perf(latency): T2.5b — NV12 compute CSC on the LINEAR/gamescope zero-copy path

design/latency-reduction-2026-07.md T2.5's Linux half: the LINEAR dmabuf path
(gamescope's only offer) fed NVENC RGB, paying its internal RGB->YUV CSC on
the SM the game is saturating — the exact contention §5.A removed everywhere
else. The Vulkan bridge now carries a buffer-to-buffer RGB->NV12 compute
shader (rgb2nv12_buf.comp, BT.709 limited, coefficient-identical to
pf-encode's rgb2yuv.comp; whole-word writes so no 8-bit-storage feature is
needed): import dmabuf -> dispatch CSC into the exportable buffer -> CUDA
de-strides both planes into a pooled two-plane NV12 buffer. PUNKTFUNK_NV12
(default-on) now covers LINEAR; a CSC failure latches RGB for the stream
(mid-frame fallback, no dropped frame); 4:4:4 LINEAR sessions stay RGB (never
silently subsample). New ImportKind::LinearNv12 rides the existing worker IPC
(appended last per the wire-tag rule); cursor stays downstream (blend_nv12).

Validated: .21 clippy -D warnings (pf-zerocopy/pf-capture/host+nvenc) + 17
zero-copy tests. Owed: on-glass gamescope session (visual + dmon sm% check).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-capture/src/linux/mod.rs

@@ -748,6 +748,9 @@ mod pipewire {
         /// (`ImportKind::Tiled444`) and feed NVENC native full-chroma YUV — takes precedence over
         /// `nv12` (a 4:4:4 session must never subsample).
         yuv444: bool,
+        /// The LINEAR (gamescope) NV12 compute CSC failed once — RGB for the rest of the stream
+        /// (T2.5b's per-stream fallback latch; cleared by the next stream's fresh `Ud`).
+        linear_nv12_failed: bool,
         /// Rate-limit counter for the latest-frame-only diagnostic log (see `.process`).
         dbg_log_n: u64,
         /// Cursor-as-metadata state, composited into the CPU de-pad path (see `consume_frame`).
@@ -1352,15 +1355,17 @@ mod pipewire {
                 // sample LINEAR).
                 let modifier = (ud.modifier != 0).then_some(ud.modifier);
                 if let Some(fourcc) = pf_frame::drm_fourcc(fmt) {
-                    // GPU converts only on the tiled EGL/GL path (`modifier.is_some()`): a 4:4:4
-                    // session gets the planar-YUV444 convert (full chroma, takes precedence over
-                    // NV12 — 4:4:4 must never subsample), otherwise `PUNKTFUNK_NV12` gets NV12 —
-                    // both feed NVENC native YUV so it skips its internal RGB→YUV CSC. The
-                    // LINEAR/Vulkan (gamescope) path stays RGB — its converts aren't wired here;
-                    // a 4:4:4 session on LINEAR frames falls to the encoder's clear-error path
-                    // (`want_444` with an RGB CUDA payload) rather than silently subsampling.
+                    // GPU converts: a 4:4:4 session gets the planar-YUV444 convert on the tiled
+                    // EGL/GL path (full chroma, takes precedence over NV12 — 4:4:4 must never
+                    // subsample), otherwise `PUNKTFUNK_NV12` gets NV12 — tiled via the EGL/GL
+                    // blit, LINEAR/gamescope via the Vulkan bridge's compute CSC (latency plan
+                    // T2.5b) — so NVENC encodes native YUV and skips its internal RGB→YUV CSC on
+                    // the contended SM. A 4:4:4 session on LINEAR frames has no convert and
+                    // stays RGB, falling to the encoder's clear-error path (`want_444` with an
+                    // RGB CUDA payload) rather than silently subsampling. A LINEAR NV12 convert
+                    // failure latches RGB for the stream (mid-frame fallback, no drop).
                     let yuv444 = ud.yuv444 && modifier.is_some();
-                    let nv12 = ud.nv12 && !yuv444 && modifier.is_some();
+                    let mut nv12 = ud.nv12 && !ud.yuv444;
                     let imported = if let Some(m) = modifier {
                         if yuv444 {
                             importer.import_yuv444(&plane, w as u32, h as u32, fourcc, Some(m))
@@ -1369,7 +1374,20 @@ mod pipewire {
                         } else {
                             importer.import(&plane, w as u32, h as u32, fourcc, Some(m))
                         }
+                    } else if nv12 && !ud.linear_nv12_failed {
+                        match importer.import_linear_nv12(&plane, w as u32, h as u32) {
+                            Ok(buf) => Ok(buf),
+                            Err(e) => {
+                                ud.linear_nv12_failed = true;
+                                nv12 = false;
+                                tracing::warn!(error = %format!("{e:#}"),
+                                    "LINEAR NV12 compute CSC failed — RGB for the rest of this \
+                                     stream (NVENC does the CSC internally)");
+                                importer.import_linear(&plane, w as u32, h as u32)
+                            }
+                        }
                     } else {
+                        nv12 = false;
                         importer.import_linear(&plane, w as u32, h as u32)
                     };
                     match imported {
@@ -1742,6 +1760,7 @@ mod pipewire {
             vaapi_passthrough,
             nv12: pf_zerocopy::nv12_enabled(),
             yuv444: want_444,
+            linear_nv12_failed: false,
             dbg_log_n: 0,
             cursor: CursorState::default(),
         };

