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Re: [PATCH v2] ARM: at91: Document new TCB bindings

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  Re: [PATCH v2] ARM: at91: Document new TCB bindings Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-04-07 14:20 +0200
    Re: [PATCH v2] ARM: at91: Document new TCB bindings Alexandre Belloni <alexandre.belloni@free-electrons.com> - 2017-04-07 14:40 +0200

#1618724 — Re: [PATCH v2] ARM: at91: Document new TCB bindings

On 13/03/2017 16:18, Nicolas Ferre wrote:
> Le 25/01/2017 à 16:11, Boris Brezillon a écrit :
>> Hi Rob,
>>
>> Sorry to revive this old discussion, but there's still one aspect I'm
>> not sure about.
>>
>> On Tue, 5 Jul 2016 10:40:22 -0500
>> Rob Herring <robh@kernel.org> wrote:
>>
>>>>>> +   - compatible: Should be "atmel,tcb-free-running-timer"
>>>>>> +   - reg: Should contain the TCB channels to be used. If the
>>>>>> +     counter width is 16 bits (at91rm9200-tcb), two consecutive
>>>>>> +     channels are needed. Else, only one channel will be used.
>>>>>> +
>>>>>> + * a clockevent device
>>>>>> +   - compatible: Should be "atmel,tcb-programmable-timer"  
>>>>>
>>>>> This still looks like assigning usage in DT. As I'm willing to accept
>>>>> that for PWM, either timer channels should be whatever channels are not
>>>>> assigned to PWM (i.e. not in DT) or they should just be "timer" and let
>>>>> the kernel decide their usage.  
>>>>
>>>> I just reviewed Alexandre's new binding, and it makes the whole thing
>>>> a lot more obscure: on older SoCs, we have to chain 2 channels to
>>>> create an acceptable wraparound time (16 bits at 5MHz is generating too
>>>> much interrupts to be acceptable).
>>>>
>>>> If we don't assign the mode from the DT, how should we know which
>>>> channels should be chained to create the free-running timer? Note that
>>>> not all channels can be chained together: they have to be part of the
>>>> same timer counter block and have to be consecutive (0+1, 1+2 or 3+0).  
>>>
>>> The driver can have this knowledge if it is just picking 2 consecutive
>>> timers. It should already know it has 16-bit timers based on the
>>> compatible string. If it gets more complicated then the features or
>>> limitations of the channels should be listed so the driver can make a
>>> choice. OMAP is a good example of lots of timers with differing
>>> features.
>>
>> Yes it's possible to do that, but what about DT overlays then? Say you
>> have some TCB channels you'd like to reserve because they are connected
>> to pins that are exposed on your board. Those pins are not connected to
>> any device yet, but extension boards can be added, and in this case you
>> might want to expose new PWM devices by dynamically loading DT overlays.
>>
>> If your clksource/clkevent driver parsed the initial DT and picked X
>> free channels randomly, it may conflicts with the one requested by the
>> DT overlay.
>>
>> What's your solution for this case?
> 
> It seems that we don't have any progress on this topic for more than 6
> months which is a pity as we now experience an issue that would have
> been addressed completely by the TC rework [1].
> 
> aka "ping"... ;-)


Hi Nicolas, Boris,

is there any news from your side ?

Thanks.

  -- Daniel


-- 
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#1618733

On 07/04/2017 at 14:15:36 +0200, Daniel Lezcano wrote:
> >> What's your solution for this case?
> > 
> > It seems that we don't have any progress on this topic for more than 6
> > months which is a pity as we now experience an issue that would have
> > been addressed completely by the TC rework [1].
> > 
> > aka "ping"... ;-)
> 
> 
> Hi Nicolas, Boris,
> 
> is there any news from your side ?
> 

I'm planning to resubmit news bindings and the full rework when I'll
have time to work on that. But I must admit I don't quite see the issue
with the proposed bindings.

Also, we are starting to see more and more flexible timer IPs that can
be used as RTC, timer, pwm, counters, quad decoders. I'm starting to
think we may need a new subsystem.


-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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