[PATCH] x86/apic: Clarify i82489DX bit overlap in APIC_LVT0

From: Thomas Gleixner <tglx@linutronix.de>
To: Daniel Vacek <neelx@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, open list <linux-kernel@vger.kernel.org>
Subject: [PATCH] x86/apic: Clarify i82489DX bit overlap in APIC_LVT0
Date: Tue, 12 Apr 2022 22:34:21 +0200	[thread overview]
Message-ID: <87ee22f3ci.ffs@tglx> (raw)
In-Reply-To: <CACjP9X_A7aLmvypyOz1UrXM571gx_X5q7=w-1j+G+MSbCteiEw@mail.gmail.com>

Daniel stumbled over the undocumented bit overlap of the i82498DX external
APIC and the TSC deadline timer configuration bit in modern APICs.

Remove the i82489DX macro maze, use a i82489DX specific define in the apic
code and document the overlap in a comment.

Reported-by: Daniel Vacek <neelx@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 arch/x86/include/asm/apicdef.h |    6 ------
 arch/x86/kernel/apic/apic.c    |   11 ++++++++++-
 2 files changed, 10 insertions(+), 7 deletions(-)

--- a/arch/x86/include/asm/apicdef.h
+++ b/arch/x86/include/asm/apicdef.h
@@ -95,12 +95,6 @@
 #define	APIC_LVTTHMR	0x330
 #define	APIC_LVTPC	0x340
 #define	APIC_LVT0	0x350
-#define		APIC_LVT_TIMER_BASE_MASK	(0x3 << 18)
-#define		GET_APIC_TIMER_BASE(x)		(((x) >> 18) & 0x3)
-#define		SET_APIC_TIMER_BASE(x)		(((x) << 18))
-#define		APIC_TIMER_BASE_CLKIN		0x0
-#define		APIC_TIMER_BASE_TMBASE		0x1
-#define		APIC_TIMER_BASE_DIV		0x2
 #define		APIC_LVT_TIMER_ONESHOT		(0 << 17)
 #define		APIC_LVT_TIMER_PERIODIC		(1 << 17)
 #define		APIC_LVT_TIMER_TSCDEADLINE	(2 << 17)
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -320,6 +320,9 @@ int lapic_get_maxlvt(void)
 #define APIC_DIVISOR 16
 #define TSC_DIVISOR  8
 
+/* i82489DX specific */
+#define		I82489DX_BASE_DIVIDER		(((0x2) << 18))
+
 /*
  * This function sets up the local APIC timer, with a timeout of
  * 'clocks' APIC bus clock. During calibration we actually call
@@ -340,8 +343,14 @@ static void __setup_APIC_LVTT(unsigned i
 	else if (boot_cpu_has(X86_FEATURE_TSC_DEADLINE_TIMER))
 		lvtt_value |= APIC_LVT_TIMER_TSCDEADLINE;
 
+	/*
+	 * The i82489DX APIC uses bit 18 and 19 for the base divider.  This
+	 * overlaps with bit 18 on integrated APICs, but is not documented
+	 * in the SDM. No problem though. i82489DX equipped systems do not
+	 * have TSC deadline timer.
+	 */
 	if (!lapic_is_integrated())
-		lvtt_value |= SET_APIC_TIMER_BASE(APIC_TIMER_BASE_DIV);
+		lvtt_value |= I82489DX_BASE_DIVIDER;
 
 	if (!irqen)
 		lvtt_value |= APIC_LVT_MASKED;
