)
 
+/*
+ * Repoort framebuffer damage
+ *
+ * Only necessary if WSFB_VRAM_NEEDS_DAMAGE is set in fbi_flags.
+ */
+
+struct wsdisplay_damage {
+       uint32_t        flags;
+#define WSDISPLAY_DAMAGE_WAIT  0x0001 /* Wait for damage flush to finish */
+       uint32_t        x;
+       uint32_t        y;
+       uint32_t        width;
+       uint32_t        height;
+};
+#define WSDISPLAYIO_DAMAGE     _IOWR('W', 109, struct wsdisplay_damage)
+
 #endif /* _DEV_WSCONS_WSCONSIO_H_ */
```

This unified approach would allow consistent damage reporting across different
device types without requiring device-specific implementations.

There are also some alternative ways to archive similar functionality, such
as implementing blit from userspace pointer, or implement page flip mechanism
for WSDISPLAYIO. I'm happy to look into them if damage reporting is deemed
inappropriate.

I'd appreciate your feedback on this proposal.

Thanks!
- Jiaxun