[RFC v6 01/23] clocksource: sh_cmt: compute rate before registration again

Nicolai Stange
2016-09-09T20:01:06Z

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 | 50 ++++++++++++++++++++++++--------------------
1 file changed, 27 insertions(+), 23 deletions(-)

diff --git a/drivers/clocksource/sh_cmt.c b/drivers/clocksource/sh_cmt.c
index 103c493..fd9caa7 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;

@@ -339,17 +339,14 @@ static int sh_cmt_enable(struct sh_cmt_channel *ch, unsigned long *rate)
sh_cmt_start_stop_ch(ch, 0);

/* configure channel, periodic mode and maximum timeout */
- if (ch->cmt->info->width == 16) {
- *rate = clk_get_rate(ch->cmt->clk) / 512;
+ if (ch->cmt->info->width == 16)
sh_cmt_write_cmcsr(ch, SH_CMT16_CMCSR_CMIE |
SH_CMT16_CMCSR_CKS512);
- } else {
- *rate = clk_get_rate(ch->cmt->clk) / 8;
+ else
sh_cmt_write_cmcsr(ch, SH_CMT32_CMCSR_CMM |
SH_CMT32_CMCSR_CMTOUT_IE |
SH_CMT32_CMCSR_CMR_IRQ |
SH_CMT32_CMCSR_CKS_RCLK8);
- }

sh_cmt_write_cmcor(ch, 0xffffffff);
sh_cmt_write_cmcnt(ch, 0);
@@ -572,7 +569,7 @@ 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 +637,9 @@ 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 +693,7 @@ 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 +704,10 @@ 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 +810,12 @@ 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 +988,18 @@ 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)
--
2.9.3