crates/pf-zerocopy/src/imp/client.rs

@@ -320,6 +320,17 @@ impl RemoteImporter {
         self.import_impl(plane, ImportKind::Linear, width, height, 0, None)
     }
 
+    /// Mirror of [`super::egl::EglImporter::import_linear_nv12`] (LINEAR dmabuf → Vulkan-bridge
+    /// compute CSC → two-plane NV12 buffer, latency plan T2.5b).
+    pub fn import_linear_nv12(
+        &mut self,
+        plane: &DmabufPlane,
+        width: u32,
+        height: u32,
+    ) -> Result<DeviceBuffer> {
+        self.import_impl(plane, ImportKind::LinearNv12, width, height, 0, None)
+    }
+
     fn import_impl(
         &mut self,
         plane: &DmabufPlane,

crates/pf-zerocopy/src/imp/cuda.rs

@@ -1386,3 +1386,50 @@ pub fn copy_pitched_to_buffer(
     // within both. Wrapper → live table.
     unsafe { copy_blocking(&copy, "cuMemcpy2DAsync_v2(ext->dev)") }
 }
+
+/// De-stride an NV12 pair from an external mapping (the Vulkan bridge's exportable buffer after
+/// its compute CSC — latency plan T2.5b) into a pooled two-plane NV12 [`DeviceBuffer`]: the Y
+/// plane (`width` bytes × `height` rows) and the interleaved UV plane (`width` bytes × ⌈h/2⌉
+/// rows), each de-strided from `src_pitch` to the pool's own plane pitches. Same contract as
+/// [`copy_pitched_to_buffer`]: the shared context must be current.
+pub fn copy_pitched_nv12_to_buffer(
+    y_src: CUdeviceptr,
+    uv_src: CUdeviceptr,
+    src_pitch: usize,
+    dst: &DeviceBuffer,
+) -> Result<()> {
+    let Some((uv_ptr, uv_pitch)) = dst.uv else {
+        anyhow::bail!("copy_pitched_nv12_to_buffer: destination is not an NV12 buffer");
+    };
+    let y = CUDA_MEMCPY2D {
+        srcMemoryType: CU_MEMORYTYPE_DEVICE,
+        srcDevice: y_src,
+        srcPitch: src_pitch,
+        dstMemoryType: CU_MEMORYTYPE_DEVICE,
+        dstDevice: dst.ptr,
+        dstPitch: dst.pitch,
+        WidthInBytes: dst.width as usize,
+        Height: dst.height as usize,
+        ..Default::default()
+    };
+    let uv = CUDA_MEMCPY2D {
+        srcMemoryType: CU_MEMORYTYPE_DEVICE,
+        srcDevice: uv_src,
+        srcPitch: src_pitch,
+        dstMemoryType: CU_MEMORYTYPE_DEVICE,
+        dstDevice: uv_ptr,
+        dstPitch: uv_pitch,
+        // W/2 interleaved UV samples × 2 bytes = `width` bytes per row.
+        WidthInBytes: dst.width as usize,
+        Height: dst.height.div_ceil(2) as usize,
+        ..Default::default()
+    };
+    // SAFETY: same contract as `copy_pitched_to_buffer` — the caller holds the shared context
+    // current; both `CUDA_MEMCPY2D`s are live locals describing spans inside the caller's live
+    // external mapping (`y_src`/`uv_src` at `src_pitch`) and `dst`'s live pooled planes; each
+    // `copy_blocking` synchronizes before returning.
+    unsafe {
+        copy_blocking(&y, "cuMemcpy2DAsync_v2(ext->dev nv12 Y)")?;
+        copy_blocking(&uv, "cuMemcpy2DAsync_v2(ext->dev nv12 UV)")
+    }
+}

