linux-kernel - Re: [PATCH v3 4/6] pinctrl: Add pinctrl binding for Broadcom Capri SoCs

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Date:	Tue, 17 Dec 2013 18:02:07 -0800
From:	Sherman Yin <syin@...adcom.com>
To:	Linus Walleij <linus.walleij@...aro.org>
CC:	Rob Herring <rob.herring@...xeda.com>,
	Heiko Stübner 
	<heiko@...ech.de>, Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Rob Landley <rob@...dley.net>,
	Christian Daudt <bcm@...thebug.org>,
	Russell King <linux@....linux.org.uk>,
	Grant Likely <grant.likely@...aro.org>,
	Matt Porter <matt.porter@...aro.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 4/6] pinctrl: Add pinctrl binding for Broadcom Capri
 SoCs

On 13-12-16 05:17 AM, Linus Walleij wrote:
> On Sat, Dec 14, 2013 at 2:16 AM, Sherman Yin <syin@...adcom.com> wrote:
>
>>>> +- bias-pull-up:                        Integer. Pull up strength in Ohm:
>>>> +                                       1: 1.2 kOhm
>>>> +                                       2: 1.8 kOhm
>>>> +                                       3: 720 Ohm
>>>> +                                       4: 2.7 kOhm
>>>> +                                       5: 831 Ohm
>>>> +                                       6: 1.08 kOhm
>>>> +                                       7: 568 Ohm
>>>
>>>
>>> Please do not use these custom enumerators 1 thru 7.
>>> Instead say that the argument is given in ohms, so for
>>> example:
>>>
>>> bias-pull-up = <12000>;
>>>
>>> Then translate this into the proper enumerator in your
>>> driver instead.
>>
>> Ok.  The 3 bits in this enumeration correspond to enabling the 3 pull up Rs
>> (1.2k, 1.8k, 2.7k) in parallel, so the enum kind of make sense. But sure I
>> can change it to actual ohms and will error out in the driver if specified
>> value is not one of the 7.
>
> Aha, that's how they do it :-)
>
> Please write this in a comment in the driver because it makes
> it a lot easier to understand what is going on, and e.g. how to
> fuzz an apropriate resistance.

Yes, I plan to explain where those odd numbers come from in this file.

> Can you tell me something about how such resistors are produced
> in silicon?

Sorry I'm not quite sure.  I only know a little bit of the circuitry 
down to the schematic level...

Regards,
Sherman
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