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

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Message-ID: <CAD=FV=VUomY-h2euRVWNO=ND0pUNLO5mZr_=sMm_oUSrwBssnA@mail.gmail.com>
Date:	Thu, 19 Jun 2014 08:49:07 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Amit Daniel Kachhap <amit.daniel@...sung.com>,
	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

Daniel,

On Thu, Jun 19, 2014 at 2:07 AM, Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
> 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

I could do that if someone told me that they'll land it.

My understanding of the current status is:
* I posed the 64-bit version that's almost as fast as the 32-bit version.
* I asked if people want the 32-bit version: no answer
* I asked if anyone is opposed to the 64-bit version: no answer

I know that you wanted me to clean up the description of the 64-bit
version so I was going to do that and repost.  If there's someone
willing to review / ack the 32-bit version I'd be happy to do that
instead.  Possibly I'll do both and a maintainer can choose which to
apply?


In the case here I was suggesting using the 32-bit version just
because on ARM32 there's totally no reason to read 64-bits.  I hadn't
given a thought to ARM64.  More on that in response to the other
messages.

-Doug
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