linux-kernel - Re: [PATCH] clocksource: exynos-mct: Register the timer for stable udelay

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Date:	Thu, 19 Jun 2014 11:07:58 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Doug Anderson <dianders@...omium.org>,
	Amit Daniel Kachhap <amit.daniel@...sung.com>
CC:	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Riley <davidriley@...omium.org>,
	Tomasz Figa <tomasz.figa@...il.com>
Subject: Re: [PATCH] clocksource: exynos-mct: Register the timer for stable
 udelay

On 06/19/2014 01:17 AM, Doug Anderson wrote:
> Amit,
>
> Thanks for posting!
>
> On Wed, Jun 18, 2014 at 4:31 AM, Amit Daniel Kachhap
> <amit.daniel@...sung.com> wrote:
>> This patch register the exynos mct clocksource as the current timer
>> as it has constant clock rate. This will generate correct udelay for the
>> exynos platform and avoid using unnecessary calibrated jiffies. This change
>> have been tested on exynos5420 based board.
>>
>> Signed-off-by: Amit Daniel Kachhap <amit.daniel@...sung.com>
>> ---
>>
>> Patches from David Riley confirmed that udelay is broken in exynos5420.
>> Link to those patches are,
>> 1) https://patchwork.kernel.org/patch/4344911/
>> 2) https://patchwork.kernel.org/patch/4344881/
>>
>>   drivers/clocksource/exynos_mct.c | 11 +++++++++++
>>   1 file changed, 11 insertions(+)
>>
>> diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
>> index 8d64200..57cb3dc 100644
>> --- a/drivers/clocksource/exynos_mct.c
>> +++ b/drivers/clocksource/exynos_mct.c
>> @@ -198,10 +198,21 @@ static u64 notrace exynos4_read_sched_clock(void)
>>          return exynos4_frc_read(&mct_frc);
>>   }
>>
>> +static struct delay_timer exynos4_delay_timer;
>> +
>> +static unsigned long exynos4_read_current_timer(void)
>> +{
>> +       return exynos4_frc_read(&mct_frc);
>
> This is terribly inefficient to read all 64-bits and then cast back to
> a 32-bit value.  Replace with:
>
> return __raw_readl(reg_base + EXYNOS4_MCT_G_CNT_L);
>
>
>> +}
>> +
>>   static void __init exynos4_clocksource_init(void)
>>   {
>>          exynos4_mct_frc_start(0, 0);
>
> Please rebase atop (1d80415 clocksource: exynos_mct: Don't reset the
> counter during boot and resume), which is in linuxnext among other
> places.
>
>>
>> +       exynos4_delay_timer.read_current_timer = &exynos4_read_current_timer;
>> +       exynos4_delay_timer.freq = clk_rate;
>> +       register_current_timer_delay(&exynos4_delay_timer);
>> +
>>          if (clocksource_register_hz(&mct_frc, clk_rate))
>>                  panic("%s: can't register clocksource\n", mct_frc.name);
>
> It does seem to work for me though.  :)

Doug,

aren't you working on a 32 bits version ? So this patch could be 
simplified ?

Thanks
   -- Daniel


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