linux-kernel - Re: [PATCH 1/2] drm/bridge: sii902x: Enable I/O and core VCC supplies if present

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Date:   Mon, 28 Sep 2020 21:03:41 +0200
From:   Sam Ravnborg <sam@...nborg.org>
To:     "Alex G." <mr.nuke.me@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        David Airlie <airlied@...ux.ie>,
        Mark Brown <broonie@...nel.org>,
        Jonas Karlman <jonas@...boo.se>, linux-kernel@...r.kernel.org,
        Andrzej Hajda <a.hajda@...sung.com>,
        Rob Herring <robh+dt@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH 1/2] drm/bridge: sii902x: Enable I/O and core VCC
 supplies if present

Hi Alex.

On Mon, Sep 28, 2020 at 12:35:01PM -0500, Alex G. wrote:
> On 9/26/20 1:49 PM, Sam Ravnborg wrote:
> > Hi Alexandru
> > 
> > On Thu, Sep 24, 2020 at 03:05:05PM -0500, Alexandru Gagniuc wrote:
> > > On the SII9022, the IOVCC and CVCC12 supplies must reach the correct
> > > voltage before the reset sequence is initiated. On most boards, this
> > > assumption is true at boot-up, so initialization succeeds.
> > > 
> > > However, when we try to initialize the chip with incorrect supply
> > > voltages, it will not respond to I2C requests. sii902x_probe() fails
> > > with -ENXIO.
> > > 
> > > To resolve this, look for the "iovcc" and "cvcc12" regulators, and
> > > make sure they are enabled before starting the reset sequence. If
> > > these supplies are not available in devicetree, then they will default
> > > to dummy-regulator. In that case everything will work like before.
> > > 
> > > This was observed on a STM32MP157C-DK2 booting in u-boot falcon mode.
> > > On this board, the supplies would be set by the second stage
> > > bootloader, which does not run in falcon mode.
> > > 
> > > Signed-off-by: Alexandru Gagniuc <mr.nuke.me@...il.com>
> > 
> > One nitpick here. The binding should be present in the tree before
> > you start using it. So this patch should be applied after the binding.
> 
> It is :)
>   * arch/arm/boot/dts/stm32mp15xx-dkx.dtsi

This is the device tree. So essentially there is part of the device
tree that is not yet documented - so in a perfect world all parts of the
device tree is documented in bindings
(Documentation/devicetree/bindings/* ) before the device tree is
committed.

	Sam