[PATCH v3 1/2] Document: dt: binding: imx: update pinctrl doc for imx6sll

[A.s. Dong]

> -----Original Message-----
> From: Linus Walleij [mailto:linus.walleij at linaro.org]
> Sent: 2018年2月7日 17:22
> To: Jacky Bai <ping.bai at nxp.com>; Lucas Stach <l.stach at pengutronix.de>; A.s.
> Dong <aisheng.dong at nxp.com>; Gary Bisson
> <gary.bisson at boundarydevices.com>; Vladimir Zapolskiy <vz at mleia.com>
> Cc: Rob Herring <robh+dt at kernel.org>; Shawn Guo <shawnguo at kernel.org>;
> Sascha Hauer <kernel at pengutronix.de>; Fabio Estevam
> <fabio.estevam at nxp.com>; Linux ARM <linux-arm-kernel at lists.infradead.org>;
> dl-linux-imx <linux-imx at nxp.com>; Jacky Bai <jacky.baip at gmail.com>
> Subject: Re: [PATCH v3 1/2] Document: dt: binding: imx: update pinctrl doc for
> imx6sll
> 
> On Wed, Feb 7, 2018 at 3:07 AM, Bai Ping <ping.bai at nxp.com> wrote:
> 
> > Add pinctrl binding doc update for imx6sll.
> >
> > Signed-off-by: Bai Ping <ping.bai at nxp.com>
> > ---
> > changes v2-v3:
> >  - add generic config binding
> 
> > +Optional Properties:
> > +- drive-strength               Integer. Controls Drive Strength
> > +                                       0: HI-Z
> > +                                       1: 260 ohm at 3.3v, 150
> ohm at 1.8v
> > +                                       2: 130 ohm at 3.3v, 75
> ohm at 1.8v
> > +                                       3: 86 ohm at 3.3v, 50
> ohm at 1.8v
> > +                                       4: 65 ohm at 3.3v, 37
> ohm at 1.8v
> > +                                       5: 52 ohm at 3.3v, 30
> ohm at 1.8v
> > +                                       6: 43 ohm at 3.3v, 25
> ohm at 1.8v
> > +                                       7: 37 ohm at 3.3v, 21
> ohm at 1.8v
> 
> HOW INTERESTING.
> 
> I am discussing this a lot with Lucas Stach right now, and in his SoC i.MX8MQ
> that first resistance is said to be 255 Ohm etc.
> 
> Plus it seems he's missing the figures for 1.8V.
> 

From the reference on my hand, it's

000  HI-Z — Output driver is disabled (Hi-Z State)
001  255_OHM — 255 Ohm @3.3V, 240 Ohm @2.5V, 230 Ohm @1.8V, 265 Ohm @1.2V
010  105_OHM — 105 Ohm @3.3V, 100 Ohm @2.5V, 85 Ohm @1.8V, 110 Ohm @1.2V
011  75_OHM — 75 Ohm @3.3V, 70 Ohm @2.5V, 60 Ohm @1.8V, 80 Ohm @1.2V
100  85_OHM — 85 Ohm @3.3V, 80 Ohm @2.5V, 75 Ohm @1.8V, 90 Ohm @1.2V
101  65_OHM — 65 Ohm @3.3V, 60 Ohm @2.5V, 55 Ohm @1.8V, 65 Ohm @1.2V
110  45_OHM — 45 Ohm @3.3V, 45 Ohm @2.5V, 40 Ohm @1.8V, 50 Ohm @1.2V
111  40_OHM — 40 Ohm @3.3V, 40 Ohm @2.5V, 33 Ohm @1.8V, 40 Ohm @1.2V

Which makes things a bit complicated as the value changes when voltage varies.

> We're discussing whether this is used for impedance matching and what not.
> We really need someone inside NXP to show us the way.
> 
> Anyways, I really need the two of you to discuss this, and I will forward my
> conversation with Lucas to you.
> 
> If you can fish around a bit and tell us what this is used for and how:
> 
> - Is the intended usage for impedance matching?
> - Does it control how many totem-poles are activated on the
>   output pad?
> 

I already forwarded the question to the IP owner from our IC design team,
just as I replied in another mail to you.

Regards
Dong Aisheng

> would all be very grateful.
> 
> > +- drive-open-drain             Bool. Enable Pin Open-drian
> 
> Open-drain?
> 
> Yours,
> Linus Walleij