Re: Re: [PATCH] ARM: dts: sun4i: Add dts file for the pov protab2-ips9 tablet

[Maxime Ripard <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>]

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On Tue, Sep 08, 2015 at 09:45:52AM +0200, Hans de Goede wrote:
> Hi,
> 
> On 07-09-15 22:56, Maxime Ripard wrote:
> >On Mon, Sep 07, 2015 at 09:05:29AM +0200, Hans de Goede wrote:
> >>>>+&reg_ldo3 {
> >>>>+	/*
> >>>>+	 * We need to always power the camera sensor, otherwhise all access
> >>>>+	 * to i2c1 is blocked.
> >>>>+	 */
> >>>>+	regulator-always-on;
> >>>>+	regulator-min-microvolt = <2800000>;
> >>>>+	regulator-max-microvolt = <2800000>;
> >>>>+	regulator-name = "vdd-csi";
> >>>>+};
> >>>
> >>>What is connected on i2c1 ? Just the camera sensor? or it has some
> >>>other devices there?
> >>
> >>The bma250 accelerometer sits there, and the kernel already has a driver
> >>for it. That driver needs to have devicetree binding support added, and
> >>then we should be able to use the accelerometer.
> >
> >Ok, so if this regulator is disable, you can't access the other
> >devices as well, right?
> 
> Right, the controller reports the bus as being stuck.

Which is pretty bad... :/

> >Do you know why? Is it the regulator providing
> >the pull-up voltage?
> 
> I've tried enabling the pull ups on the SoC i2c pins, so I do not think
> that it is that, it seems that somehow when not powered the camera sensor is
> actively keeping the lines low. Either it has multiple power planes, or
> it is using normally-on fet-s between ground and its i2c lines.

Well, if a regulator is powered down, it's also tied to the ground,
which means you would actually have pull-down. Maybe the in-SoC
pullups simply aren't strong enough in such a case.

Anyway. In both cases, the regulator really shouldn't be drifting
along like this. If the i2c bus needs a regulator to be operationaly,
then we can just add an optional bus-supply property or something to
give that to the i2c driver so that it can enable it when needed.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com