From e5d60b297ed77b55c35171a950033bcacfc5faeb Mon Sep 17 00:00:00 2001
From: 81reap <prayag.bhakar@gmail.com>
Date: Thu, 15 Oct 2026 21:08:55 -0400
Subject: [PATCH] ucc: gate building NCCL on unsupported systems

NVIDIA does not support NCCL on pre-Thor Jetson devices (ie :: Jetson Orin and Jetson Xavier), so `cudaPackages.nccl` fails to evaluate and also fails downstream comusmers like `ucc` and `openmpi`.

Now we have an `enableNccl` flag that gates if NCCL is available, simmilar to how onnxruntime does. It is accessible via `--with-nccl`/`--without-nccl`.

Builds wihtout CUDA are unaffected.

Assisted-By: Claude-Code
---
 pkgs/by-name/uc/ucc/package.nix | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/pkgs/by-name/uc/ucc/package.nix b/pkgs/by-name/uc/ucc/package.nix
index 4e55618df4b3..23937dba5dc8 100644
--- a/pkgs/by-name/uc/ucc/package.nix
+++ b/pkgs/by-name/uc/ucc/package.nix
@@ -11,13 +11,14 @@ inputs@{
   # Configuration options
   enableAvx ? stdenv.hostPlatform.avxSupport,
   enableCuda ? config.cudaSupport,
+  enableNccl ? enableCuda && cudaPackages.nccl.meta.available,
   enableSse41 ? stdenv.hostPlatform.sse4_1Support,
   enableSse42 ? stdenv.hostPlatform.sse4_2Support,
 }:
 let
   inherit (lib.attrsets) getOutput;
   inherit (lib.lists) optionals;
-  inherit (lib.strings) concatStringsSep;
+  inherit (lib.strings) concatStringsSep withFeature;
 
   inherit (cudaPackages)
     cccl
@@ -80,8 +81,8 @@ effectiveStdenv.mkDerivation (finalAttrs: {
     cccl
     cuda_cudart
     cuda_nvml_dev
-    nccl
-  ];
+  ]
+  ++ optionals enableNccl [ nccl ];
 
   # NOTE: With `__structuredAttrs` enabled, `LDFLAGS` must be set under `env` so it is assured to be a string;
   # otherwise, we might have forgotten to convert it to a string and Nix would make LDFLAGS a shell variable
@@ -106,6 +107,7 @@ effectiveStdenv.mkDerivation (finalAttrs: {
     ++ optionals enableCuda [
       "--with-cuda=${cuda_nvcc}"
       "--with-nvcc-gencode=${concatStringsSep " " flags.gencode}"
+      (withFeature enableNccl "nccl")
     ];
 
   postInstall = ''