linux-kernel - [PATCH] x86/platform/uv/BAU: disable BAU on single hub configurations

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Date:   Tue, 18 Jul 2017 16:51:33 -0500
From:   Andrew Banman <abanman@....com>
To:     mingo@...hat.com
Cc:     tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        rja@....com, mike.travis@....com, Andrew Banman <abanman@....com>
Subject: [PATCH] x86/platform/uv/BAU: disable BAU on single hub configurations

The BAU confers no benefit to a UV system running with only one hub/socket.
Permanently disable the BAU driver if there are less than two hubs online
to avoid BAU overhead. We have observed failed boots on single-socket UV4
systems caused by BAU that are avoided with this patch.

Signed-off-by: Andrew Banman <abanman@....com>
Acked-by: Russ Anderson <rja@....com>
Acked-by: Mike Travis <mike.travis@....com>
---
 arch/x86/platform/uv/tlb_uv.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/platform/uv/tlb_uv.c b/arch/x86/platform/uv/tlb_uv.c
index 2511a28..88216cc 100644
--- a/arch/x86/platform/uv/tlb_uv.c
+++ b/arch/x86/platform/uv/tlb_uv.c
@@ -2251,6 +2251,12 @@ static int __init uv_bau_init(void)
 	}
 
 	nuvhubs = uv_num_possible_blades();
+	if (nuvhubs < 2) {
+		pr_crit("UV: BAU disabled - insufficient hub count\n");
+		set_bau_off();
+		nobau_perm = 1;
+		return 0;
+	}
 
 	uv_base_pnode = 0x7fffffff;
 	for (uvhub = 0; uvhub < nuvhubs; uvhub++) {
-- 
1.8.2.1