Re: [PATCH] dt-bindings: soc: imx8mp-media-blk-ctrl: Add LDB into DT example

From: Rob Herring <robh@kernel.org>
To: Marek Vasut <marex@denx.de>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
	linux-arm-kernel@lists.infradead.org,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Lucas Stach <l.stach@pengutronix.de>,
	NXP Linux Team <linux-imx@nxp.com>,
	Paul Elder <paul.elder@ideasonboard.com>,
	Peng Fan <peng.fan@nxp.com>,
	devicetree@vger.kernel.org
Subject: Re: [PATCH] dt-bindings: soc: imx8mp-media-blk-ctrl: Add LDB into DT example
Date: Thu, 30 Jun 2022 18:09:59 -0600	[thread overview]
Message-ID: <20220701000959.GA3588170-robh@kernel.org> (raw)
In-Reply-To: <247d48ae-d22f-4adc-07c0-74dbccfc9390@denx.de>

On Thu, Jun 30, 2022 at 09:50:31PM +0200, Marek Vasut wrote:
> On 6/30/22 21:31, Krzysztof Kozlowski wrote:
> 
> Hi,
> 
> [...]
> 
> > > > > diff --git a/Documentation/devicetree/bindings/soc/imx/fsl,imx8mp-media-blk-ctrl.yaml b/Documentation/devicetree/bindings/soc/imx/fsl,imx8mp-media-blk-ctrl.yaml
> > > > > index b246d8386ba4a..05a19d3229830 100644
> > > > > --- a/Documentation/devicetree/bindings/soc/imx/fsl,imx8mp-media-blk-ctrl.yaml
> > > > > +++ b/Documentation/devicetree/bindings/soc/imx/fsl,imx8mp-media-blk-ctrl.yaml
> > > > > @@ -18,11 +18,18 @@ properties:
> > > > >      compatible:
> > > > >        items:
> > > > >          - const: fsl,imx8mp-media-blk-ctrl
> > > > > +      - const: simple-mfd
> > > > 
> > > > Not really... simple-mfd means devices is really simple and you just use
> > > > it to instantiate children. However this is not simple - it's a power
> > > > domain controller with several clocks and power domains as input.
> > > > 
> > > > It's not a simple MFD, but a regular device.
> > > 
> > > I don't understand this comment. The LDB bridge is literally two
> > > registers with a few bits in this media block controller register area.
> > > Can you expand on why the simple-mfd is unsuitable and what should it be
> > > instead ?
> > 
> > Looking at the bindings you have there 10 power domains, 10 input clocks
> > and a domain provider. The driver is also not that simple which is
> > another argument that this is not simple-mfd. Simply, it is not simple.
> > 
> > What I meant, is that probably you should populate children from the
> > driver instead of adding simple-mfd compatible. Once you add simple-mfd,
> > you cannot remove it and children cannot use anything from the parent.
> 
> No, I don't think so.
> 
> The block controller provides those 10 power domains, those are separate
> things controlled by separate registers within the block control register
> space.
> 
> This LDB bridge are two more completely unrelated registers which have
> nothing to do with those power domains . They are just in the same register
> block because they had to put those registers somewhere. And they are mixed
> literally in the middle of the register block, because there was space it
> seems. Hence the simple-mfd is I think the right thing here.

Was 'simple-mfd' missing or LDB bridge wasn't a child node? I though 
this was a no functional change patch. Seems more than just update the 
example.

'simple-mfd' is saying the child has 0 dependence on the parent. IMO, 
'syscon' contradicts that, but that's an all to common pattern. You are 
saying all the clocks (or any other resources) in the parent can be off 
and the LDB bridge is still functional.

Rob