redox-drm virtio: implement feature negotiation + display detection

Replaced hardcoded 1280x720 with real VirtIO GPU config space
display detection:

- Added VirtIO MMIO transport register constants (spec v1.2 §4.2.2)
- Added VirtIO GPU config space offsets (scanout dimensions)
- Implemented feature negotiation:
  ACKNOWLEDGE → DRIVER → read device features →
  acknowledge VIRGL + EDID → FEATURES_OK → DRIVER_OK
- Read display dimensions from GPU config space at BAR+0x100:
  num_scanouts → scanout[0] enabled/width/height
- Fallback to 1280x720 if scanout is disabled or absent
- Renamed _mmio→mmio (was unused underscore-prefixed field)

Previously: hardcoded 1280x720, no virtio negotiation,
_mmio field stored but never accessed. Now: reads real
display dimensions from the virtio-gpu device config.

local/recipes/gpu/redox-drm/source/src/drivers/virtio/mod.rs

@@ -13,9 +13,39 @@ use crate::kms::connector::{synthetic_edid, Connector};
 use crate::kms::crtc::Crtc;
 use crate::kms::{ConnectorInfo, ConnectorStatus, ConnectorType, ModeInfo};
 
+// VirtIO MMIO transport register offsets (VirtIO spec v1.2 §4.2.2).
+const VIRTIO_MMIO_MAGIC: usize = 0x000;
+const VIRTIO_MMIO_VERSION: usize = 0x004;
+const VIRTIO_MMIO_DEVICE_ID: usize = 0x008;
+const VIRTIO_MMIO_DEVICE_FEATURES: usize = 0x010;
+const VIRTIO_MMIO_DEVICE_FEATURES_SEL: usize = 0x014;
+const VIRTIO_MMIO_DRIVER_FEATURES: usize = 0x020;
+const VIRTIO_MMIO_DRIVER_FEATURES_SEL: usize = 0x024;
+const VIRTIO_MMIO_GUEST_PAGE_SIZE: usize = 0x028;
+const VIRTIO_MMIO_STATUS: usize = 0x070;
+const VIRTIO_MMIO_CONFIG_GENERATION: usize = 0x0FC;
+
+// VirtIO GPU config space offsets (after virtio MMIO transport).
+// The GPU config region starts at offset 0x100 from the BAR base.
+const VIRTIO_GPU_CONFIG_OFFSET: usize = 0x100;
+const VIRTIO_GPU_NUM_SCANOUTS: usize = 0x00;
+const VIRTIO_GPU_SCANOUT_WIDTH: usize = 0x04;
+const VIRTIO_GPU_SCANOUT_HEIGHT: usize = 0x08;
+const VIRTIO_GPU_SCANOUT_ENABLED: usize = 0x0C;
+
+// VirtIO device status bits (§2.1).
+const VIRTIO_STATUS_ACKNOWLEDGE: u32 = 1;
+const VIRTIO_STATUS_DRIVER: u32 = 2;
+const VIRTIO_STATUS_DRIVER_OK: u32 = 4;
+const VIRTIO_STATUS_FEATURES_OK: u32 = 8;
+
+// VirtIO GPU feature bits.
+const VIRTIO_GPU_F_VIRGL: u32 = 1 << 0;
+const VIRTIO_GPU_F_EDID: u32 = 1 << 1;
+
 pub struct VirtioDriver {
     info: PciDeviceInfo,
-    _mmio: MmioRegion,
+    mmio: MmioRegion,
     irq_handle: Mutex<Option<InterruptHandle>>,
     width: u32,
     height: u32,
@@ -53,7 +83,7 @@ impl VirtioDriver {
         }
 
         let fb_bar = find_fb_bar(&info)?;
-        let _mmio = map_bar(&fb_bar, "VirtIO FB BAR")?;
+        let mmio = map_bar(&fb_bar, "VirtIO FB BAR")?;
 
         info!(
             "redox-drm: VirtIO GPU at {}: {} MiB BAR at {:#x}",
@@ -62,12 +92,49 @@ impl VirtioDriver {
             fb_bar.addr,
         );
 
+        // Negotiate VirtIO features: ACKNOWLEDGE → DRIVER → FEATURES_OK → DRIVER_OK
+        mmio.write32(VIRTIO_MMIO_STATUS, VIRTIO_STATUS_ACKNOWLEDGE);
+        mmio.write32(VIRTIO_MMIO_STATUS, VIRTIO_STATUS_DRIVER);
+
+        // Read device features and acknowledge Virgl + EDID support.
+        mmio.write32(VIRTIO_MMIO_DEVICE_FEATURES_SEL, 0);
+        let dev_features = mmio.read32(VIRTIO_MMIO_DEVICE_FEATURES);
+        let driver_features = dev_features & (VIRTIO_GPU_F_VIRGL | VIRTIO_GPU_F_EDID);
+        mmio.write32(VIRTIO_MMIO_DRIVER_FEATURES_SEL, 0);
+        mmio.write32(VIRTIO_MMIO_DRIVER_FEATURES, driver_features);
+        mmio.write32(VIRTIO_MMIO_STATUS, VIRTIO_STATUS_FEATURES_OK);
+
+        // Read the device status back to confirm negotiation.
+        let status = mmio.read32(VIRTIO_MMIO_STATUS);
+        if status & VIRTIO_STATUS_FEATURES_OK == 0 {
+            warn!("redox-drm: VirtIO GPU feature negotiation failed (status={:#x})", status);
+        }
+        mmio.write32(VIRTIO_MMIO_STATUS, VIRTIO_STATUS_DRIVER_OK);
+
+        // Read display dimensions from the VirtIO GPU config space.
+        let gpu_config = VIRTIO_GPU_CONFIG_OFFSET;
+        let num_scanouts = mmio.read32(gpu_config + VIRTIO_GPU_NUM_SCANOUTS);
+        let (width, height) = if num_scanouts > 0 {
+            let enabled = mmio.read32(gpu_config + VIRTIO_GPU_SCANOUT_ENABLED);
+            if enabled != 0 {
+                let w = mmio.read32(gpu_config + VIRTIO_GPU_SCANOUT_WIDTH);
+                let h = mmio.read32(gpu_config + VIRTIO_GPU_SCANOUT_HEIGHT);
+                info!("redox-drm: VirtIO GPU scanout 0: {}x{} ({} scanouts, enabled={})",
+                    w, h, num_scanouts, enabled);
+                (w.max(640), h.max(480))
+            } else {
+                (1280, 720)
+            }
+        } else {
+            (1280, 720)
+        };
+
         Ok(Self {
             info,
-            _mmio,
+            mmio,
             irq_handle: Mutex::new(None),
-            width: 1280,
-            height: 720,
+            width,
+            height,
             gem: Mutex::new(GemManager::new()),
             connectors: Mutex::new(Vec::new()),
             crtcs: Mutex::new(Vec::new()),