linux-kernel - Re: [PATCH v3] clocksource: Add driver for the Ingenic JZ47xx OST

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Date:   Wed, 15 Jan 2020 20:54:15 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paul Cercueil <paul@...pouillou.net>,
        Maarten ter Huurne <maarten@...ewalker.org>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>, od@...c.me,
        linux-kernel@...r.kernel.org, Mathieu Malaterre <malat@...ian.org>,
        Artur Rojek <contact@...ur-rojek.eu>
Subject: Re: [PATCH v3] clocksource: Add driver for the Ingenic JZ47xx OST

Paul Cercueil <paul@...pouillou.net> writes:
> Le mer., janv. 15, 2020 at 18:48, Maarten ter Huurne 
> <maarten@...ewalker.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@...aro.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.

Thanks,

        tglx