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Subject: Re: [PATCH v3] clocksource: Add driver for the Ingenic JZ47xx OST
To:     Paul Cercueil <paul@crapouillou.net>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     Maarten ter Huurne <maarten@treewalker.org>, od@zcrc.me,
        linux-kernel@vger.kernel.org, Mathieu Malaterre <malat@debian.org>,
        Artur Rojek <contact@artur-rojek.eu>
References: <1579110897.3.0@crapouillou.net>
 <87y2u8xzq0.fsf@nanos.tec.linutronix.de> <1579121936.3.1@crapouillou.net>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
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Message-ID: <eef21d5f-334c-81b4-19b1-6498df4fca30@linaro.org>
Date:   Sat, 8 Feb 2020 08:09:45 +0100
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Hi Paul,

On 15/01/2020 21:58, Paul Cercueil wrote:
> 
> 
> Le mer., janv. 15, 2020 at 20:54, Thomas Gleixner <tglx@linutronix.de> a
> écrit :
>> Paul Cercueil <paul@crapouillou.net> writes:
>>>  Le mer., janv. 15, 2020 at 18:48, Maarten ter Huurne
>>>  <maarten@treewalker.org> a écrit :
>>>>  On Wednesday, 15 January 2020 14:57:01 CET Paul Cercueil wrote:
>>>>>   Le mer., janv. 15, 2020 at 14:44, Daniel Lezcano
>>>>>   <daniel.lezcano@linaro.org> a écrit :
>>>>>   > Is the JZ47xx OST really a mfd needing a regmap? (Note regmap_read
>>>>>   > will take a lock).
>>>>>
>>>>>   Yes, the TCU_REG_OST_TCSR register is shared with the clocks driver.
>>>>
>>>>  The TCU_REG_OST_TCSR register is only used in the probe though.
>>>>
>>>>  To get the counter value from TCU_REG_OST_CNTL/TCU_REG_OST_CNTH you
>>>>  could technically do it by reading the register directly, if
>>>>  performance
>>>>  concerns make it necessary to bypass the usual kernel infrastructure
>>>>  for
>>>>  dealing with shared registers.
>>>
>>>  In theory yes, in practice there's no easy way to do that (the
>>>  underlying mmio pointer is not obtainable from the regmap), and
>>>  besides, the lock is just a spinlock and not a mutex.
>>
>> That lock still a massive contention point as clock readouts can be
>> pretty
>> frequent depending on workloads. Just think about tracing ...
>>
>> So I really would avoid both the lock and that ugly 64bit readout thing.
> 
> The 64bit readout thing is gone in V3.
> 
> The lock cannot go away unless we have a way to retrieve the underlying
> mmio pointer from the regmap, which the regmap maintainers will never
> accept. So I can't really change that now. Besides,
> drivers/clocksource/ingenic-timer.c also registers a clocksource that's
> read with the regmap, and nobody complained.

Is there any progress on this? Having a lock in this code path is very
impacting.


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