fix(pcie): stop misreporting empty/vanished slots as speed degradation

pcie-link's retrain check compared post-retrain speed against a port's
max capability without checking whether anything was actually seated
downstream. A switch downstream port with no card plugged in retrains
to zero lanes but still reports a reset-state speed, so it was
misreported as "retrained to Gen1, capable of Gen5" — indistinguishable
from a real degraded link.

Now a port that was already empty before the retrain is skipped (same
treatment as a disabled device: legitimately unpopulated slots are
common and shouldn't fail the check), while a device that answered
before the retrain and disappeared right after it still fails, with an
honest "no device detected downstream" message instead of a fabricated
speed reading.

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

audit/internal/platform/pcie_link_check.go

@@ -35,6 +35,7 @@ type pcieLinkFinding struct {
 	Width       int
 	MaxWidth    int
 	Degraded    bool
+	NotPresent  bool // true when the slot trained to zero lanes: nothing is plugged in (or it fell off the bus), not a speed regression
 }
 
 // RunPCIeLinkCheckPack forces every enabled PCIe device to retrain its link
@@ -139,6 +140,20 @@ func retrainAndSamplePCIeDevice(ctx context.Context, verboseLog, bdf string, log
 		return f
 	}
 
+	if width == 0 {
+		// A downstream switch/root port with nothing seated reads zero
+		// trained lanes even before we touch it. Plenty of legitimate
+		// configs leave slots like this unpopulated (not every server ships
+		// every NIC/riser slot filled), so this is not by itself evidence
+		// of anything wrong — retraining an empty slot can't produce a
+		// meaningful speed reading, and there's no baseline here to say
+		// "this used to have a card." Skip it exactly like a disabled
+		// device: nothing to verify, no reason to fail the run over it.
+		f.NotPresent = true
+		f.Skipped = "no device present downstream (empty slot/riser — nothing to retrain)"
+		return f
+	}
+
 	if err := retrainPCIeLink(ctx, verboseLog, bdf, logFunc); err != nil {
 		f.Skipped = "retrain failed: " + err.Error()
 		f.AfterSpeed = before
@@ -151,6 +166,17 @@ func retrainAndSamplePCIeDevice(ctx context.Context, verboseLog, bdf string, log
 
 	after, _ := readPCIeSysfsString(bdf, "current_link_speed")
 	widthAfter, _ := readPCIeSysfsInt(bdf, "current_link_width")
+	if widthAfter == 0 {
+		// The device answered before the retrain but is gone immediately
+		// after it (fell off the bus mid-check) — unlike the pre-retrain
+		// case above, this had a live link a moment ago, so it's worth
+		// surfacing rather than silently skipping.
+		f.AfterSpeed = after
+		f.Width = widthAfter
+		f.NotPresent = true
+		f.Degraded = true
+		return f
+	}
 	f.AfterSpeed = after
 	f.Width = widthAfter
 	f.Degraded = after != maxSpeed
@@ -315,11 +341,17 @@ func renderPCIeLinkCheckSummary(findings []pcieLinkFinding) string {
 		fmt.Fprintln(&b, "overall_status=FAILED")
 		var reasons []string
 		for _, f := range findings {
-			if f.Degraded {
+			if !f.Degraded {
+				continue
+			}
+			if f.NotPresent {
+				reasons = append(reasons, fmt.Sprintf("%s (%s): no device detected downstream (link down / empty slot or riser, capable of %s)",
+					f.BDF, nonEmptyOr(f.Description, "unknown device"), f.MaxSpeed))
+				continue
+			}
 			reasons = append(reasons, fmt.Sprintf("%s (%s): retrained to %s, capable of %s",
 				f.BDF, nonEmptyOr(f.Description, "unknown device"), f.AfterSpeed, f.MaxSpeed))
 		}
-		}
 		fmt.Fprintf(&b, "warnings=%s\n", strings.Join(reasons, "; "))
 	} else {
 		fmt.Fprintln(&b, "overall_status=OK")
@@ -340,7 +372,10 @@ func renderPCIeLinkCheckReport(findings []pcieLinkFinding) string {
 			continue
 		}
 		verdict := "OK"
-		if f.Degraded {
+		switch {
+		case f.NotPresent:
+			verdict = "FELL OFF BUS"
+		case f.Degraded:
 			verdict = "DEGRADED"
 		}
 		fmt.Fprintf(&b, "    %s: before=%s after=%s max=%s width=%d/%d\n",

audit/internal/platform/pcie_link_check_test.go

@@ -64,3 +64,51 @@ func TestRenderPCIeLinkCheckSummaryAllClean(t *testing.T) {
 		t.Fatalf("disabled/skipped-only device should not produce an other_status line, got:\n%s", summary)
 	}
 }
+
+func TestRenderPCIeLinkCheckSummaryEmptySlotDoesNotFail(t *testing.T) {
+	// An unpopulated switch downstream port (no card ever seated) is a
+	// normal, common configuration — it must not fail the SAT the way a
+	// genuinely degraded link does.
+	findings := []pcieLinkFinding{
+		{BDF: "0000:0d:00.0", IsGPU: true, GPUVendor: "nvidia", BeforeSpeed: "Gen5", AfterSpeed: "Gen5", MaxSpeed: "Gen5"},
+		{BDF: "0000:2a:00.0", Description: "PEX890xx PCIe Gen 5 Switch",
+			NotPresent: true, Skipped: "no device present downstream (empty slot/riser — nothing to retrain)"},
+	}
+	summary := renderPCIeLinkCheckSummary(findings)
+	if !strings.Contains(summary, "overall_status=OK") {
+		t.Fatalf("expected overall_status=OK for an empty slot, got:\n%s", summary)
+	}
+	if strings.Contains(summary, "other_status=") {
+		t.Fatalf("empty-slot-only device should not produce an other_status line, got:\n%s", summary)
+	}
+
+	report := renderPCIeLinkCheckReport(findings)
+	if !strings.Contains(report, "skipped: no device present downstream") {
+		t.Fatalf("expected empty slot to be reported as skipped, not degraded, got:\n%s", report)
+	}
+	if strings.Contains(report, "retrained to") {
+		t.Fatalf("empty slot must not be reported with a fabricated retrain speed, got:\n%s", report)
+	}
+}
+
+func TestRenderPCIeLinkCheckSummaryDeviceFellOffBusStillFails(t *testing.T) {
+	// A device that had a live link before the retrain and is gone right
+	// after it is a real regression, distinct from a slot that was never
+	// populated — this must still fail the check.
+	findings := []pcieLinkFinding{
+		{BDF: "0000:ab:00.0", Description: "PEX890xx PCIe Gen 5 Switch",
+			BeforeSpeed: "Gen5", AfterSpeed: "Gen5", MaxSpeed: "Gen5", Width: 0, NotPresent: true, Degraded: true},
+	}
+	summary := renderPCIeLinkCheckSummary(findings)
+	if !strings.Contains(summary, "overall_status=FAILED") {
+		t.Fatalf("expected overall_status=FAILED when a device disappears after retrain, got:\n%s", summary)
+	}
+	if !strings.Contains(summary, "no device detected downstream") {
+		t.Fatalf("expected an honest 'no device detected' reason, not a fabricated speed, got:\n%s", summary)
+	}
+
+	report := renderPCIeLinkCheckReport(findings)
+	if !strings.Contains(report, "FELL OFF BUS") {
+		t.Fatalf("expected FELL OFF BUS verdict in report, got:\n%s", report)
+	}
+}