crates/pf-zerocopy/src/imp/egl.rs

@@ -504,6 +504,9 @@ pub struct EglImporter {
     /// created lazily on the first LINEAR frame, + the destination pool.
     vk: Option<super::vulkan::VkBridge>,
     linear_pool: Option<cuda::BufferPool>,
+    /// NV12 twin of [`linear_pool`](Self::linear_pool) for the bridge's compute-CSC output
+    /// (T2.5b) — separate pools because a session may fall back RGB mid-stream.
+    linear_nv12_pool: Option<cuda::BufferPool>,
     gbm: *mut c_void,
     render_fd: c_int,
 }
@@ -647,6 +650,7 @@ impl EglImporter {
             yuv444_blit: None,
             vk: None,
             linear_pool: None,
+            linear_nv12_pool: None,
             gbm,
             render_fd,
         })
@@ -677,6 +681,38 @@ impl EglImporter {
         )
     }
 
+    /// Like [`import_linear`](Self::import_linear), but the bridge's compute CSC converts to a
+    /// two-plane **NV12** buffer (latency plan T2.5b) — the gamescope/LINEAR analogue of
+    /// [`import_nv12`](Self::import_nv12), so NVENC encodes native YUV on the dedicated-session
+    /// path too instead of paying its internal RGB→YUV CSC on the contended SM.
+    pub fn import_linear_nv12(
+        &mut self,
+        plane: &DmabufPlane,
+        width: u32,
+        height: u32,
+    ) -> Result<DeviceBuffer> {
+        cuda::make_current()?;
+        if self
+            .linear_nv12_pool
+            .as_ref()
+            .map(|p| (p.width(), p.height()))
+            != Some((width, height))
+        {
+            self.linear_nv12_pool = Some(cuda::BufferPool::new_nv12(width, height)?);
+        }
+        if self.vk.is_none() {
+            self.vk = Some(super::vulkan::VkBridge::new()?);
+        }
+        self.vk.as_mut().unwrap().import_linear_nv12(
+            plane.fd,
+            plane.offset,
+            plane.stride,
+            width,
+            height,
+            self.linear_nv12_pool.as_ref().unwrap(),
+        )
+    }
+
     /// Drop the Vulkan bridge's cached per-fd import (see [`super::vulkan::VkBridge::forget_fd`]).
     /// No-op when the bridge hasn't been built (tiled-only captures).
     pub fn forget_linear_fd(&mut self, fd: i32) {

crates/pf-zerocopy/src/imp/mod.rs

@@ -166,6 +166,20 @@ impl Importer {
         }
     }
 
+    /// LINEAR dmabuf → Vulkan-bridge compute CSC → two-plane NV12 buffer (latency plan T2.5b —
+    /// the gamescope analogue of [`import_nv12`](Self::import_nv12)).
+    pub fn import_linear_nv12(
+        &mut self,
+        plane: &DmabufPlane,
+        width: u32,
+        height: u32,
+    ) -> anyhow::Result<DeviceBuffer> {
+        match self {
+            Importer::Remote(r) => r.import_linear_nv12(plane, width, height),
+            Importer::InProc(i) => i.import_linear_nv12(plane, width, height),
+        }
+    }
+
     /// True once the worker process is gone/wedged (every further call fails fast). Always
     /// `false` in-process — an in-process driver fault doesn't return.
     pub fn dead(&self) -> bool {

crates/pf-zerocopy/src/imp/proto.rs

@@ -41,6 +41,10 @@ pub enum ImportKind {
     /// variants' wire tags must never shift — an old worker receiving this fails the decode and
     /// the import-fail machinery handles it like any other worker error.
     Tiled444,
+    /// LINEAR dmabuf → Vulkan-bridge compute CSC → two-plane NV12 CUDA buffer (latency plan
+    /// T2.5b — the gamescope analogue of [`TiledNv12`](Self::TiledNv12)). Appended last, same
+    /// wire-tag rule as [`Tiled444`](Self::Tiled444).
+    LinearNv12,
 }
 
