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Subject[RFC v8 01/28] clocksource: sh_cmt: compute rate before registration again
Date
With the upcoming NTP correction related rate adjustments to be implemented
in the clockevents core, the latter needs to get informed about every rate
change of a clockevent device made after its registration.

Currently, sh_cmt violates this requirement in that it registers its
clockevent device with a dummy rate and sets its final ->mult and ->shift
values from its ->set_state_oneshot() and ->set_state_periodic() functions
respectively.

This patch moves the setting of the clockevent device's ->mult and ->shift
values to before its registration.

Note that there has been some back and forth regarding this question with
respect to the clocksource also provided by this driver:
commit f4d7c3565c16 ("clocksource: sh_cmt: compute mult and shift before
registration")
moves the rate determination from the clocksource's ->enable() function to
before its registration. OTOH, the later
commit 3593f5fe40a1 ("clocksource: sh_cmt: __clocksource_updatefreq_hz()
update")
basically reverts this, saying
"Without this patch the old code uses clocksource_register() together
with a hack that assumes a never changing clock rate."

However, I checked all current sh_cmt users in arch/sh as well as in
arch/arm/mach-shmobile carefully and right now, none of them changes any
rate in any clock tree relevant to sh_cmt after their respective
time_init(). Since all sh_cmt instances are created after time_init(), none
of them should ever observe any clock rate changes.

What's more, both, a clocksource as well as a clockevent device, can
immediately get selected for use at their registration and thus, enabled
at this point already. So it's probably safer to assume a "never changing
clock rate" here.

- Move the struct sh_cmt_channel's ->rate member to struct sh_cmt_device:
it's a property of the underlying clock which is in turn specific to
the sh_cmt_device.
- Determine the ->rate value in sh_cmt_setup() at device probing rather
than at first usage.
- Set the clockevent device's ->mult and ->shift values right before its
registration.
- Although not strictly necessary for the upcoming clockevent core changes,
set the clocksource's rate at its registration for consistency.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---

Notes:
For a detailed analysis of the current sh_cmt users, please see
https://nicst.de/ced-clk-rate-change-analysis/sh_cmt-cgitted.html

Compile-only tested on ARCH=sh and ARCH=arm.

drivers/clocksource/sh_cmt.c | 45 ++++++++++++++++++++++++--------------------
1 file changed, 25 insertions(+), 20 deletions(-)

diff --git a/drivers/clocksource/sh_cmt.c b/drivers/clocksource/sh_cmt.c
index 103c493..3038885 100644
--- a/drivers/clocksource/sh_cmt.c
+++ b/drivers/clocksource/sh_cmt.c
@@ -103,7 +103,6 @@ struct sh_cmt_channel {
unsigned long match_value;
unsigned long next_match_value;
unsigned long max_match_value;
- unsigned long rate;
raw_spinlock_t lock;
struct clock_event_device ced;
struct clocksource cs;
@@ -118,6 +117,7 @@ struct sh_cmt_device {

void __iomem *mapbase;
struct clk *clk;
+ unsigned long rate;

raw_spinlock_t lock; /* Protect the shared start/stop register */

@@ -320,7 +320,7 @@ static void sh_cmt_start_stop_ch(struct sh_cmt_channel *ch, int start)
raw_spin_unlock_irqrestore(&ch->cmt->lock, flags);
}

-static int sh_cmt_enable(struct sh_cmt_channel *ch, unsigned long *rate)
+static int sh_cmt_enable(struct sh_cmt_channel *ch)
{
int k, ret;

@@ -340,11 +340,9 @@ static int sh_cmt_enable(struct sh_cmt_channel *ch, unsigned long *rate)

