linux-kernel - Re: [v1 1/3] media: dt-bindings: ov8856: Document YAML bindings

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Date:   Thu, 12 Mar 2020 11:13:59 +0100
From:   Robert Foss <robert.foss@...aro.org>
To:     Fabio Estevam <festevam@...il.com>
Cc:     ben.kao@...el.com, Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan.Cameron@...wei.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-media <linux-media@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        linux-mediatek@...ts.infradead.org,
        Dongchun Zhu <dongchun.zhu@...iatek.com>,
        Tomasz Figa <tfiga@...omium.org>
Subject: Re: [v1 1/3] media: dt-bindings: ov8856: Document YAML bindings

On Tue, 10 Mar 2020 at 16:51, Robert Foss <robert.foss@...aro.org> wrote:
>
> Hey Fabio,
>
> Thanks for having a look at this series so quickly.
>
> On Tue, 10 Mar 2020 at 14:57, Fabio Estevam <festevam@...il.com> wrote:
> >
> > Hi Robert,
> >
> > On Tue, Mar 10, 2020 at 10:46 AM Robert Foss <robert.foss@...aro.org> wrote:
> >
> > > +    ov8856: camera-sensor@10 {
> > > +        compatible = "ovti,ov8856";
> > > +        reg = <0x10>;
> > > +        reset-gpios = <&pio 111 GPIO_ACTIVE_HIGH>;
> >
> > Could you double check this is correct? Other OmniVision sensors have
> > reset-gpios as active low.
>
> I have tested this, unfortunately I don't have access to a ov8856
> datasheet that includes
> this level of detail. But I have tested this.
>
> >
> > I suspect that the driver has also an inverted logic, so that's why it works.
>
> That could explain it still working. Let me have a look into the
> driver and see what it does.
>

I had a look at some of OmniVision drivers, and there does seem to be
a logical inversion in some of them,
but not all of them.

ov7251:
- enable-gpios: Chip enable GPIO. Polarity is GPIO_ACTIVE_HIGH. This corresponds
  to the hardware pin XSHUTDOWN which is physically active low.

ov5640:
- reset-gpios: reference to the GPIO connected to the reset pin, if any.
           This is an active low signal to the OV5640.

I think the confusion stems from the XSHUTDOWN pin being mapped to a
GPIO called reset, and the two being logically inverted. Currently
this series does several mappings.

XSHUTDOWN -> reset-gpio -> n_shutdn_gpio
       ^                           ^                      ^
Physical Pin               DT                Driver

I think changing to what ov5640 does makes the most sense.
XSHUTDOWN -> reset-gpio -> reset_gpio

> >
> > I don't have access to the datasheet though, so I am just guessing.
>
> Me neither unfortunately, if anyone does have a link for it, I would
> very much appreciate it.