apple: program the NHI interrupt throttle via ring->interval_nsec

flake.nix

@@ -120,6 +120,7 @@
               tools/ci/distro-package.sh \
               tools/ci/vm-guest-smoke.sh \
               tools/ci/vm-smoke.sh \
+              tools/ci/kernel-compat-probe-test.sh \
               userspace/bench/tbv_vllm_smoke.sh
             python -m py_compile \
               userspace/bench/tbv_perftest_runner.py \
@@ -134,6 +135,43 @@
             runHook postInstall
           '';
 
+          meta = {
+            maintainers = with pkgs.lib.maintainers; [ georgewhewell ];
+          };
+        };
+      # The kernel compat probe decides whether to compile code against a
+      # struct member only some kernels have. It once failed OPEN -- a missing
+      # header made it claim the member was present -- and broke the build on
+      # linux 6.18.37. This runs the probe against synthetic kernel trees so
+      # that regression cannot reach a builder again.
+      mkKernelCompatProbeCheck =
+        pkgs:
+        pkgs.stdenv.mkDerivation {
+          pname = "thunderbolt-ibverbs-kernel-compat-probe";
+          version = "0.3.4";
+          src = ./.;
+
+          nativeBuildInputs = [
+            pkgs.bash
+            pkgs.gnumake
+            pkgs.gnugrep
+            pkgs.gnused
+          ];
+
+          dontConfigure = true;
+
+          buildPhase = ''
+            runHook preBuild
+            bash tools/ci/kernel-compat-probe-test.sh "$PWD/kernel/Makefile"
+            runHook postBuild
+          '';
+
+          installPhase = ''
+            runHook preInstall
+            mkdir -p "$out"
+            runHook postInstall
+          '';
+
           meta = {
             maintainers = with pkgs.lib.maintainers; [ georgewhewell ];
           };
@@ -426,6 +464,7 @@
           thunderbolt-ibverbs = pkgsAt.thunderbolt-ibverbs;
           portable-kernel-patches = mkPortableKernelPatchCheck pkgs;
           script-syntax = mkScriptSyntaxCheck pkgs;
+          kernel-compat-probe = mkKernelCompatProbeCheck pkgs;
           proto-smoke = mkProtoSmoke pkgs;
           rdma-core-usb4 = pkgsAt.rdma-core-usb4;
           verbs-smoke-build = mkVerbsSmokeBuild pkgs;

kernel/Makefile

@@ -11,7 +11,50 @@ thunderbolt_ibverbs-y := main.o profile.o backend.o configfs.o core.o debugfs.o
 
 ccflags-y := -Wall -Wextra -Wno-unused-parameter -I$(src) -I$(src)/..
 
-.PHONY: all clean modules modules_install help
+# COMPAT PROBE. The Asahi USB4 tree carries struct tb_ring.interval_nsec, which
+# stock kernels do not. Probe for it rather than requiring a patched kernel, so
+# one source builds against either.
+#
+# Two properties this probe must have, both of them paid for by a CI failure:
+#
+#  1. Look under BOTH $(KDIR)/include and $(KDIR)/source/include. A kernel
+#     packaged as a SPLIT build/source tree keeps only generated headers under
+#     build/ and the real ones under source/ -- which is what nixpkgs ships and
+#     what the Hydra builders run. Those two are SIBLINGS
+#     (/lib/modules/$(KVER)/{build,source}), though some packagings nest
+#     source/ inside build/, so both spellings are tried.
+#
+#  2. FAIL CLOSED. The previous form was
+#         ifneq ($(shell grep -c interval_nsec $(TBV_TB_HDR) 2>/dev/null),0)
+#     and grep on a missing file prints NOTHING, so $(shell ...) expanded to the
+#     empty string, "" != "0" was true, and the flag was defined precisely when
+#     the probe could not check anything. Against linux 6.18.37 that produced
+#     "error: 'struct tb_ring' has no member named 'interval_nsec'" -- a probe
+#     whose failure mode was to assert the thing it exists to doubt.
+#     grep -l prints a filename on a match and nothing otherwise, so a missing
+#     or unreadable header can only ever yield "absent".
+#
+# The pattern matches a member DECLARATION (identifier followed by ; , or :)
+# rather than the bare string, so prose in a comment cannot enable the code.
+TBV_TB_HDR := $(firstword $(wildcard $(KDIR)/include/linux/thunderbolt.h \
+                                     $(KDIR)/source/include/linux/thunderbolt.h \
+                                     $(KDIR)/../source/include/linux/thunderbolt.h))
+tbv-kernel-has = $(if $(TBV_TB_HDR),$(shell grep -lE '$(1)[[:space:]]*[;,:]' $(TBV_TB_HDR) 2>/dev/null))
+
+ifneq ($(call tbv-kernel-has,interval_nsec),)
+ccflags-y += -DTBV_HAVE_RING_INTERVAL_NSEC
+endif
+
+.PHONY: all clean modules modules_install help compat-report
+
+# Report what the COMPAT PROBE above concluded, without building anything.
+# tools/ci/kernel-compat-probe-test.sh drives this against synthetic kernel
+# trees; it is also the fastest way for a human to answer "why did this build
+# think my kernel had that member?".
+compat-report:
+	@echo "TBV_TB_HDR=$(TBV_TB_HDR)"
+	@echo "interval_nsec=$(if $(findstring TBV_HAVE_RING_INTERVAL_NSEC,$(ccflags-y)),yes,no)"
+
 
