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

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Date:   Wed, 15 Jan 2020 14:54:57 -0300
From:   Paul Cercueil <paul@...pouillou.net>
To:     Maarten ter Huurne <maarten@...ewalker.org>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>, 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



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.

-Paul