/* configure channel, periodic mode and maximum timeout */
if (ch->cmt->info->width == 16) {
- *rate = clk_get_rate(ch->cmt->clk) / 512;
sh_cmt_write_cmcsr(ch, SH_CMT16_CMCSR_CMIE |
SH_CMT16_CMCSR_CKS512);
} else {
- *rate = clk_get_rate(ch->cmt->clk) / 8;
sh_cmt_write_cmcsr(ch, SH_CMT32_CMCSR_CMM |
SH_CMT32_CMCSR_CMTOUT_IE |
SH_CMT32_CMCSR_CMR_IRQ |
@@ -572,7 +570,7 @@ static int sh_cmt_start(struct sh_cmt_channel *ch, unsigned long flag)
raw_spin_lock_irqsave(&ch->lock, flags);

if (!(ch->flags & (FLAG_CLOCKEVENT | FLAG_CLOCKSOURCE)))
- ret = sh_cmt_enable(ch, &ch->rate);
+ ret = sh_cmt_enable(ch);

if (ret)
goto out;
@@ -640,10 +638,9 @@ static int sh_cmt_clocksource_enable(struct clocksource *cs)
ch->total_cycles = 0;

ret = sh_cmt_start(ch, FLAG_CLOCKSOURCE);
- if (!ret) {
- __clocksource_update_freq_hz(cs, ch->rate);
+ if (!ret)
ch->cs_enabled = true;
- }
+
return ret;
}

@@ -697,8 +694,7 @@ static int sh_cmt_register_clocksource(struct sh_cmt_channel *ch,
dev_info(&ch->cmt->pdev->dev, "ch%u: used as clock source\n",
ch->index);

- /* Register with dummy 1 Hz value, gets updated in ->enable() */
- clocksource_register_hz(cs, 1);
+ clocksource_register_hz(cs, ch->cmt->rate);
return 0;
}

@@ -709,19 +705,10 @@ static struct sh_cmt_channel *ced_to_sh_cmt(struct clock_event_device *ced)

static void sh_cmt_clock_event_start(struct sh_cmt_channel *ch, int periodic)
{
- struct clock_event_device *ced = &ch->ced;
-
sh_cmt_start(ch, FLAG_CLOCKEVENT);

- /* TODO: calculate good shift from rate and counter bit width */
-
- ced->shift = 32;
- ced->mult = div_sc(ch->rate, NSEC_PER_SEC, ced->shift);
- ced->max_delta_ns = clockevent_delta2ns(ch->max_match_value, ced);
- ced->min_delta_ns = clockevent_delta2ns(0x1f, ced);
-
if (periodic)
- sh_cmt_set_next(ch, ((ch->rate + HZ/2) / HZ) - 1);
+ sh_cmt_set_next(ch, ((ch->cmt->rate + HZ/2) / HZ) - 1);
else
sh_cmt_set_next(ch, ch->max_match_value);
}
@@ -824,6 +811,12 @@ static int sh_cmt_register_clockevent(struct sh_cmt_channel *ch,
ced->suspend = sh_cmt_clock_event_suspend;
ced->resume = sh_cmt_clock_event_resume;

+ /* TODO: calculate good shift from rate and counter bit width */
+ ced->shift = 32;
+ ced->mult = div_sc(ch->cmt->rate, NSEC_PER_SEC, ced->shift);
+ ced->max_delta_ns = clockevent_delta2ns(ch->max_match_value, ced);
+ ced->min_delta_ns = clockevent_delta2ns(0x1f, ced);
+
dev_info(&ch->cmt->pdev->dev, "ch%u: used for clock events\n",
ch->index);
clockevents_register_device(ced);
@@ -996,6 +989,18 @@ static int sh_cmt_setup(struct sh_cmt_device *cmt, struct platform_device *pdev)
if (ret < 0)
goto err_clk_put;

+ /* Determine clock rate. */
+ ret = clk_enable(cmt->clk);
+ if (ret < 0)
+ goto err_clk_unprepare;
+
+ if (cmt->info->width == 16)
+ cmt->rate = clk_get_rate(cmt->clk) / 512;
+ else
+ cmt->rate = clk_get_rate(cmt->clk) / 8;
+
+ clk_disable(cmt->clk);
+
/* Map the memory resource(s). */
ret = sh_cmt_map_memory(cmt);
if (ret < 0)
--
2.10.2
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