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SubjectRe: [PATCH 5/7] clocksource/cadence_ttc: Overhaul clocksource frequency adjustment
On 11/08/2013 10:21 PM, Soren Brinkmann wrote:
> The currently used method adjusting the clocksource to a changing input
> frequency does not work on kernels from 3.11 on.
> The new approach is to keep the timer frequency as constant as possible.
> I.e.
> - due to the TTC's prescaler limitations, allow frequency changes
> only if the frequency scales by a power of 2
> - adjust the counter's divider on the fly when a frequency change
> occurs
>
> When suspending though, the driver should not prevent rate changes in
> order to allow the system to enter its low power state. For that
> reason a PM notifier is added so rate changes can be ignored during
> suspend/resume.

It sounds very weird you have to add a PM notifier in this driver.

Have you been facing an issue or do you assume it could happen ?

> This limits cpufreq to scale by certain factors only.
> But we may keep the time base somewhat constant, so that sleep() & co
> keep working as expected, while supporting cpufreq and suspend.
>
> Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
> ---
> drivers/clocksource/cadence_ttc_timer.c | 142 +++++++++++++++++++++++++++-----
> 1 file changed, 122 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/clocksource/cadence_ttc_timer.c b/drivers/clocksource/cadence_ttc_timer.c
> index 68a336038d8f..421b942dd707 100644
> --- a/drivers/clocksource/cadence_ttc_timer.c
> +++ b/drivers/clocksource/cadence_ttc_timer.c
> @@ -16,12 +16,14 @@
> */
>
> #include <linux/clk.h>
> +#include <linux/clk-provider.h>
> #include <linux/interrupt.h>
> #include <linux/clockchips.h>
> #include <linux/of_address.h>
> #include <linux/of_irq.h>
> #include <linux/slab.h>
> #include <linux/sched_clock.h>
> +#include <linux/suspend.h>
>
> /*
> * This driver configures the 2 16-bit count-up timers as follows:
> @@ -52,6 +54,8 @@
> #define TTC_CNT_CNTRL_DISABLE_MASK 0x1
>
> #define TTC_CLK_CNTRL_CSRC_MASK (1 << 5) /* clock source */
> +#define TTC_CLK_CNTRL_PSV_MASK 0x1e
> +#define TTC_CLK_CNTRL_PSV_SHIFT 1
>
> /*
> * Setup the timers to use pre-scaling, using a fixed value for now that will
> @@ -63,6 +67,8 @@
> #define CLK_CNTRL_PRESCALE_EN 1
> #define CNT_CNTRL_RESET (1 << 4)
>
> +#define MAX_F_ERR 50
> +
> /**
> * struct ttc_timer - This definition defines local timer structure
> *
> @@ -82,8 +88,13 @@ struct ttc_timer {
> container_of(x, struct ttc_timer, clk_rate_change_nb)
>
> struct ttc_timer_clocksource {
> + int scale_dir;
> + u32 scale_clk_ctrl_reg_old;
> + u32 scale_clk_ctrl_reg_new;
> + int suspending;
> struct ttc_timer ttc;
> struct clocksource cs;
> + struct notifier_block suspend_nb;
> };
>
> #define to_ttc_timer_clksrc(x) \
> @@ -228,33 +239,120 @@ static int ttc_rate_change_clocksource_cb(struct notifier_block *nb,
> struct ttc_timer_clocksource *ttccs = container_of(ttc,
> struct ttc_timer_clocksource, ttc);
>
> + if (ttccs->suspending)
> + return NOTIFY_OK;

I don't see how it couldn't be racy. What prevents suspend to occur
right after this check ?

> +
> switch (event) {
> - case POST_RATE_CHANGE:
> + case PRE_RATE_CHANGE:
> + {
> + u32 psv;
> + unsigned long factor, rate_low, rate_high;
> +
> + if (ndata->new_rate == ndata->old_rate) {
> + ttccs->scale_dir = 0;
> + return NOTIFY_OK;
> + }
> +
> + if (ndata->new_rate > ndata->old_rate) {
> + factor = DIV_ROUND_CLOSEST(ndata->new_rate,
> + ndata->old_rate);
> + ttccs->scale_dir = 1;
> + rate_low = ndata->old_rate;
> + rate_high = ndata->new_rate;
> + } else {
> + factor = DIV_ROUND_CLOSEST(ndata->old_rate,
> + ndata->new_rate);
> + ttccs->scale_dir = -1;
> + rate_low = ndata->new_rate;
> + rate_high = ndata->old_rate;
> + }

Hmm, it could be interesting if Viresh can give its opinion on this
patch [cc'ed].

