linux-kernel - [RFC PATCH v3 09/14] m68k: hp300: Handle timer counter overflow

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Date:   Sat, 24 Nov 2018 14:09:03 +1100
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Philip Blundell <philb@....org>
Cc:     Andreas Schwab <schwab@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        Stephen N Chivers <schivers@....com.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kars de Jong <jongk@...ux-m68k.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Michael Schmitz <schmitzmic@...il.com>,
        John Stultz <john.stultz@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH v3 09/14] m68k: hp300: Handle timer counter overflow

Because hp300_read_clk() never checks the timer interrupt flag it may
fail to notice that the timer has wrapped, allowing the clock to jump
backwards. This is not a new problem.

This is resolved by checking the interrupt flag and, if need be,
taking wrap-around into account. The interrupt handler clears the flag
when it eventually executes.

Signed-off-by: Finn Thain <fthain@...egraphics.com.au>
---
TODO: find a spare counter for the clocksource, rather than hanging
it off the HZ timer.
---
 arch/m68k/hp300/time.c | 44 +++++++++++++++++++++++++-----------------
 1 file changed, 26 insertions(+), 18 deletions(-)

diff --git a/arch/m68k/hp300/time.c b/arch/m68k/hp300/time.c
index 90982803a629..ce6b5fc8d4a9 100644
--- a/arch/m68k/hp300/time.c
+++ b/arch/m68k/hp300/time.c
@@ -41,9 +41,12 @@ static u32 clk_total;
 #define	CLKCR3		CLKCR1
 #define	CLKSR		CLKCR2
 #define	CLKMSB1		0x5
+#define	CLKLSB1		0x7
 #define	CLKMSB2		0x9
 #define	CLKMSB3		0xD
 
+#define	CLKSR_INT1	BIT(0)
+
 /* This is for machines which generate the exact clock. */
 
 #define HP300_TIMER_CLOCK_FREQ 250000
@@ -70,24 +73,29 @@ static irqreturn_t hp300_tick(int irq, void *dev_id)
 
 static u64 hp300_read_clk(struct clocksource *cs)
 {
-  unsigned long flags;
-  unsigned char lsb, msb1, msb2;
-  u32 ticks;
-
-  local_irq_save(flags);
-  /* Read current timer 1 value */
-  msb1 = in_8(CLOCKBASE + 5);
-  lsb = in_8(CLOCKBASE + 7);
-  msb2 = in_8(CLOCKBASE + 5);
-  if (msb1 != msb2)
-    /* A carry happened while we were reading.  Read it again */
-    lsb = in_8(CLOCKBASE + 7);
-
-  ticks = INTVAL - ((msb2 << 8) | lsb);
-  ticks += clk_total;
-  local_irq_restore(flags);
-
-  return ticks;
+	unsigned long flags;
+	unsigned char lsb, msb, msb_new;
+	u16 offset = 0;
+	u32 ticks;
+
+	local_irq_save(flags);
+	/* Read current timer 1 value */
+	msb = in_8(CLOCKBASE + CLKMSB1);
+again:
+	if ((in_8(CLOCKBASE + CLKSR) & CLKSR_INT1) && msb > 0)
+		offset = INTVAL;
+	lsb = in_8(CLOCKBASE + CLKLSB1);
+	msb_new = in_8(CLOCKBASE + CLKMSB1);
+	if (msb_new != msb) {
+		msb = msb_new;
+		goto again;
+	}
+
+	ticks = INTVAL - ((msb << 8) | lsb);
+	ticks += offset + clk_total;
+	local_irq_restore(flags);
+
+	return ticks;
 }
 
 void __init hp300_sched_init(irq_handler_t vector)
-- 
2.18.1