3 files changed, 103 insertions, 0 deletions
diff --git a/crates/wayland/src/udev/gpu.rs b/crates/wayland/src/udev/gpu.rs
new file mode 100644
index 0000000..5b21171
--- /dev/null
+++ b/crates/wayland/src/udev/gpu.rs
@@ -0,0 +1,97 @@
+//! Opt-in (`SRDWM_GPU=1`) capability probe for a real GBM+EGL GPU render
+//! path on the udev backend - see [`probe`]'s own doc comment for exactly
+//! what this does and does not do yet.
+
+use std::os::fd::{AsFd, OwnedFd};
+
+use smithay::backend::egl::{EGLDevice, EGLDisplay};
+use smithay::reexports::gbm::Device as GbmDevice;
+use smithay::reexports::rustix;
+
+use super::Card;
+
+/// Marker returned by a successful probe. Carries no data yet - see
+/// [`probe`]'s own doc comment for why - this exists purely to answer,
+/// definitively and safely, "does GBM+EGL actually initialize on this exact
+/// machine's DRM device" (logged from inside `probe` itself) as a plain
+/// `bool`-shaped `Option` a future caller can match on once there's
+/// something to actually do with a `Some`.
+pub(crate) struct GpuProbe;
+
+/// Attempts GBM device creation plus an EGL display/device query against
+/// `card`'s DRM fd, gated behind `SRDWM_GPU=1` (unset - the default --
+/// skips this entirely: zero cost, zero risk, identical to every session
+/// before this one). Returns `None` on *any* failure at any step, each
+/// logged with which step failed and why, so a machine without working
+/// KMS+3D driver support (or a VM with only dumb-buffer scanout - this
+/// backend's own long-standing default target, see `udev/mod.rs`'s module
+/// doc comment) gets one clear, harmless log line instead of anything
+/// touching the actual rendering/modesetting this backend already does.
+///
+/// Deliberately does **not** yet create an `EGLContext`, a `GlesRenderer`,
+/// or touch scanout at all. Reading smithay's own reference compositor
+/// (`anvil/src/udev.rs`) confirmed it wires GBM+EGL rendering together with
+/// *atomic*-KMS scanout as one unit via `DrmCompositor`, not as a renderer
+/// swapped into the existing legacy `set_crtc`/`page_flip` flip loop this
+/// backend uses today. Adopting `DrmCompositor` is real, separate,
+/// larger-scoped work than "swap the renderer" - it replaces the same
+/// `UdevHead` mode-set/flip machinery this session's own VT-switch fixes
+/// (`register_session_notifier`'s `ActivateSession` arm, `copy_and_flip`'s
+/// retry backoff) live in, and needs its own plan. This probe answers the
+/// *first* question - does the hardware even support it at all - safely,
+/// before that larger integration is scoped and attempted.
+pub(crate) fn probe(card: &Card) -> Option<GpuProbe> {
+    if std::env::var("SRDWM_GPU").as_deref() != Ok("1") {
+        return None;
+    }
+    // GBM wants its own fd, not the one this backend's KMS control-plane
+    // ioctls (`set_crtc`, `page_flip`, property gets/sets) already share
+    // via `Rc<Card>` - duplicated here rather than reusing `card`'s own
+    // fd directly, both because `gbm::Device`'s `EGLNativeDisplay` impl
+    // needs `T: Send + 'static` (an `Rc`-shared `Card` is neither) and
+    // because giving GBM a fd genuinely separate from the control-plane
+    // one matches real compositor practice, not just this one's
+    // convenience.
+    let dup_fd: OwnedFd = match rustix::io::dup(card.as_fd()) {
+        Ok(fd) => fd,
+        Err(e) => {
+            log::warn!("udev: SRDWM_GPU=1 but failed to dup the DRM fd for GBM: {e} - staying on the software (Pixman) render path");
+            return None;
+        }
+    };
+    let gbm = match GbmDevice::new(dup_fd) {
+        Ok(gbm) => gbm,
+        Err(e) => {
+            log::warn!("udev: SRDWM_GPU=1 but GBM device creation failed: {e} - staying on the software (Pixman) render path");
+            return None;
+        }
+    };
+    // Safety: `gbm` owns its own fd (the dup above), stays alive for as
+    // long as this function needs it, and is a real GBM device - exactly
+    // what `EGLDisplay::new`'s own safety contract (a native display handle
+    // that outlives the `EGLDisplay`) requires.
+    let display = match unsafe { EGLDisplay::new(gbm) } {
+        Ok(d) => d,
+        Err(e) => {
+            log::warn!("udev: SRDWM_GPU=1 but EGL display creation failed: {e} - staying on the software (Pixman) render path");
+            return None;
+        }
+    };
+    let egl_device = match EGLDevice::device_for_display(&display) {
+        Ok(d) => d,
+        Err(e) => {
+            log::warn!("udev: SRDWM_GPU=1 but no EGL device found for this display: {e} - staying on the software (Pixman) render path");
+            return None;
+        }
+    };
+    if egl_device.is_software() {
+        log::warn!("udev: SRDWM_GPU=1 but the only EGL device found is a software rasterizer - staying on the software (Pixman) render path");
+        return None;
+    }
+    let render_node = egl_device.try_get_render_node().ok().flatten();
+    log::info!(
+        "udev: SRDWM_GPU=1 - GBM+EGL capability confirmed on this hardware (render node: {render_node:?}). \
+         Full GPU rendering isn't wired up yet (see gpu::probe's own doc comment); still rendering via Pixman for now."
+    );
+    Some(GpuProbe)
+}
diff --git a/crates/wayland/src/udev/mod.rs b/crates/wayland/src/udev/mod.rs
index 4762992..b5529c4 100644
--- a/crates/wayland/src/udev/mod.rs
+++ b/crates/wayland/src/udev/mod.rs
@@ -531,6 +531,7 @@ mod copy_damaged_rows_tests {
 
 mod capture;
 mod drm;
+pub(crate) mod gpu;
 mod outputs;
 mod platform;
 mod render;
diff --git a/crates/wayland/src/udev/platform.rs b/crates/wayland/src/udev/platform.rs
index 96d5cf2..3f25c2e 100644
--- a/crates/wayland/src/udev/platform.rs
+++ b/crates/wayland/src/udev/platform.rs
@@ -36,6 +36,11 @@ impl UdevPlatform {
             .open(&gpu_path, rustix::fs::OFlags::RDWR | rustix::fs::OFlags::CLOEXEC)
             .map_err(err)?;
         let card = Rc::new(Card(fd));
+        // Opt-in only (`SRDWM_GPU=1`, unset by default) - see `gpu::probe`'s
+        // own doc comment for exactly what this does and does not do yet.
+        // A no-op unless that variable is set, so this line changes nothing
+        // about any session that doesn't set it.
+        let _gpu_probe = super::gpu::probe(&card);
 
         // Every connected connector becomes a head, laid out left-to-right.
         let connected = probe_connected(&card)?;