 /// host → worker.

crates/pf-zerocopy/src/imp/rgb2nv12_buf.comp

@@ -0,0 +1,77 @@
+#version 450
+// LINEAR BGRx dmabuf bytes -> NV12 (BT.709 limited range), BUFFER to BUFFER — the Vulkan-bridge
+// CSC for the gamescope path (latency plan T2.5b). The bridge deals in VkBuffers (NVIDIA's EGL
+// can't sample LINEAR and CUDA rejects raw dmabuf fds), so unlike `pf-encode`'s image-based
+// `rgb2yuv.comp` sibling this reads packed texels straight out of the imported dmabuf buffer and
+// writes the two NV12 planes into the exportable buffer CUDA reads. Same BT.709 coefficients as
+// the sibling shader — keep the two in sync.
+//
+// One invocation converts a 4x2 luma block: 4 Y bytes = exactly one u32 word per row, and its
+// two chroma samples = one u32 word — so every store is a whole word and no two invocations
+// touch the same word (an 8-bit-storage-free way to write byte planes race-free). All pitches
+// and the UV offset are in WORDS and must be word-aligned (the Rust side sizes them so).
+//
+// Rebuild: glslc rgb2nv12_buf.comp -o rgb2nv12_buf.spv
+layout(local_size_x = 8, local_size_y = 8) in;
+
+layout(std430, binding = 0) readonly buffer Src {
+    uint spx[]; // packed 4-byte texels, byte order B,G,R,X (little-endian XRGB8888/ARGB8888)
+};
+layout(std430, binding = 1) writeonly buffer Dst {
+    uint dw[]; // [0, uv_off_w): Y rows at y_pitch_w; [uv_off_w, ...): interleaved UV rows
+};
+
+layout(push_constant) uniform Push {
+    uint width;       // source pixels
+    uint height;
+    uint src_off_w;   // dmabuf plane offset, in words (offset bytes / 4)
+    uint src_pitch_w; // dmabuf row stride, in words
+    uint y_pitch_w;   // dst Y row pitch, in words
+    uint uv_off_w;    // dst UV plane offset, in words
+    uint uv_pitch_w;  // dst UV row pitch, in words
+} pc;
+
+// Edge-clamped fetch: alignment padding (x >= width from the 4-wide block, odd-height chroma)
+// duplicates the last real texel instead of reading out of bounds.
+vec3 texel(uint x, uint y) {
+    x = min(x, pc.width - 1u);
+    y = min(y, pc.height - 1u);
+    uint t = spx[pc.src_off_w + y * pc.src_pitch_w + x];
+    // B,G,R,X byte order: r = bits 16..23, g = 8..15, b = 0..7.
+    return vec3(float((t >> 16) & 0xFFu), float((t >> 8) & 0xFFu), float(t & 0xFFu)) / 255.0;
+}
+
+// BT.709 limited range — numerically identical to rgb2yuv.comp's lumaY/U/V, scaled to bytes.
+uint lumaB(vec3 c) {
+    return uint(clamp(16.0 + 255.0 * (0.1826 * c.r + 0.6142 * c.g + 0.0620 * c.b) + 0.5, 0.0, 255.0));
+}
+
+void main() {
+    uint bx = gl_GlobalInvocationID.x; // 4-px column block
+    uint by = gl_GlobalInvocationID.y; // 2-row block
+    uint x0 = bx * 4u;
+    uint y0 = by * 2u;
+    if (x0 >= pc.width || y0 >= pc.height) {
+        return;
+    }
+    // Two Y words (4 bytes each, rows y0 and y0+1).
+    for (uint row = 0u; row < 2u; row++) {
+        uint y = y0 + row;
+        uint w = lumaB(texel(x0, y))
+            | (lumaB(texel(x0 + 1u, y)) << 8)
+            | (lumaB(texel(x0 + 2u, y)) << 16)
+            | (lumaB(texel(x0 + 3u, y)) << 24);
+        dw[y * pc.y_pitch_w + bx] = w;
+    }
+    // One UV word: the block's two chroma samples (each a 2x2 average).
+    uint uvw = 0u;
+    for (uint s = 0u; s < 2u; s++) {
+        uint cx = x0 + s * 2u;
+        vec3 a = (texel(cx, y0) + texel(cx + 1u, y0) + texel(cx, y0 + 1u) + texel(cx + 1u, y0 + 1u))
+            * 0.25;
+        uint U = uint(clamp(128.0 + 255.0 * (-0.1006 * a.r - 0.3386 * a.g + 0.4392 * a.b) + 0.5, 0.0, 255.0));
+        uint V = uint(clamp(128.0 + 255.0 * (0.4392 * a.r - 0.3989 * a.g - 0.0403 * a.b) + 0.5, 0.0, 255.0));
+        uvw |= (U | (V << 8)) << (16u * s);
+    }
+    dw[pc.uv_off_w + by * pc.uv_pitch_w + bx] = uvw;
+}

