linux-kernel - Re: [PATCH] [RFC] apic: fix timer base macro definitions

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Date:   Wed, 06 Apr 2022 13:47:21 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Daniel Vacek <neelx@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RFC] apic: fix timer base macro definitions

Daniel,

On Wed, Feb 02 2022 at 15:02, Daniel Vacek wrote:
> I was wondering if the aliasing of APIC_TIMER_BASE_TMBASE and
> APIC_LVT_TIMER_TSCDEADLINE was intentional or we need to << 19?

That's intentional. This is only used for the !lapic_is_integrated()
case, which is the ancient i82489DX.

Something like the below should make this more clear.

Thanks,

        tglx
---
--- 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;