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To:     Daniel Lezcano <daniel.lezcano@linaro.org>
Subject: Re: [PATCH v4 7/8] clocksource: Add a new timer-ingenic driver
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Date:   Sat, 31 Mar 2018 19:46:06 +0200
From:   Paul Cercueil <paul@crapouillou.net>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Jason Cooper <jason@lakedaemon.net>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Lee Jones <lee.jones@linaro.org>,
        Ralf Baechle <ralf@linux-mips.org>,
        Rob Herring <robh+dt@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Mark Rutland <mark.rutland@arm.com>,
        James Hogan <jhogan@kernel.org>,
        Maarten ter Huurne <maarten@treewalker.org>,
        linux-clk@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mips@linux-mips.org,
        linux-doc@vger.kernel.org
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Le 2018-03-31 10:10, Daniel Lezcano a écrit :
> On 29/03/2018 16:52, Paul Cercueil wrote:
>> 
>> 
>> Le mer. 28 mars 2018 à 18:25, Daniel Lezcano 
>> <daniel.lezcano@linaro.org>
>> a écrit :
>>> On 28/03/2018 17:15, Paul Cercueil wrote:
>>>>  Le 2018-03-24 07:26, Daniel Lezcano a écrit :
>>>>>  On 18/03/2018 00:29, Paul Cercueil wrote:
>>>>>>  This driver will use the TCU (Timer Counter Unit) present on the
>>>>>> Ingenic
>>>>>>  JZ47xx SoCs to provide the kernel with a clocksource and timers.
>>>>> 
>>>>>  Please provide a more detailed description about the timer.
>>>> 
>>>>  There's a doc file for that :)
>>> 
>>> Usually, when there is a new driver I ask for a description in the
>>> changelog for reference.
>>> 
>>>>>  Where is the clocksource ?
>>>> 
>>>>  Right, there is no clocksource, just timers.
>>>> 
>>>>>  I don't see the point of using channel idx and pwm checking here.
>>>>> 
>>>>>  There is one clockevent, why create multiple channels ? Can't you
>>>>> stick
>>>>>  to the usual init routine for a timer.
>>>> 
>>>>  So the idea is that we use all the TCU channels that won't be used
>>>> for PWM
>>>>  as timers. Hence the PWM checking. Why is this bad?
>>> 
>>> It is not bad but arguable. By checking the channels used by the pwm 
>>> in
>>> the code, you introduce an adherence between two subsystems even if 
>>> it
>>> is just related to the DT parsing part.
>>> 
>>> As it is not needed to have more than one timer in the time framework
>>> (at least with the same characteristics), the pwm channels check is
>>> pointless. We can assume the author of the DT file is smart enough to
>>> prevent conflicts and define a pwm and a timer properly instead of
>>> adding more code complexity.
>>> 
>>> In addition, simplifying the code will allow you to use the timer-of
>>> code and reduce very significantly the init function.
>> 
>> That's what I had in my V1 and V2, my DT node for the timer-ingenic 
>> driver
>> had a "timers" property (e.g. "timers = <4 5>;") to select the 
>> channels
>> that
>> should be used as timers. Then Rob told me I shouldn't do that, and 
>> instead
>> detect the channels that will be used for PWM.
>> 
> 
> [ ... ]
> 
> How do you specify the channels used for PWM ?

To detect the channels that will be used as PWM I parse the whole 
devicetree
searching for "pwms" properties; check that the PWM handle is for our 
TCU PWM
driver; then read the PWM number from there.

Of course it's hackish, and it only works for devicetree. I preferred 
the
method with the "timers" property.

>>>>>> 
>>>>>>  +config INGENIC_TIMER
>>>>>>  +    bool "Clocksource/timer using the TCU in Ingenic JZ SoCs"
>>>>>>  +    depends on MACH_INGENIC || COMPILE_TEST
>>>>> 
>>>>>  bool "Clocksource/timer using the TCU in Ingenic JZ SoCs" if
>>>>> COMPILE_TEST
>>>>> 
>>>>>  Remove the depends MACH_INGENIC.
>>>> 
>>>>  This driver is not useful on anything else than Ingenic SoCs, why
>>>> should I
>>>>  remove MACH_INGENIC then?
>>> 
>>> For COMPILE_TEST on x86.
>> 
>> Well that's a logical OR right here, so it will work...
> 
> Right, I missed the second part of the condition. For consistency
> reason, we don't add a dependency on the platform. The platform will
> select it. Look the other timer options and you will see there is no
> MACH deps. I'm trying consolidating all these options to have same
> format and hopefully factor them out.

I'm all for factorisation, but what I dislike with not depending on
MACH_INGENIC, is that the driver now appears in the menuconfig for
every arch, even if it only applies to one MIPS SoC.

Regards,
-Paul