crates/pf-zerocopy/src/imp/vulkan.rs

@@ -40,6 +40,20 @@ struct DstBuf {
     cuda: cuda::ExternalDmabuf,
 }
 
+/// The lazy compute-CSC pipeline (`rgb2nv12_buf.comp`) for [`VkBridge::import_linear_nv12`].
+struct Csc {
+    module: vk::ShaderModule,
+    dset_layout: vk::DescriptorSetLayout,
+    playout: vk::PipelineLayout,
+    pipeline: vk::Pipeline,
+    dpool: vk::DescriptorPool,
+    dset: vk::DescriptorSet,
+}
+
+/// The buffer-to-buffer RGB→NV12 compute shader (see `rgb2nv12_buf.comp` beside this file;
+/// rebuild with `glslc rgb2nv12_buf.comp -o rgb2nv12_buf.spv`).
+const CSC_SPV: &[u8] = include_bytes!("rgb2nv12_buf.spv");
+
 pub struct VkBridge {
     _entry: ash::Entry,
     instance: ash::Instance,
@@ -52,6 +66,9 @@ pub struct VkBridge {
     mem_props: vk::PhysicalDeviceMemoryProperties,
     src_cache: HashMap<i32, SrcBuf>,
     dst: Option<DstBuf>,
+    /// Built on the first [`import_linear_nv12`](Self::import_linear_nv12); RGB-only bridges
+    /// never pay for it.
+    csc: Option<Csc>,
 }
 
 // SAFETY: `VkBridge` owns ash Vulkan handles (instance/device/queue/command pool+buffer/fence), a
@@ -94,18 +111,15 @@ impl VkBridge {
                 .ok_or_else(|| anyhow!("no NVIDIA Vulkan device"))?;
             let mem_props = instance.get_physical_device_memory_properties(phys);
 
-            // Any queue family supporting transfer (graphics/compute imply it).
+            // A COMPUTE-capable family (compute implies transfer): the copy path only needs
+            // transfer, but the NV12 CSC dispatch (T2.5b) needs compute — on every NVIDIA
+            // device family 0 is graphics+compute+transfer, so this picks the same family the
+            // old transfer-only predicate did.
             let qf = instance
                 .get_physical_device_queue_family_properties(phys)
                 .iter()
-                .position(|q| {
-                    q.queue_flags.intersects(
-                        vk::QueueFlags::TRANSFER
-                            | vk::QueueFlags::GRAPHICS
-                            | vk::QueueFlags::COMPUTE,
-                    )
-                })
-                .ok_or_else(|| anyhow!("no transfer-capable queue family"))?
+                .position(|q| q.queue_flags.contains(vk::QueueFlags::COMPUTE))
+                .ok_or_else(|| anyhow!("no compute-capable queue family"))?
                 as u32;
 
