linux-kernel - [PATCH 2/2 v4] MIPS: Ingenic: Disable broken BTB lookup optimization.

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Message-Id: <1564734457-48801-3-git-send-email-zhouyanjie@zoho.com>
Date:   Fri,  2 Aug 2019 16:27:37 +0800
From:   Zhou Yanjie <zhouyanjie@...o.com>
To:     linux-mips@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, ralf@...ux-mips.org,
        paul@...pouillou.net, paul.burton@...s.com, jhogan@...nel.org,
        malat@...ian.org, gregkh@...uxfoundation.org, tglx@...utronix.de,
        allison@...utok.net, syq@...ian.org, chenhc@...ote.com,
        jiaxun.yang@...goat.com
Subject: [PATCH 2/2 v4] MIPS: Ingenic: Disable broken BTB lookup optimization.

In order to further reduce power consumption, the XBurst core
by default attempts to avoid branch target buffer lookups by
detecting & special casing loops. This feature will cause
BogoMIPS and lpj calculate in error. Set cp0 config7 bit 4 to
disable this feature.

Signed-off-by: Zhou Yanjie <zhouyanjie@...o.com>
---
 arch/mips/include/asm/mipsregs.h | 4 ++++
 arch/mips/kernel/cpu-probe.c     | 7 +++++++
 2 files changed, 11 insertions(+)

diff --git a/arch/mips/include/asm/mipsregs.h b/arch/mips/include/asm/mipsregs.h
index 1e6966e..bdbdc19 100644
--- a/arch/mips/include/asm/mipsregs.h
+++ b/arch/mips/include/asm/mipsregs.h
@@ -689,6 +689,9 @@
 #define MIPS_CONF7_IAR		(_ULCAST_(1) << 10)
 #define MIPS_CONF7_AR		(_ULCAST_(1) << 16)
 
+/* Ingenic Config7 bits */
+#define MIPS_CONF7_BTB_LOOP_EN	(_ULCAST_(1) << 4)
+
 /* Config7 Bits specific to MIPS Technologies. */
 
 /* Performance counters implemented Per TC */
@@ -2813,6 +2816,7 @@ __BUILD_SET_C0(status)
 __BUILD_SET_C0(cause)
 __BUILD_SET_C0(config)
 __BUILD_SET_C0(config5)
+__BUILD_SET_C0(config7)
 __BUILD_SET_C0(intcontrol)
 __BUILD_SET_C0(intctl)
 __BUILD_SET_C0(srsmap)
diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
index eb527a1..2bdd3e1 100644
--- a/arch/mips/kernel/cpu-probe.c
+++ b/arch/mips/kernel/cpu-probe.c
@@ -1964,6 +1964,13 @@ static inline void cpu_probe_ingenic(struct cpuinfo_mips *c, unsigned int cpu)
 		c->cputype = CPU_XBURST;
 		c->writecombine = _CACHE_UNCACHED_ACCELERATED;
 		__cpu_name[cpu] = "Ingenic XBurst";
+		/*
+		 * The XBurst core by default attempts to avoid branch target
+		 * buffer lookups by detecting & special casing loops. This
+		 * feature will cause BogoMIPS and lpj calculate in error.
+		 * Set cp0 config7 bit 4 to disable this feature.
+		 */
+		set_c0_config7(MIPS_CONF7_BTB_LOOP_EN);
 		break;
 	default:
 		panic("Unknown Ingenic Processor ID!");
-- 
2.7.4