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

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Date:   Mon, 13 Mar 2017 16:18:12 +0100
From:   Nicolas Ferre <nicolas.ferre@...el.com>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Rob Herring <robh@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>
CC:     Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Linux PWM List <linux-pwm@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2] ARM: at91: Document new TCB bindings

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@...nel.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"... ;-)

Best regards,

[1]
http://lists.infradead.org/pipermail/linux-arm-kernel/2017-March/492080.html
-- 
Nicolas Ferre