Re: [PATCH 2/2] rtc: tegra: Implement suspend clock source

[Thierry Reding]

On Fri, Jun 14, 2019 at 04:49:44PM +0300, Dmitry Osipenko wrote:
> 14.06.2019 16:41, Thierry Reding ÐÐÑÐÑ:
> > On Fri, Jun 14, 2019 at 03:01:13PM +0300, Dmitry Osipenko wrote:
> >> 14.06.2019 13:47, Thierry Reding ÐÐÑÐÑ:
> >>> From: Thierry Reding <treding@xxxxxxxxxx>
> >>>
> >>> The suspend clock source for Tegra210 and earlier is currently
> >>> implemented in the Tegra timer driver. However, the suspend clock source
> >>> code accesses registers that are part of the RTC hardware block, so both
> >>> can step on each others' toes. In practice this isn't an issue, but
> >>> there is no reason why the RTC driver can't implement the clock source,
> >>> so move the code over to the tegra-rtc driver.
> >>>
> >>> Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>
> >>> ---
> >>>  drivers/clocksource/timer-tegra.c | 44 -------------------------------
> >>>  drivers/rtc/rtc-tegra.c           | 42 +++++++++++++++++++++++++++++
> >>>  2 files changed, 42 insertions(+), 44 deletions(-)
> >>>
> >>> diff --git a/drivers/clocksource/timer-tegra.c b/drivers/clocksource/timer-tegra.c
> >>> index e6608141cccb..87eac618924d 100644
> >>> --- a/drivers/clocksource/timer-tegra.c
> >>> +++ b/drivers/clocksource/timer-tegra.c
> >>> @@ -21,10 +21,6 @@
> >>>  
> >>>  #include "timer-of.h"
> >>>  
> >>> -#define RTC_SECONDS		0x08
> >>> -#define RTC_SHADOW_SECONDS	0x0c
> >>> -#define RTC_MILLISECONDS	0x10
> >>> -
> >>>  #define TIMERUS_CNTR_1US	0x10
> >>>  #define TIMERUS_USEC_CFG	0x14
> >>>  #define TIMERUS_CNTR_FREEZE	0x4c
> >>> @@ -164,34 +160,6 @@ static struct delay_timer tegra_delay_timer = {
> >>>  };
> >>>  #endif
> >>>  
> >>> -static struct timer_of suspend_rtc_to = {
> >>> -	.flags = TIMER_OF_BASE | TIMER_OF_CLOCK,
> >>> -};
> >>> -
> >>> -/*
> >>> - * tegra_rtc_read - Reads the Tegra RTC registers
> >>> - * Care must be taken that this function is not called while the
> >>> - * tegra_rtc driver could be executing to avoid race conditions
> >>> - * on the RTC shadow register
> >>> - */
> >>> -static u64 tegra_rtc_read_ms(struct clocksource *cs)
> >>> -{
> >>> -	void __iomem *reg_base = timer_of_base(&suspend_rtc_to);
> >>> -
> >>> -	u32 ms = readl_relaxed(reg_base + RTC_MILLISECONDS);
> >>> -	u32 s = readl_relaxed(reg_base + RTC_SHADOW_SECONDS);
> >>> -
> >>> -	return (u64)s * MSEC_PER_SEC + ms;
> >>> -}
> >>> -
> >>> -static struct clocksource suspend_rtc_clocksource = {
> >>> -	.name	= "tegra_suspend_timer",
> >>> -	.rating	= 200,
> >>> -	.read	= tegra_rtc_read_ms,
> >>> -	.mask	= CLOCKSOURCE_MASK(32),
> >>> -	.flags	= CLOCK_SOURCE_IS_CONTINUOUS | CLOCK_SOURCE_SUSPEND_NONSTOP,
> >>> -};
> >>> -
> >>>  static inline unsigned int tegra_base_for_cpu(int cpu, bool tegra20)
> >>>  {
> >>>  	if (tegra20) {
> >>> @@ -385,15 +353,3 @@ static int __init tegra20_init_timer(struct device_node *np)
> >>>  	return tegra_init_timer(np, true, rating);
> >>>  }
> >>>  TIMER_OF_DECLARE(tegra20_timer, "nvidia,tegra20-timer", tegra20_init_timer);
> >>> -
> >>> -static int __init tegra20_init_rtc(struct device_node *np)
> >>> -{
> >>> -	int ret;
> >>> -
> >>> -	ret = timer_of_init(np, &suspend_rtc_to);
> >>> -	if (ret)
> >>> -		return ret;
> >>> -
> >>> -	return clocksource_register_hz(&suspend_rtc_clocksource, 1000);
> >>> -}
