linux-kernel - Re: [PATCH v5 01/10] pinctrl: generic: Add bi-directional and output-enable

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Date:   Mon, 8 May 2017 20:47:17 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     jmondi <jacopo@...ndi.org>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Chris Brandt <Chris.Brandt@...esas.com>,
        Jacopo Mondi <jacopo+renesas@...ndi.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 01/10] pinctrl: generic: Add bi-directional and output-enable

On Mon, May 8, 2017 at 8:25 PM, jmondi <jacopo@...ndi.org> wrote:
> Andy,
>
> On Mon, May 08, 2017 at 07:08:32PM +0300, Andy Shevchenko wrote:
>> On Mon, May 8, 2017 at 7:01 PM, jmondi <jacopo@...ndi.org> wrote:
>> > On Sun, May 07, 2017 at 09:52:49AM +0200, Linus Walleij wrote:
>> >> On Fri, Apr 28, 2017 at 4:53 PM, Andy Shevchenko
>> >> <andy.shevchenko@...il.com> wrote:
>> >>
>> >> > Linus, for me it looks like better to revert that change, until we
>> >> > will have clear picture why existing configuration parameters can't
>> >> > work.
>> >>
>> >> Yeah I'll revert the binding for fixes.
>>
>> > As it seems we won't be able to proceed with the currently proposed solution,
>> > would that be acceptable now that we use the "pinmux" property to add
>> > flags as BIDIR
>>
>> Can you explain what does this *electrically* mean?
>
> I really don't know what to add to what Chris said in his 2 previous
> replies to the same question, and I don't know if I can even get this
> information as the most detailed drawing I can provide is what you
> have seen already at page 2696 Fig. 54.1 of the following document.
>
> https://www.renesas.com/en-us/doc/products/mpumcu/doc/rz/r01uh0403ej0300_rz_a1h.pdf?key=ccbb2d79446f1cbd015031061140507c

I didn't see before this document. (I had downloaded what Chris
referred to, which has less than 1200 pages).

The figure you pointed to is really nice and explains it, thank you.

So, BiDi in this hardware is just helper to enable Input
simultaneously when you enable output.

This makes me wonder what prevents you to do this in two steps in software?
So, basically in terms of pin control framework you define this pin
configuration as

1. PIN_CONFIG_INPUT_ENABLE:
2. PIN_CONFIG_OUTPUT:

(or wise versa)

> From my perspective these flags are configurations internal to the pin
> controller hardware used to enable/disable input buffers when a pin needs to
> perform in both direction.

> The level of detail I can provide on this is the logical diagram we have pointed
> you to already.
>
> As I assume you are trying to get this answer from us in order to
> avoid duplicating things in pin controller sub-system, and I
> understand this, but my question here was "can we have those flags as part
> of the pinmux property argument list, as that property description
> seems to allow us to do that, instead of making them generic pin
> configuration properties and upset other developers?"

I guess Linus is better than me could answer to this.

> Anyway, I still fail to see why those configuration flags, only
> affecting the way the pin controller hardware enables/disables
> its internal buffers and its internal operations have to be
> described in term of their externally visible electrically characteristics.
>
>> Second question, what makes it differ to what already exists?
>
> To me, what already exists are pin configuration properties generic to
> the whole pin controller subsystem, and I understand you don't want to
> see duplication there.
>
> At the same time, to me, those flags are settings the pin controller
> wants to have specified by software to overcome its hw design flaws,
> and are intended to configure its internal buffers in a way it cannot
> do by itself for some very specific operation modes (they are listed
> in the hw reference manual, it's not something you can chose to
> configure or not, if you want a pin working in i2c mode, you HAVE to
> pass those flags to pin controller).

So, when you configuring pinmux to use group of pins to be i2c, what
does prevent you to apply those settings implicitly?

-- 
With Best Regards,
Andy Shevchenko