Re: [PATCH v7 4/6] dt-bindings: mailbox: imx-mu: add i.MX6SX and i.MX7S SoCs.

From: Lucas Stach <l.stach@pengutronix.de>
To: Jassi Brar <jassisinghbrar@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	"A.s. Dong" <aisheng.dong@nxp.com>,
	Devicetree List <devicetree@vger.kernel.org>,
	Oleksij Rempel <o.rempel@pengutronix.de>,
	Rob Herring <robh+dt@kernel.org>,
	Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>, ",
	Sascha Hauer" <kernel@pengutronix.de>,
	Fabio Estevam <fabio.estevam@nxp.com>,
	Shawn Guo <shawnguo@kernel.org>, ",
	linux-arm-kernel@lists.infradead.org,
	 linux-mediatek@lists.infradead.org,
	srv_heupstream" <linux-arm-kernel@lists.infradead.org>,
	dl-linux-imx <linux-imx@nxp.com>
Subject: Re: [PATCH v7 4/6] dt-bindings: mailbox: imx-mu: add i.MX6SX and i.MX7S SoCs.
Date: Thu, 26 Jul 2018 13:35:37 +0200	[thread overview]
Message-ID: <1532604937.32306.30.camel@pengutronix.de> (raw)
In-Reply-To: <CABb+yY1GEmw_8q2HaF8y5VfFJX_c_0_MbWtPUE_j_o-zS8iZ_g@mail.gmail.com>

Am Donnerstag, den 26.07.2018, 16:45 +0530 schrieb Jassi Brar:
> On Thu, Jul 26, 2018 at 4:11 PM, Lucas Stach <l.stach@pengutronix.de>
> wrote:
> > Hi Jassi,
> > 
> > Am Donnerstag, den 26.07.2018, 15:25 +0530 schrieb Jassi Brar:
> > > On Thu, Jul 26, 2018 at 12:23 PM, Oleksij Rempel
> > > > <o.rempel@pengutronix.de> wrote:
> > > > This are currently tested SoCs with imx-mailbox driver.
> > > > 
> > > > > > Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> > > > 
> > > > ---
> > > >  Documentation/devicetree/bindings/mailbox/fsl,mu.txt | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git
> > > > a/Documentation/devicetree/bindings/mailbox/fsl,mu.txt
> > > > b/Documentation/devicetree/bindings/mailbox/fsl,mu.txt
> > > > index 113d6ab931ef..5616d2afca45 100644
> > > > --- a/Documentation/devicetree/bindings/mailbox/fsl,mu.txt
> > > > +++ b/Documentation/devicetree/bindings/mailbox/fsl,mu.txt
> > > > @@ -18,7 +18,7 @@ Messaging Unit Device Node:
> > > >  Required properties:
> > > >  -------------------
> > > >  - compatible : should be "fsl,<chip>-mu", the supported chips
> > > > include
> > > > -               imx8qxp, imx8qm.
> > > > +               imx6sx, imx7s, imx8qxp, imx8qm.
> > > > 
> > > 
> > > This is not scalable. Do we add every new SoC that contains the
> > > same controller?
> > 
> > Yes, we do. This is a policy direction from the DT maintainers.
> > 
> 
> I would love to read the post/documentation.
> 
> Consider the same h/w - controller and platforms, but only the the MU
> chapter said the controller name is, say, 'MU121'. I am sure now you
> will see it correct to call it "fsl,mu121" compatible.
> What changed? just the name, right?
> 
> 
> > If we
> > ever going to want to validate DTs against the binding, all
> > compatibles
> > used in the DTs must be specified in the binding.
> > 
> > As we can't really tell if the controller is exactly the same or
> > even
> > has some SoC integration bugs, we generally add a new compatible
> > for
> > each SoC to key off any workarounds necessary in the driver without
> > the
> > need to change the DTs, breaking compatibility.
> > 
> 
> I think if the h/w resources and behaviour remain the same and the
> documentation does not call it by a different name -- it is safe to
> assume its the same IP. Especially when the driver is absolutely
> indifferent to the 5 SoC names.

Even if it is the same IP core, the SoC integration might have bugs
that need different behavior from the driver. We've already had that
case with the i.MX6 SPI controller.

> If/when we find the controller changes, we could revisit the binding
> and add another compatible option and modify the driver to catch that
> and adapt.

That's way too late. If we want to keep DTs stable the DT must include
forward looking compatibles, to allow the driver to key any required
behavior changes from already present compatibles. Otherwise you
require the DT to be updated in step-lock with the kernel or need much
more awkward detection in the drivers. IP integration bugs might be
found way after the initial DT has been written.

Regards,
Lucas

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