 all: modules
 

kernel/path.c

@@ -4,6 +4,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/jiffies.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/slab.h>
 #include <linux/string.h>
 #include <linux/thunderbolt.h>
@@ -1288,6 +1289,52 @@ int tbv_path_alloc_rings(struct tbv_path *path, struct tb_xdomain *xd,
 	return ret;
 }
 
+/*
+ * Apple NHI interrupt throttle (0xd004c, 256 ns units).
+ *
+ * The stock/Intel register (0x38c00) is programmed through
+ * tb_ring_throttling(), and MUST NOT be written on an Apple NHI -- the register
+ * lives elsewhere there. On Apple hardware the kernel's own ring activation
+ * writes it: apple_nhi_ring_interrupt_active() programs
+ * APPLE_CIO_NHI_IRQ_THROTTLE from ring->interval_nsec, and SKIPS the write
+ * when that field is zero. A zero module parameter therefore proves nothing
+ * was written, and the register keeps whatever it held -- 255 * 256 ns =
+ * 65.28 us on the machine this was measured on, visible as a latency floor
+ * that does not move with message size (2 B through 4 KB all ~65.3 us,
+ * ib_send_lat across a USB4 link).
+ *
+ * This module never set ring->interval_nsec, so on an Apple NHI the throttle
+ * was silently skipped forever. This applies to every ring on an Apple NHI --
+ * including native-backend rings toward a Linux peer -- because the NHI is
+ * Apple's regardless of who is on the other end of the cable.
+ */
+static bool tbv_host_is_apple(void)
+{
+	return of_machine_is_compatible("apple,arm-platform");
+}
+
+static void tbv_path_apply_ring_interval(struct tbv_path *path)
+{
+#ifdef TBV_HAVE_RING_INTERVAL_NSEC
+	unsigned int interval = READ_ONCE(nhi_interrupt_throttle_ns);
+	u32 before, after;
+
+	if (!interval || !tbv_host_is_apple())
+		return;
+
+	if (!path->tx_ring || !path->rx_ring)
+		return;
+
+	before = path->tx_ring->interval_nsec;
+	WRITE_ONCE(path->tx_ring->interval_nsec, interval);
+	WRITE_ONCE(path->rx_ring->interval_nsec, interval);
+	after = path->tx_ring->interval_nsec;
+
+	pr_info("apple ring throttle: interval_nsec %u -> %u ns (kernel programs it at activation)\n",
+		before, after);
+#endif
+}
+
 int tbv_path_start_rings(struct tbv_path *path)
 {
 	u32 i;
@@ -1296,6 +1343,10 @@ int tbv_path_start_rings(struct tbv_path *path)
 	if (path->state != TBV_PATH_RING_ALLOCATED)
 		return -EINVAL;
 
