linux-kernel - Re: [PATCH 4/4] ARM: dts: sun8i: add OTG function to Lichee Pi Zero

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Date:   Thu, 12 Jan 2017 08:50:14 -0600
From:   Bin Liu <b-liu@...com>
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>
CC:     Icenowy Zheng <icenowy@...c.xyz>, Chen-Yu Tsai <wens@...e.org>,
        Kishon Vijay Abraham I <kishon@...com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCH 4/4] ARM: dts: sun8i: add OTG function to Lichee Pi Zero

On Wed, Jan 11, 2017 at 10:06:38PM +0100, Maxime Ripard wrote:
> On Wed, Jan 11, 2017 at 02:08:11PM -0600, Bin Liu wrote:
> > On Thu, Jan 12, 2017 at 03:55:33AM +0800, Icenowy Zheng wrote:
> > > 
> > > 
> > > 11.01.2017, 04:24, "Bin Liu" <b-liu@...com>:
> > > > On Tue, Jan 03, 2017 at 11:25:34PM +0800, Icenowy Zheng wrote:
> > > >>  Lichee Pi Zero features a USB OTG port.
> > > >>
> > > >>  Add support for it.
> > > >>
> > > >>  Note: in order to use the Host mode, the board must be powered via the
> > > >>  +5V and GND pins.
> > > >>
> > > >>  Signed-off-by: Icenowy Zheng <icenowy@...c.xyz>
> > > >>  ---
> > > >>   arch/arm/boot/dts/sun8i-v3s-licheepi-zero.dts | 10 ++++++++++
> > > >>   1 file changed, 10 insertions(+)
> > > >>
> > > >>  diff --git a/arch/arm/boot/dts/sun8i-v3s-licheepi-zero.dts b/arch/arm/boot/dts/sun8i-v3s-licheepi-zero.dts
> > > >>  index 0099affc6ce3..3d9168cbaeca 100644
> > > >>  --- a/arch/arm/boot/dts/sun8i-v3s-licheepi-zero.dts
> > > >>  +++ b/arch/arm/boot/dts/sun8i-v3s-licheepi-zero.dts
> > > >>  @@ -71,3 +71,13 @@
> > > >>           pinctrl-names = "default";
> > > >>           status = "okay";
> > > >>   };
> > > >>  +
> > > >>  +&usb_otg {
> > > >>  + dr_mode = "otg";
> > > >
> > > > Why not set this default mode in dtsi instead?
> > > >
> > > > Regards,
> > > > -Bin.
> > > 
> > > There's possibly boards which do not have OTG functions.
> > 
> > That is board specific.
> 
> Exactly, and this is why it should be done in the board DT.

I am just suggesting based on the common practice. If a .dtsi exists for
a family, the .dtsi describes the device and common properties for all
possible boards, and each board .dts adds or overrides its specific
implementation. Kernel has many devices/boards done in this way - define
the default dr_mode in .dtsi.

In this case, I suggest to set the common dr_mode in .dtsi, then each
board .dts only overrides it if the implementation is different. 

> 
> The controller in the Allwinner SoCs do not handle directly the ID pin
> and VBUS, but rather rely on a GPIO to do so.
> 
> So boards with OTG will need setup anyway, at least to tell which
> GPIOs are used. There's no point in enforcing a default if it doesn't
> work by default.

Then define a default which supposes to work for most boards.

Why I suggest this, is because defining a default dr_mode which works
for most cases in dtsi could prevent a little surprise in MUSB function.
If someone designs a new board but forgets to define dr_mode in the new
board DT, the MUSB driver will default to org mode, which might not be
intended.

Regards,
-Bin.