[bugfix]: build fastvideo-kernel on GPU-less Docker runners (#1437)

Author: SolitaryThinker

docker/Dockerfile.python3.10

@@ -55,12 +55,14 @@ RUN source $HOME/.local/bin/env && \
     echo 'source /opt/venv/bin/activate' >> /root/.bashrc && \
     echo 'if [ -n "$ZSH_VERSION" ] && [ -f ~/.zshrc ]; then . ~/.zshrc; elif [ -f ~/.bashrc ]; then . ~/.bashrc; fi' > /root/.profile
 
-# Install FastVideo Unified Kernel
+# Install FastVideo Unified Kernel.
+# This build machine has no GPU, so target Hopper (sm_90a) explicitly instead
+# of probing a live device for the arch (matches the released kernel wheel).
 RUN source $HOME/.local/bin/env && \
     source /opt/venv/bin/activate && \
     cd fastvideo-kernel && \
     git submodule update --init --recursive && \
-    ./build.sh
+    TORCH_CUDA_ARCH_LIST=9.0a ./build.sh
 
 
 EXPOSE 22

docker/Dockerfile.python3.11

@@ -55,12 +55,14 @@ RUN source $HOME/.local/bin/env && \
     echo 'source /opt/venv/bin/activate' >> /root/.bashrc && \
     echo 'if [ -n "$ZSH_VERSION" ] && [ -f ~/.zshrc ]; then . ~/.zshrc; elif [ -f ~/.bashrc ]; then . ~/.bashrc; fi' > /root/.profile
 
-# Install FastVideo Unified Kernel
+# Install FastVideo Unified Kernel.
+# This build machine has no GPU, so target Hopper (sm_90a) explicitly instead
+# of probing a live device for the arch (matches the released kernel wheel).
 RUN source $HOME/.local/bin/env && \
     source /opt/venv/bin/activate && \
     cd fastvideo-kernel && \
     git submodule update --init --recursive && \
-    ./build.sh
+    TORCH_CUDA_ARCH_LIST=9.0a ./build.sh
 
 
 EXPOSE 22

docker/Dockerfile.python3.12

@@ -55,11 +55,13 @@ RUN source $HOME/.local/bin/env && \
     echo 'source /opt/venv/bin/activate' >> /root/.bashrc && \
     echo 'if [ -n "$ZSH_VERSION" ] && [ -f ~/.zshrc ]; then . ~/.zshrc; elif [ -f ~/.bashrc ]; then . ~/.bashrc; fi' > /root/.profile
 
-# Install FastVideo Unified Kernel
+# Install FastVideo Unified Kernel.
+# This build machine has no GPU, so target Hopper (sm_90a) explicitly instead
+# of probing a live device for the arch (matches the released kernel wheel).
 RUN source $HOME/.local/bin/env && \
     source /opt/venv/bin/activate && \
     cd fastvideo-kernel && \
     git submodule update --init --recursive && \
-    ./build.sh
+    TORCH_CUDA_ARCH_LIST=9.0a ./build.sh
 
 EXPOSE 22

docker/Dockerfile.python3.12.cuda12.9.1

@@ -55,12 +55,14 @@ RUN source $HOME/.local/bin/env && \
     echo 'source /opt/venv/bin/activate' >> /root/.bashrc && \
     echo 'if [ -n "$ZSH_VERSION" ] && [ -f ~/.zshrc ]; then . ~/.zshrc; elif [ -f ~/.bashrc ]; then . ~/.bashrc; fi' > /root/.profile
 
-# Install FastVideo Unified Kernel
+# Install FastVideo Unified Kernel.
+# This build machine has no GPU, so target Hopper (sm_90a) explicitly instead
+# of probing a live device for the arch (matches the released kernel wheel).
 RUN source $HOME/.local/bin/env && \
     source /opt/venv/bin/activate && \
     cd fastvideo-kernel && \
     git submodule update --init --recursive && \
-    ./build.sh
+    TORCH_CUDA_ARCH_LIST=9.0a ./build.sh
 
 
 EXPOSE 22

fastvideo-kernel/build.sh

@@ -78,16 +78,26 @@ print(f'{mj}.{mn}')"
 }
 
 if [ "${GPU_BACKEND}" = "CUDA" ]; then
-    detected_cc="$(detect_with_torch)" || {
-        echo "ERROR: torch-based CUDA arch detection failed in uv environment." >&2
-        echo "       Ensure torch is installed and CUDA is available in the uv-selected Python." >&2
-        exit 1
-    }
-
-    cc_major="${detected_cc%%.*}"
-    cc_minor="${detected_cc##*.}"
+    # Compute capability drives the arch/TK defaults below. Prefer an explicit
+    # TORCH_CUDA_ARCH_LIST (works on GPU-less build machines such as CI/Docker);
+    # only probe a live GPU via torch when no arch was provided.
+    if [ -n "${TORCH_CUDA_ARCH_LIST:-}" ]; then
+        echo "Using TORCH_CUDA_ARCH_LIST=${TORCH_CUDA_ARCH_LIST} (skipping torch GPU probe)"
+        first_arch="${TORCH_CUDA_ARCH_LIST%%[;, ]*}"   # first entry, e.g. 9.0a
+        first_arch="${first_arch%[af]}"                # strip trailing a/f suffix
+        cc_major="${first_arch%%.*}"
+        cc_minor="${first_arch##*.}"
+    else
+        detected_cc="$(detect_with_torch)" || {
+            echo "ERROR: torch-based CUDA arch detection failed and TORCH_CUDA_ARCH_LIST is unset." >&2
+            echo "       Set TORCH_CUDA_ARCH_LIST (e.g. 9.0a) for GPU-less builds, or build where CUDA is available." >&2
+            exit 1
+        }
+        cc_major="${detected_cc%%.*}"
+        cc_minor="${detected_cc##*.}"
+        echo "Detected compute capability via torch: ${detected_cc}"
+    fi
     cmake_arch="${cc_major}${cc_minor}"
-    echo "Detected compute capability via torch: ${detected_cc} (sm_${cmake_arch})"
 
     # Respect explicit overrides.
     if [ -z "${TORCH_CUDA_ARCH_LIST:-}" ]; then