             let exts = [
@@ -161,6 +175,7 @@ impl VkBridge {
                 mem_props,
                 src_cache: HashMap::new(),
                 dst: None,
+                csc: None,
             })
         }
     }
@@ -189,7 +204,11 @@ impl VkBridge {
             .create_buffer(
                 &vk::BufferCreateInfo::default()
                     .size(size)
-                    .usage(vk::BufferUsageFlags::TRANSFER_SRC)
+                    // STORAGE so the NV12 compute CSC can read it as an SSBO (T2.5b); harmless
+                    // for the plain copy path.
+                    .usage(
+                        vk::BufferUsageFlags::TRANSFER_SRC | vk::BufferUsageFlags::STORAGE_BUFFER,
+                    )
                     .push_next(&mut ext_info),
                 None,
             )
@@ -256,7 +275,10 @@ impl VkBridge {
             .create_buffer(
                 &vk::BufferCreateInfo::default()
                     .size(size)
-                    .usage(vk::BufferUsageFlags::TRANSFER_DST)
+                    // STORAGE so the NV12 compute CSC can write it as an SSBO (T2.5b).
+                    .usage(
+                        vk::BufferUsageFlags::TRANSFER_DST | vk::BufferUsageFlags::STORAGE_BUFFER,
+                    )
                     .push_next(&mut ext_info),
                 None,
             )
@@ -302,6 +324,246 @@ impl VkBridge {
         Ok(())
     }
 