> +
> + if (!is_power_of_2(factor))
> + return NOTIFY_BAD;
> +
> + if (abs(rate_high - (factor * rate_low)) > MAX_F_ERR)
> + return NOTIFY_BAD;
> +
> + factor = __ilog2_u32(factor);
> +
> /*
> - * Do whatever is necessary to maintain a proper time base
> - *
> - * I cannot find a way to adjust the currently used clocksource
> - * to the new frequency. __clocksource_updatefreq_hz() sounds
> - * good, but does not work. Not sure what's that missing.
> - *
> - * This approach works, but triggers two clocksource switches.
> - * The first after unregister to clocksource jiffies. And
> - * another one after the register to the newly registered timer.
> - *
> - * Alternatively we could 'waste' another HW timer to ping pong
> - * between clock sources. That would also use one register and
> - * one unregister call, but only trigger one clocksource switch
> - * for the cost of another HW timer used by the OS.
> + * store timer clock ctrl register so we can restore it in case
> + * of an abort.
> */
> - clocksource_unregister(&ttccs->cs);
> - clocksource_register_hz(&ttccs->cs,
> - ndata->new_rate / PRESCALE);
> - /* fall through */
> - case PRE_RATE_CHANGE:
> + ttccs->scale_clk_ctrl_reg_old =
> + __raw_readl(ttccs->ttc.base_addr +
> + TTC_CLK_CNTRL_OFFSET);
> +
> + psv = (ttccs->scale_clk_ctrl_reg_old &
> + TTC_CLK_CNTRL_PSV_MASK) >>
> + TTC_CLK_CNTRL_PSV_SHIFT;
> + if (ttccs->scale_dir < 0)
> + psv -= factor;
> + else
> + psv += factor;
> +
> + /* prescaler within legal range? */
> + if (psv & ~(TTC_CLK_CNTRL_PSV_MASK >> TTC_CLK_CNTRL_PSV_SHIFT))
> + return NOTIFY_BAD;
> +
> + ttccs->scale_clk_ctrl_reg_new = ttccs->scale_clk_ctrl_reg_old &
> + ~TTC_CLK_CNTRL_PSV_MASK;
> + ttccs->scale_clk_ctrl_reg_new |= psv << TTC_CLK_CNTRL_PSV_SHIFT;
> +
> +
> + /* scale down: adjust divider in post-change notification */
> + if (ttccs->scale_dir < 0)
> + return NOTIFY_DONE;
> +
> + /* scale up: adjust divider now - before frequency change */
> + __raw_writel(ttccs->scale_clk_ctrl_reg_new,
> + ttccs->ttc.base_addr + TTC_CLK_CNTRL_OFFSET);
> + break;
> + }
> + case POST_RATE_CHANGE:
> + /* scale up: pre-change notification did the adjustment */
> + if (ttccs->scale_dir >= 0)
> + return NOTIFY_OK;
> +
> + /* scale down: adjust divider now - after frequency change */
> + __raw_writel(ttccs->scale_clk_ctrl_reg_new,
> + ttccs->ttc.base_addr + TTC_CLK_CNTRL_OFFSET);
> + break;
> +
> case ABORT_RATE_CHANGE:
> + /* we have to undo the adjustment in case we scale up */
> + if (ttccs->scale_dir <= 0)
> + return NOTIFY_OK;
> +
> + /* restore original register value */
> + __raw_writel(ttccs->scale_clk_ctrl_reg_old,
> + ttccs->ttc.base_addr + TTC_CLK_CNTRL_OFFSET);
> + /* fall through */
> default:
> return NOTIFY_DONE;
> }
> +
> + return NOTIFY_DONE;
> +}
> +
> +static int ttc_suspend_notifier_cb(struct notifier_block *nb,
> + unsigned long event, void *data)
> +{
> + struct ttc_timer_clocksource *ttccs = container_of(nb,
> + struct ttc_timer_clocksource, suspend_nb);
> +
> + switch (event) {
> + case PM_SUSPEND_PREPARE:
> + ttccs->suspending = 1;
> + break;
> + case PM_POST_SUSPEND:
> + ttccs->suspending = 0;
> + break;
> + default:
> + break;
> + }
> +
> + return NOTIFY_DONE;
> }
>
> static void __init ttc_setup_clocksource(struct clk *clk, void __iomem *base)
> @@ -283,6 +381,10 @@ static void __init ttc_setup_clocksource(struct clk *clk, void __iomem *base)
> &ttccs->ttc.clk_rate_change_nb))
> pr_warn("Unable to register clock notifier.\n");
>
> + ttccs->suspend_nb.notifier_call = ttc_suspend_notifier_cb;
> + if (register_pm_notifier(&ttccs->suspend_nb))
> + pr_warn("Unable to register PM notifier.\n");
> +
> ttccs->ttc.base_addr = base;
> ttccs->cs.name = "ttc_clocksource";
> ttccs->cs.rating = 200;
>


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