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

[Paul Cercueil <paul@crapouillou.net>]

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.

-Paul