+    /// Build the RGB→NV12 compute pipeline once (T2.5b): two-SSBO descriptor set + a 28-byte
+    /// push-constant block matching `rgb2nv12_buf.comp`'s `Push`.
+    unsafe fn ensure_csc(&mut self) -> Result<()> {
+        if self.csc.is_some() {
+            return Ok(());
+        }
+        let words: Vec<u32> = CSC_SPV
+            .chunks_exact(4)
+            .map(|c| u32::from_le_bytes(c.try_into().unwrap()))
+            .collect();
+        let module = self
+            .device
+            .create_shader_module(&vk::ShaderModuleCreateInfo::default().code(&words), None)
+            .context("create CSC shader module")?;
+        let bindings = [
+            vk::DescriptorSetLayoutBinding::default()
+                .binding(0)
+                .descriptor_type(vk::DescriptorType::STORAGE_BUFFER)
+                .descriptor_count(1)
+                .stage_flags(vk::ShaderStageFlags::COMPUTE),
+            vk::DescriptorSetLayoutBinding::default()
+                .binding(1)
+                .descriptor_type(vk::DescriptorType::STORAGE_BUFFER)
+                .descriptor_count(1)
+                .stage_flags(vk::ShaderStageFlags::COMPUTE),
+        ];
+        let dset_layout = self
+            .device
+            .create_descriptor_set_layout(
+                &vk::DescriptorSetLayoutCreateInfo::default().bindings(&bindings),
+                None,
+            )
+            .context("create CSC dset layout")?;
+        let pc = [vk::PushConstantRange::default()
+            .stage_flags(vk::ShaderStageFlags::COMPUTE)
+            .size(28)];
+        let layouts = [dset_layout];
+        let playout = self
+            .device
+            .create_pipeline_layout(
+                &vk::PipelineLayoutCreateInfo::default()
+                    .set_layouts(&layouts)
+                    .push_constant_ranges(&pc),
+                None,
+            )
+            .context("create CSC pipeline layout")?;
+        let entry = c"main";
+        let stage = vk::PipelineShaderStageCreateInfo::default()
+            .stage(vk::ShaderStageFlags::COMPUTE)
+            .module(module)
+            .name(entry);
+        let pipeline = self
+            .device
+            .create_compute_pipelines(
+                vk::PipelineCache::null(),
+                &[vk::ComputePipelineCreateInfo::default()
+                    .stage(stage)
+                    .layout(playout)],
+                None,
+            )
+            .map_err(|(_, e)| anyhow!("create CSC pipeline: {e}"))?[0];
+        let sizes = [vk::DescriptorPoolSize::default()
+            .ty(vk::DescriptorType::STORAGE_BUFFER)
+            .descriptor_count(2)];
+        let dpool = self
+            .device
+            .create_descriptor_pool(
+                &vk::DescriptorPoolCreateInfo::default()
+                    .max_sets(1)
+                    .pool_sizes(&sizes),
+                None,
+            )
+            .context("create CSC descriptor pool")?;
+        let dset = self
+            .device
+            .allocate_descriptor_sets(
+                &vk::DescriptorSetAllocateInfo::default()
+                    .descriptor_pool(dpool)
+                    .set_layouts(&layouts),
+            )
+            .context("allocate CSC descriptor set")?[0];
+        self.csc = Some(Csc {
+            module,
+            dset_layout,
+            playout,
+            pipeline,
+            dpool,
+            dset,
+        });
+        tracing::info!("Vulkan-bridge NV12 compute CSC ready (LINEAR path feeds NVENC native YUV)");
+        Ok(())
+    }
+
+    /// Bridge one LINEAR dmabuf frame into a pooled NV12 CUDA buffer (latency plan T2.5b):
+    /// instead of the plain byte copy, the compute CSC reads the imported RGB texels and writes
+    /// both NV12 planes into the exportable buffer, so NVENC on the gamescope path encodes
+    /// native YUV (its internal RGB→YUV CSC on the contended SM disappears). `pool` must be an
+    /// NV12 pool ([`cuda::BufferPool::new_nv12`]).
+    pub fn import_linear_nv12(
+        &mut self,
+        fd: i32,
+        offset: u32,
+        stride: u32,
+        width: u32,
+        height: u32,
+        pool: &cuda::BufferPool,
+    ) -> Result<DeviceBuffer> {
+        anyhow::ensure!(
+            offset % 4 == 0 && stride % 4 == 0,
+            "LINEAR dmabuf offset/stride not word-aligned ({offset}/{stride})"
+        );
+        // Exportable-buffer NV12 layout the shader writes: 4-aligned Y pitch, UV plane (⌈h/2⌉
+        // rows at the same pitch) directly after the Y plane.
+        let y_pitch = (width as u64 + 3) & !3;
+        let uv_off = y_pitch * height as u64;
+        let dst_size = uv_off + y_pitch * height.div_ceil(2) as u64;
+        // SAFETY: same structure and proofs as `import_linear` — `fd` is the caller's live dmabuf
+        // (dup'd by `import_src`), sizes are checked (`import_src` asserts the fd covers
+        // `offset + stride*height`; `ensure_dst(dst_size)` makes the exportable buffer at least
+        // the shader's whole write range, whose last word is `dst_size - 4`). The descriptor
+        // update binds the live cached src buffer and the live dst buffer WHOLE_SIZE; every
+        // `*Info`/array is a local outliving its synchronous call; `cmd`/`queue`/`fence` are this
+        // bridge's own single-thread handles. The dispatch covers ⌈w/32⌉×⌈h/16⌉ groups of 8×8
+        // invocations, each writing only whole words inside the proven dst range (shader
+        // contract). The host `wait_for_fences` retires the compute pass (with a shader-write →
+        // memory barrier recorded before end) BEFORE CUDA reads the shared memory.
+        unsafe {
+            let span = offset as u64 + stride as u64 * height as u64;
+            if !self.src_cache.contains_key(&fd) {
+                let size = libc::lseek(fd, 0, libc::SEEK_END);
+                anyhow::ensure!(size > 0, "lseek(dmabuf)");
