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

Dmitry Osipenko
2019-06-14T16:42:02Z

14.06.2019 17:14, Thierry Reding пишет:
> On Fri, Jun 14, 2019 at 04:49:44PM +0300, Dmitry Osipenko wrote:
>> 14.06.2019 16:41, Thierry Reding пишет:
>>> On Fri, Jun 14, 2019 at 03:01:13PM +0300, Dmitry Osipenko wrote:
>>>> 14.06.2019 13:47, Thierry Reding пишет:
>>>>> From: Thierry Reding <treding@nvidia.com>
>>>>>
>>>>> 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@nvidia.com>
>>>>> ---
>>>>> 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.

I'm a bit puzzled now. Looks problematic that wraparound will happen unexpectedly for
the timekeeping. Although please bare in mind that I'm not expert in the area of
timekeeping, actually I know very little about it.

Maybe tegra_rtc_read_ms() should track wraparound case itself and return a
monotonically incrementing value?

static u64 tegra_rtc_read_ms(struct clocksource *clksrc)
{
struct tegra_rtc_info *info = to_tegra_rtc(clksrc);
u64 ms, s, now;

ms = readl_relaxed(info->base + TEGRA_RTC_REG_MILLI_SECONDS);
s = readl_relaxed(info->base + TEGRA_RTC_REG_SHADOW_SECONDS);

now = s * MSEC_PER_SEC + ms;

if (now < info->last)
info->ms_cnt += 0x3e8000003e8ull - info->last + now;
else
info->ms_cnt += now - info->last;

info->last = now;

return info->ms_cnt;
}

and then simply CLOCKSOURCE_MASK(64).