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

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Message-Id: <1581168224.3.0@crapouillou.net>
Date:   Sat, 08 Feb 2020 10:23:44 -0300
From:   Paul Cercueil <paul@...pouillou.net>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Maarten ter Huurne <maarten@...ewalker.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

Hi Daniel,

Le sam., févr. 8, 2020 at 08:09, Daniel Lezcano 
<daniel.lezcano@...aro.org> a écrit :
> Hi Paul,
> 
> On 15/01/2020 21:58, Paul Cercueil wrote:
>> 
>> 
>>  Le mer., janv. 15, 2020 at 20:54, Thomas Gleixner 
>> <tglx@...utronix.de> a
>>  écrit :
>>>  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.
>> 
>>  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.

I have a v4 ready, I'm just waiting for 5.6-rc1 to start to rebase on 
top and send it.

-Paul