Add a real general.gpu config option for the GPU render path

SRDWM_GPU=1 was the only way to opt into the udev backend's GBM+EGL
GPU render path - an env var, not a real config option, with no way
to enable it from init.lua the way every other general.* flag works.

WindowManager::gpu_enabled (plain bool, false by default - unlike
rounded_corners_enabled's Option<bool>, GPU rendering has one
unambiguous default regardless of which backend ends up connecting,
so there's no "let the backend decide" case to preserve) is read from
general.gpu in apply_general_settings, same as every other general.*
key. gpu::probe now takes an explicit enabled: bool instead of
checking the env var itself; udev/platform.rs's call site computes it
as wm.gpu_enabled OR SRDWM_GPU=1, so the env var still works as a
quick manual override for testing without touching config, on top of
the new persistent option.

Falls back to the existing software (Pixman) path exactly as before
on any failure at any step (no GBM device, no atomic-modesetting
support, a software-only EGL renderer, ...) - gpu::probe's own
fallback behavior is unchanged, only how the initial enabled/disabled
decision gets made.

2 files changed, 35 insertions, 23 deletions

diff --git a/crates/wayland/src/udev/gpu.rs b/crates/wayland/src/udev/gpu.rs
index 904a420..8617af1 100644
--- a/crates/wayland/src/udev/gpu.rs
+++ b/crates/wayland/src/udev/gpu.rs
@@ -1,6 +1,7 @@
-//! Opt-in (`SRDWM_GPU=1`) GBM+EGL+`DrmCompositor` GPU render path for the
-//! udev backend - see [`probe`]'s own doc comment for exactly what this
-//! does and does not do yet. Past the original Phase 2 (one output,
+//! Opt-in (`general.gpu` in config, or the lower-level `SRDWM_GPU=1` env
+//! var) GBM+EGL+`DrmCompositor` GPU render path for the udev backend --
+//! see [`probe`]'s own doc comment for exactly what this does and does
+//! not do yet. Past the original Phase 2 (one output,
 //! clear-color only - see the plan this was built from, `snappy-
 //! percolating-boole.md`, for that phase's own scoping): every head
 //! `initialize_output` succeeds for gets driven, not just the first
@@ -106,15 +107,20 @@ impl GpuContext {
 const COLOR_FORMATS: [DrmFourcc; 2] = [DrmFourcc::Argb8888, DrmFourcc::Xrgb8888];
 
 /// Attempts the full GBM+EGL+GLES+`DrmDevice`+`DrmOutputManager` chain
-/// 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.
+/// against `card`'s DRM fd, gated behind `enabled` (the caller's own
+/// combined decision - `udev/platform.rs`'s call site attempts this
+/// whenever *either* `general.gpu` in config or the lower-level
+/// `SRDWM_GPU=1` env-var override says to; both unset/`false` is the
+/// default, and skips this entirely: zero cost, zero risk, identical to
+/// every session before this option existed). 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, and falls straight back to
+/// that same always-available software path with no further action
+/// needed from whoever enabled this.
 ///
 /// `DrmDevice::new`'s own `disable_connectors` parameter (passed `false`
 /// here, matching the existing legacy path's own connector handling) is
@@ -132,8 +138,8 @@ const COLOR_FORMATS: [DrmFourcc; 2] = [DrmFourcc::Argb8888, DrmFourcc::Xrgb8888]
 /// is building a ready-to-use `GpuContext`; which head (if any) actually
 /// gets driven through it is a per-session, per-head decision the caller
 /// (`udev/platform.rs`'s startup path) makes once this succeeds.
-pub(crate) fn probe(card: &Card) -> Option<(GpuContext, DrmDeviceNotifier)> {
-    if std::env::var("SRDWM_GPU").as_deref() != Ok("1") {
+pub(crate) fn probe(card: &Card, enabled: bool) -> Option<(GpuContext, DrmDeviceNotifier)> {
+    if !enabled {
         return None;
     }
     // One `DrmDeviceFd` used for everything below - GBM, EGL, and the
diff --git a/crates/wayland/src/udev/platform.rs b/crates/wayland/src/udev/platform.rs
index 19991e8..691c8de 100644
--- a/crates/wayland/src/udev/platform.rs
+++ b/crates/wayland/src/udev/platform.rs
@@ -36,15 +36,21 @@ 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. `gpu_notifier` is
-        // registered as its own calloop event source further down
-        // (alongside `register_drm_fd`'s own registration for the
-        // existing legacy heads); `gpu_context` is stored on `UdevState`
-        // below and consulted by `render_udev_frame`.
-        let (mut gpu_context, gpu_notifier) = match super::gpu::probe(&card) {
+        // Opt-in only - `general.gpu` in config (`wm.gpu_enabled`,
+        // `false` by default) or the lower-level `SRDWM_GPU=1` env var,
+        // whichever says yes - see `gpu::probe`'s own doc comment for
+        // exactly what this does and does not do yet. A no-op unless
+        // either says to, so this line changes nothing about any session
+        // that leaves both alone. The env var stays as a quick manual
+        // override for testing without touching config (still works even
+        // if `general.gpu` is explicitly `false`) - it does not gate
+        // config *off*, only ever adds an extra way to opt *in*.
+        // `gpu_notifier` is registered as its own calloop event source
+        // further down (alongside `register_drm_fd`'s own registration
+        // for the existing legacy heads); `gpu_context` is stored on
+        // `UdevState` below and consulted by `render_udev_frame`.
+        let gpu_enabled = wm.borrow().gpu_enabled || std::env::var("SRDWM_GPU").as_deref() == Ok("1");
+        let (mut gpu_context, gpu_notifier) = match super::gpu::probe(&card, gpu_enabled) {
             Some((ctx, notifier)) => (Some(ctx), Some(notifier)),
             None => (None, None),
         };