linux-kernel - Re: [PATCH 4/4] iio: hi8435: cleanup reset gpio

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Message-ID: <5eec10c5-9e13-b0fc-fe40-a8b74a4fdbb0@cogentembedded.com>
Date:   Wed, 24 May 2017 14:27:50 +0300
From:   Vladimir Barinov <vladimir.barinov@...entembedded.com>
To:     Nikita Yushchenko <nikita.yoush@...entembedded.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Matt Ranostay <mranostay@...il.com>,
        Gregor Boirie <gregor.boirie@...rot.com>,
        Sanchayan Maity <maitysanchayan@...il.com>
Cc:     linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jeff White <Jeff.White@....aero>,
        Chris Healy <Chris.Healy@....aero>
Subject: Re: [PATCH 4/4] iio: hi8435: cleanup reset gpio

On 23.05.2017 11:18, Nikita Yushchenko wrote:
>>> Reset GPIO is active low.
>>>
>>> Currently driver uses gpiod_set_value(1) to clean reset, which depends
>>> on device tree to contain GPIO_ACTIVE_HIGH - that does not match reality.
>>>
>>> This fixes driver to use _raw version of gpiod_set_value() to enforce
>>> active-low semantics despite of what's written in device tree. Allowing
>>> device tree to override that only opens possibility for errors and does
>>> not add any value.
>>>
>>> Additionally, use _cansleep version to make things work with i2c-gpio
>>> and other sleeping gpio drivers.
>> The reset gpio comes from platform hence it should be handled by DTS.
>>
>> In driver the gpio should not be raw.
>>
>> Even the hi8435 is active low but platform may invert signal (f.e. by
>> adding trigger on the circuit path).
> I see.  However - isn't this pure theoretic?  Does such case exist?
I assure you that this is frequently used.

Simply search google for "simple voltage level shifter"
It might be on PNP or NPN transistor, hence logic might be inverted.

>
> In vast majority of cases, GPIO polarity is chip-specific, not
> chip-use-specific.  Thus this knowlege belongs to driver and not to
> device tree describing particular chip usage.  Having this always
> defined at usage side is IMO major source of errors.
GPIO comes from SoC then "circuit path" and finally chip reset input.

What do you propose if h/w circuit path has simple voltage level shifter 
on transistor. How to differentiate PNP and NPN cases?

Regards,
Vladimir