[linux-sunxi] [PATCH 1/3] ARM: dts: sun4i: Allow to use the PH6 pin for GPIO on pcDuino1/2

[Siarhei Siamashka]

On Mon, 5 Oct 2015 08:39:40 +0300
Siarhei Siamashka <siarhei.siamashka at gmail.com> wrote:

> On Mon, 5 Oct 2015 09:55:28 +0800
> Chen-Yu Tsai <wens at csie.org> wrote:
> 
> > On Mon, Oct 5, 2015 at 2:58 AM, Siarhei Siamashka
> > <siarhei.siamashka at gmail.com> wrote:
> > > The pcDuino1 board does not use any power switches at all for its
> > > two USB host ports and the VBUS pins are always connected to 5V.
> > >
> > > The pcDuino2 board uses the RT9701GB power switch for its single
> > > USB host port, but the USB_EN pin (PD2) is pulled up with a 10K
> > > resistor. So that the USB power is still enabled by default even
> > > if nobody bothers to configure the PD2 pin or runs the pcDuino1
> > > firmware.
> > 
> > Seems like it would be better if you had a regulator controlled
> > by PD2. At least can shut down VBUS power when it wants to?
> 
> That's a good question.
> 
> Describing the regulator controlled by PD2 in the dts file is surely
> the right solution for pcDuino2 boards. But in the case of using this
> dts for pcDuino1, the kernel would think that it can shut down VBUS
> power, while in fact this is not true.
> 
> The RT9701GB switch also provides the current limiting feature in
> addition to the ability to enable/disable the VBUS power. Probably
> this was a real reason why it was added to the board.
> 
> Everything boils down to the question whether we want to have a
> common dts file for pcDuino1 and pcDuino2 or decide to split them.
> Based on the schematics comparison, there do not seem to be any
> substantial differences between these boards if we ignore the PD2
> pin altogether. LinkSprite says that "Ubuntu images are same for
> both pcDuino1 and pcDuino2" at their website:
>     http://www.linksprite.com/?page_id=809
> 
> And I actually like their decision to have the PD2 pin pulled-up.

I'm not sure if everyone would like this, but the following trick
works. If we configure the PD2 pin as input with pull-down on the SoC
side and read it, then it still reads as 1 on pcDuino2. Which means
that the pull-up is apparently stronger (by how much and whether this
is really reliable is another question). It should read as 0 on
pcDuino1 and we might use this to detect the board type at runtime.

Still it is probably an overkill for just this really minor thing :-)

-- 
Best regards,
Siarhei Siamashka