linux-kernel - Re: [PATCH v6 3/9] docs: Add Generic Counter interface documentation

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Message-ID: <cd045792-8ed2-8d83-231a-661975ffc7c5@st.com>
Date:   Fri, 25 May 2018 11:26:11 +0200
From:   Fabrice Gasnier <fabrice.gasnier@...com>
To:     Jonathan Cameron <jic23@...nel.org>,
        William Breathitt Gray <vilhelm.gray@...il.com>
CC:     <benjamin.gaignard@...com>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v6 3/9] docs: Add Generic Counter interface documentation

On 05/22/2018 07:08 PM, Jonathan Cameron wrote:
>>>> +* Quadrature x2 Rising:
>>>> +  Rising edges on either quadrature pair signals updates the respective
>>>> +  count. Quadrature encoding determines the direction.  
>>> This one I've never met.  Really? There are devices who do this form
>>> of crazy? It gives really uneven counting and I'm failing to see when
>>> it would ever make sense...  References for these odd corner cases
>>> would be good.
>>>
>>>
>>> __|---|____|-----|____
>>> ____|----|____|-----|____
>>>
>>> 001122222223334444444  
>> That's the same reaction I had when I discovered this -- in fact the
>> STM32 LP Timer is the first time I've come across such a quadrature
>> mode. I'm not sure of the use case for this mode, because positioning
>> wouldn't be precise as you've pointed out. Perhaps Fabrice or Benjamin
>> can probe the ST guys responsible for this design choice to figure out
>> the rationale.
> Hmm.  My inclination would be to not support it unless someone can up
> with a meaningful use.  We are adding ABI (be it not much) for a case
> that to us makes no sense.

Hi Jonathan, William,

Sorry for the late reply. To follow your advise, we can probably drop
this for now. I think simple counter, or quadrature x4 will be mostly
used for now. As you pointed out, there's not much ABI for x2
rising/falling cases. It will not be a big deal to add it later if needed.

I can help to update (remove & test) this in LP-Timer counter driver if
you wish.

Please let me know,

Thanks,
Fabrice

> 
> Looks rather like the sort of thing that is a side effect of the
> implementation rather than deliberate.
> 
>> I'm leaving in these modes for now, as they do exist in the STM32 LP
>> Timer, but it does make me curious what the intentions for them were
>> (perhaps use cases outside of traditional quadrature encoder
>> positioning).
>>
> Sure if there is a usecase then fair enough.
> 
> Jonathan
> 
>