+                anyhow::ensure!(size as u64 >= span, "dmabuf smaller than frame span");
+                self.import_src(fd, size as u64)?;
+            }
+            let src_buffer = self.src_cache[&fd].buffer;
+            self.ensure_dst(dst_size)?;
+            self.ensure_csc()?;
+            let (dst_buffer, dst_cuda_ptr) = {
+                let d = self.dst.as_ref().unwrap();
+                (d.buffer, d.cuda.ptr)
+            };
+            let csc = self.csc.as_ref().unwrap();
+
+            let src_info = [vk::DescriptorBufferInfo::default()
+                .buffer(src_buffer)
+                .range(vk::WHOLE_SIZE)];
+            let dst_info = [vk::DescriptorBufferInfo::default()
+                .buffer(dst_buffer)
+                .range(vk::WHOLE_SIZE)];
+            let writes = [
+                vk::WriteDescriptorSet::default()
+                    .dst_set(csc.dset)
+                    .dst_binding(0)
+                    .descriptor_type(vk::DescriptorType::STORAGE_BUFFER)
+                    .buffer_info(&src_info),
+                vk::WriteDescriptorSet::default()
+                    .dst_set(csc.dset)
+                    .dst_binding(1)
+                    .descriptor_type(vk::DescriptorType::STORAGE_BUFFER)
+                    .buffer_info(&dst_info),
+            ];
+            self.device.update_descriptor_sets(&writes, &[]);
+
+            self.device
+                .begin_command_buffer(
+                    self.cmd,
+                    &vk::CommandBufferBeginInfo::default()
+                        .flags(vk::CommandBufferUsageFlags::ONE_TIME_SUBMIT),
+                )
+                .context("begin cmd")?;
+            self.device
+                .cmd_bind_pipeline(self.cmd, vk::PipelineBindPoint::COMPUTE, csc.pipeline);
+            self.device.cmd_bind_descriptor_sets(
+                self.cmd,
+                vk::PipelineBindPoint::COMPUTE,
+                csc.playout,
+                0,
+                &[csc.dset],
+                &[],
+            );
+            let push: [u32; 7] = [
+                width,
+                height,
+                offset / 4,
+                stride / 4,
+                (y_pitch / 4) as u32,
+                (uv_off / 4) as u32,
+                (y_pitch / 4) as u32,
+            ];
+            let push_bytes: &[u8] = std::slice::from_raw_parts(push.as_ptr().cast(), 28);
+            self.device.cmd_push_constants(
+                self.cmd,
+                csc.playout,
+                vk::ShaderStageFlags::COMPUTE,
+                0,
+                push_bytes,
+            );
+            self.device
+                .cmd_dispatch(self.cmd, width.div_ceil(32), height.div_ceil(16), 1);
+            // Make the shader writes available before the external (CUDA) read.
+            let barrier = vk::MemoryBarrier::default()
+                .src_access_mask(vk::AccessFlags::SHADER_WRITE)
+                .dst_access_mask(vk::AccessFlags::MEMORY_READ);
+            self.device.cmd_pipeline_barrier(
+                self.cmd,
+                vk::PipelineStageFlags::COMPUTE_SHADER,
+                vk::PipelineStageFlags::BOTTOM_OF_PIPE,
+                vk::DependencyFlags::empty(),
+                &[barrier],
+                &[],
+                &[],
+            );
+            self.device
+                .end_command_buffer(self.cmd)
+                .context("end cmd")?;
+            let cmds = [self.cmd];
+            let submit = vk::SubmitInfo::default().command_buffers(&cmds);
+            self.device
+                .queue_submit(self.queue, &[submit], self.fence)
+                .context("queue submit")?;
+            self.device
+                .wait_for_fences(&[self.fence], true, 1_000_000_000)
+                .context("fence wait")?;
+            self.device
+                .reset_fences(&[self.fence])
+                .context("reset fence")?;
+
+            // De-stride both NV12 planes from the CUDA view into a pooled two-plane buffer.
+            cuda::make_current()?;
+            let out = pool.get()?;
+            cuda::copy_pitched_nv12_to_buffer(
+                dst_cuda_ptr,
+                dst_cuda_ptr + uv_off,
+                y_pitch as usize,
+                &out,
+            )?;
+            Ok(out)
+        }
+    }
+
     /// Drop the cached import for `fd` (the PipeWire buffer it wrapped is gone — pool recycle /
     /// renegotiation — or the caller is about to store a different dmabuf under the same slot).
     /// Without this the cache could serve a stale imported buffer for a reused fd number, or
@@ -414,6 +676,14 @@ impl Drop for VkBridge {
                 self.device.destroy_buffer(d.buffer, None);
                 self.device.free_memory(d.memory, None);
             }
+            if let Some(c) = self.csc.take() {
+                self.device.destroy_pipeline(c.pipeline, None);
+                self.device.destroy_pipeline_layout(c.playout, None);
+                self.device.destroy_descriptor_pool(c.dpool, None); // frees `c.dset` with it
+                self.device
+                    .destroy_descriptor_set_layout(c.dset_layout, None);
+                self.device.destroy_shader_module(c.module, None);
+            }
             self.device.destroy_fence(self.fence, None);
             self.device.destroy_command_pool(self.cmd_pool, None);
             self.device.destroy_device(None);

crates/pf-zerocopy/src/imp/worker.rs

@@ -299,6 +299,9 @@ impl EglBackend {
                 req.modifier,
             )?,
             ImportKind::Linear => self.importer.import_linear(&plane, req.width, req.height)?,
+            ImportKind::LinearNv12 => self
+                .importer
+                .import_linear_nv12(&plane, req.width, req.height)?,
         };
         // Assign / look up the buffer's id and export its CUDA IPC identity on first delivery.
         cuda::make_current()?;