[RFC linux dev-4.19] clocksource: fttmr010: Detect back-pressure for Aspeed controllers

[Andrew Jeffery]

Signed-off-by: Andrew Jeffery <andrew at aj.id.au>
---

See the cover letter on the corresponding qemu RFC series, to be sent after
this patch.

 arch/arm/mach-aspeed/Kconfig         | 1 +
 drivers/clocksource/timer-fttmr010.c | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/arch/arm/mach-aspeed/Kconfig b/arch/arm/mach-aspeed/Kconfig
index 2d5570e6e186..6b82d5416cf0 100644
--- a/arch/arm/mach-aspeed/Kconfig
+++ b/arch/arm/mach-aspeed/Kconfig
@@ -7,6 +7,7 @@ menuconfig ARCH_ASPEED
 	select FTTMR010_TIMER
 	select MFD_SYSCON
 	select PINCTRL
+	select GENERIC_CLOCKEVENTS_MIN_ADJUST
 	help
 	  Say Y here if you want to run your kernel on an ASpeed BMC SoC.
 
diff --git a/drivers/clocksource/timer-fttmr010.c b/drivers/clocksource/timer-fttmr010.c
index cf93f6419b51..f15386b98289 100644
--- a/drivers/clocksource/timer-fttmr010.c
+++ b/drivers/clocksource/timer-fttmr010.c
@@ -124,6 +124,7 @@ static int fttmr010_timer_set_next_event(unsigned long cycles,
 {
 	struct fttmr010 *fttmr010 = to_fttmr010(evt);
 	u32 cr;
+	u32 l;
 
 	/* Stop */
 	cr = readl(fttmr010->base + TIMER_CR);
@@ -136,6 +137,12 @@ static int fttmr010_timer_set_next_event(unsigned long cycles,
 		 * into TIMER1_COUNT register when the timer is re-enabled.
 		 */
 		writel(cycles, fttmr010->base + TIMER1_LOAD);
+#if CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST
+		/* QEMU model may provide back-pressure on requested period */
+		l = readl(fttmr010->base + TIMER1_LOAD);
+		if (l > cycles)
+			return -EINVAL;
+#endif
 	} else {
 		/* Setup the match register forward in time */
 		cr = readl(fttmr010->base + TIMER1_COUNT);
-- 
2.19.1