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Date:   Wed, 15 Jan 2020 17:58:56 -0300
From:   Paul Cercueil <paul@crapouillou.net>
Subject: Re: [PATCH v3] clocksource: Add driver for the Ingenic JZ47xx OST
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Maarten ter Huurne <maarten@treewalker.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>, od@zcrc.me,
        linux-kernel@vger.kernel.org, Mathieu Malaterre <malat@debian.org>,
        Artur Rojek <contact@artur-rojek.eu>
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Le mer., janv. 15, 2020 at 20:54, Thomas Gleixner <tglx@linutronix.de>=20
a =E9crit :
> Paul Cercueil <paul@crapouillou.net> writes:
>>  Le mer., janv. 15, 2020 at 18:48, Maarten ter Huurne
>>  <maarten@treewalker.org> a =E9crit :
>>>  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 =E9crit :
>>>>   > Is the JZ47xx OST really a mfd needing a regmap? (Note=20
>>>> regmap_read
>>>>   > will take a lock).
>>>>=20
>>>>   Yes, the TCU_REG_OST_TCSR register is shared with the clocks=20
>>>> driver.
>>>=20
>>>  The TCU_REG_OST_TCSR register is only used in the probe though.
>>>=20
>>>  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=20
>>> infrastructure
>>>  for
>>>  dealing with shared registers.
>>=20
>>  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.
>=20
> That lock still a massive contention point as clock readouts can be=20
> pretty
> frequent depending on workloads. Just think about tracing ...
>=20
> So I really would avoid both the lock and that ugly 64bit readout=20
> thing.

The 64bit readout thing is gone in V3.

The lock cannot go away unless we have a way to retrieve the underlying=20
mmio pointer from the regmap, which the regmap maintainers will never=20
accept. So I can't really change that now. Besides,=20
drivers/clocksource/ingenic-timer.c also registers a clocksource that's=20
read with the regmap, and nobody complained.

-Paul

=