> >>> -TIMER_OF_DECLARE(tegra20_rtc, "nvidia,tegra20-rtc", tegra20_init_rtc);
> >>> diff --git a/drivers/rtc/rtc-tegra.c b/drivers/rtc/rtc-tegra.c
> >>> index 8fa1b3febf69..6da54264a27a 100644
> >>> --- a/drivers/rtc/rtc-tegra.c
> >>> +++ b/drivers/rtc/rtc-tegra.c
> >>> @@ -6,6 +6,7 @@
> >>>   */
> >>>  
> >>>  #include <linux/clk.h>
> >>> +#include <linux/clocksource.h>
> >>>  #include <linux/delay.h>
> >>>  #include <linux/init.h>
> >>>  #include <linux/io.h>
> >>> @@ -52,8 +53,15 @@ struct tegra_rtc_info {
> >>>  	struct clk *clk;
> >>>  	int irq; /* alarm and periodic IRQ */
> >>>  	spinlock_t lock;
> >>> +
> >>> +	struct clocksource clksrc;
> >>>  };
> >>>  
> >>> +static struct tegra_rtc_info *to_tegra_rtc(struct clocksource *clksrc)
> >>> +{
> >>> +	return container_of(clksrc, struct tegra_rtc_info, clksrc);
> >>> +}
> >>> +
> >>>  /*
> >>>   * RTC hardware is busy when it is updating its values over AHB once every
> >>>   * eight 32 kHz clocks (~250 us). Outside of these updates the CPU is free to
> >>> @@ -268,6 +276,17 @@ static const struct rtc_class_ops tegra_rtc_ops = {
> >>>  	.alarm_irq_enable = tegra_rtc_alarm_irq_enable,
> >>>  };
> >>>  
> >>> +static u64 tegra_rtc_read_ms(struct clocksource *clksrc)
> >>> +{
> >>> +	struct tegra_rtc_info *info = to_tegra_rtc(clksrc);
> >>> +	u32 ms, s;
> >>> +
> >>> +	ms = readl_relaxed(info->base + TEGRA_RTC_REG_MILLI_SECONDS);
> >>> +	s = readl_relaxed(info->base + TEGRA_RTC_REG_SHADOW_SECONDS);
> >>> +
> >>> +	return (u64)s * MSEC_PER_SEC + ms;
> >>> +}
> >>> +
> >>>  static const struct of_device_id tegra_rtc_dt_match[] = {
> >>>  	{ .compatible = "nvidia,tegra20-rtc", },
> >>>  	{}
> >>> @@ -339,6 +358,28 @@ static int tegra_rtc_probe(struct platform_device *pdev)
> >>>  		goto disable_clk;
> >>>  	}
> >>>  
> >>> +	/*
> >>> +	 * The Tegra RTC is the only reliable clock source that persists
> >>> +	 * across an SC7 transition (VDD_CPU and VDD_CORE off) on Tegra210
> >>> +	 * and earlier. Starting with Tegra186, the ARM v8 architected timer
> >>> +	 * is in an always on power partition and its reference clock keeps
> >>> +	 * running during SC7. Therefore, we technically don't need to have
> >>> +	 * the RTC register as a clock source on Tegra186 and later, but it
> >>> +	 * doesn't hurt either, so we just register it unconditionally here.
> >>> +	 */
> >>> +	info->clksrc.name = "tegra_rtc";
> >>> +	info->clksrc.rating = 200;
> >>> +	info->clksrc.read = tegra_rtc_read_ms;
> >>> +	info->clksrc.mask = CLOCKSOURCE_MASK(32);
> >>
> >> Hm.. shouldn't this be CLOCKSOURCE_MASK(52)? Given that there are 32 bits for seconds and
> >> 10bits for milliseconds.
> > 
> > Did you mean to say CLOCKSOURCE_MASK(42)? Yeah, that's probably better
> > here.
> 
> Yes, 42 :)

I'm wondering if that could perhaps be a little problematic because
we're not actually using all of the 10 bits for the milliseconds. So the
maximum value that we can return is:

	4294967296 * 1000 + 999 = 4294967296999

However, the maximum value for a 42 bit mask is:

	2^42 - 1 = 4398046511103

That mask is only used in order to wrap around in delta computations. So
I can imagine a situation where we'd end up with a wrong value in the
delta. I suppose this can only really happen if the two samples are very
far apart in time, so maybe this isn't worth worrying about.

Thierry

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