+	/* Must be set before activation: the kernel reads ring->interval_nsec
+	 * when it programs the Apple NHI throttle register. */
+	tbv_path_apply_ring_interval(path);
+
 	tb_ring_start(path->tx_ring);
 	tb_ring_start(path->rx_ring);
 	path->state = TBV_PATH_RING_STARTED;

tools/ci/kernel-compat-probe-test.sh

@@ -0,0 +1,81 @@
+#!/usr/bin/env bash
+#
+# Prove the COMPAT PROBE in kernel/Makefile behaves against synthetic kernel
+# trees, without needing a real kernel to build against.
+#
+# Why this test exists. The probe decides whether to compile the code that
+# writes struct tb_ring.interval_nsec, a member the Asahi USB4 tree carries and
+# stock kernels do not. It used to be:
+#
+#     ifneq ($(shell grep -c interval_nsec $(KDIR)/include/linux/thunderbolt.h),0)
+#
+# which has two defects that combine into a build failure:
+#
+#   * it looks only under $(KDIR)/include, and a kernel packaged as a SPLIT
+#     build/source tree (nixpkgs, and therefore the Hydra builders) keeps the
+#     real headers under $(KDIR)/source/include; and
+#   * grep on a missing file prints nothing, so $(shell ...) is "", "" != "0"
+#     is true, and the flag was defined precisely when the probe could not
+#     check -- a probe whose failure mode is to assert what it exists to doubt.
+#
+# Against linux 6.18.37 that produced:
+#     path.c: error: 'struct tb_ring' has no member named 'interval_nsec'
+#
+# The rule this encodes: a capability probe MUST fail closed.
+set -euo pipefail
+
+repo_root=$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)
+makefile=${1:-$repo_root/kernel/Makefile}
+
+work=$(mktemp -d)
+trap 'rm -rf "$work"' EXIT
+
+# $1 name  $2 layout(flat|split|nested|none)  $3 header body
+fixture() {
+  local name=$1 layout=$2 body=$3 inc
+  case $layout in
+    flat)  inc=$work/$name/include/linux ; kdir=$work/$name ;;
+    split) inc=$work/$name/source/include/linux ; kdir=$work/$name/build ;;
+    nested) inc=$work/$name/build/source/include/linux ; kdir=$work/$name/build ;;
+    none)  mkdir -p "$work/$name/build" ; echo "$work/$name/build" ; return ;;
+  esac
+  mkdir -p "$inc" "$kdir"
+  printf '%s\n' "$body" > "$inc/thunderbolt.h"
+  echo "$kdir"
+}
+
+probe() {
+  make -s -f "$makefile" KDIR="$1" compat-report \
+    | sed -n 's/^interval_nsec=//p'
+}
+
+fail=0
+expect() { # $1 description  $2 KDIR  $3 expected(yes|no)
+  local got; got=$(probe "$2")
+  if [ "$got" = "$3" ]; then
+    printf '  ok      %-46s -> %s\n' "$1" "$got"
+  else
+    printf '  FAIL    %-46s -> %s (want %s)\n' "$1" "$got" "$3"
+    fail=1
+  fi
+}
+
+HAS='struct tb_ring { int hop; unsigned int interval_nsec; };'
+HASNT='struct tb_ring { int hop; };'
+COMMENT='/* the kernel programs the throttle from ring->interval_nsec */
+struct tb_ring { int hop; };'
+
+echo "COMPAT PROBE matrix ($makefile)"
+expect "split tree, member absent (the Hydra case)" "$(fixture split-absent split "$HASNT")"  no
+expect "split tree, member present"                 "$(fixture split-has    split "$HAS")"    yes
+expect "nested source/ packaging, member present"    "$(fixture nested-has   nested "$HAS")"    yes
+expect "flat tree, member absent (stock kernel)"    "$(fixture flat-absent  flat  "$HASNT")"  no
+expect "flat tree, member present (Asahi USB4)"     "$(fixture flat-has     flat  "$HAS")"    yes
+expect "header mentions it only in a comment"       "$(fixture comment-only flat  "$COMMENT")" no
+expect "no header at all (must fail CLOSED)"        "$(fixture missing      none  '')"        no
+
+if [ "$fail" -ne 0 ]; then
+  echo "kernel compat probe FAILED" >&2
+  exit 1
+fi
+echo "kernel compat probe: all cases correct"