linux-kernel - Re: [PATCH 1/2] arm64: dts: imx8mp-verdin: add dsi to hdmi functionality

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Message-ID: <YxNM75ZtHVKDjTa/@livingston.pivistrello.it>
Date:   Sat, 3 Sep 2022 14:47:43 +0200
From:   Francesco Dolcini <francesco@...cini.it>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Francesco Dolcini <francesco.dolcini@...adex.com>,
        Max Krummenacher <max.oss.09@...il.com>,
        Max Krummenacher <max.krummenacher@...adex.com>,
        Fabio Estevam <festevam@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Marcel Ziswiler <marcel.ziswiler@...adex.com>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Philippe Schenker <philippe.schenker@...adex.com>,
        Rob Herring <robh+dt@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64: dts: imx8mp-verdin: add dsi to hdmi
 functionality

On Sat, Sep 03, 2022 at 03:24:51AM +0300, Laurent Pinchart wrote:
> Hi Francesco,
> 
> On Fri, Sep 02, 2022 at 05:57:20PM +0200, Francesco Dolcini wrote:
> > Hello Laurent,
> > answering here for both patches (1/2 and 2/2).
> > 
> > On Thu, Sep 01, 2022 at 09:07:49PM +0300, Laurent Pinchart wrote:
> > > On Thu, Sep 01, 2022 at 05:40:50PM +0200, Max Krummenacher wrote:
> > > > From: Max Krummenacher <max.krummenacher@...adex.com>
> > > > 
> > > > Add the hdmi connector present on the dsi to hdmi adapter now
> > > > required by the upstream lontium bridge driver.
> > > > The dsi to hdmi adapter is enabled in an device tree overlay.
> > > 
> > > Shouldn't the connector also be in the overlay ? There's certainly no
> > > physical HDMI connector on the i.MX8MP Verdin SoM :-)
> > 
> > Toradex DTS include and overlay files structure so far has been a little
> > bit different and not following the expectation you just stated here,
> > you can just check the current *toradex*dts* files and you'll see that there
> > is other stuff that is not strictly part of the module.
> > 
> > Copying from a previous email thread on a very similar discussion [0]
> > some of the reasons:
> > 
> >  - The SoM dtsi representing not only the functionality implemented into
> >    the SoM, but the whole connector pinout to the carrier makes very easy
> >    to just include a different som.dtsi in the carrier board dts and just
> >    switch SoM, for example from a colibri-imx6 to a colibri-imx7.
> 
> That's fine, but I don't see how that's related to the issue at hand.
> The DSI to HDMI bridge wouldn't be present on either SoM, would it ?
> 
> >  - We avoid code duplication
> > 
> > This is working for us pretty well so far and the majority of the users
> > of ours modules rely on this structure, we would prefer not to change that.
> 
> It may work for your current use cases, but it doesn't make it right :-)

Most of engineering is about compromise, being consistent with what we
did so far and the end-user experience need to be taken into account.

> Someone can integrate a Verdin SoM with a carrier board that has no DSI
> to HDMI (or LVDS) bridge, there should thus be no such device in the
> device tree. The SoM has DSI signals present on its connector, that's
> what the SoM .dtsi should expose.

Just for the record Verdin i.MX8M Plus do have both HDMI and LVDS on the
connector (in addition to DSI) [1], of course we do have also the option to
have LVDS or HDMI using an external add-on DSI bridge as this patches are
about.

Said that it's true that sometime we describe peripherals that are part of the
SOM family into the SOM dtsi, this avoid quite a lot of duplications given the
amount of carrier board that are available on the market that use just the same
building blocks (and this was one of the 2 points I mentioned as a reasoning
for our current DTS files structure).

Of course, we keep these stuff disabled by default, so apart for some small size
increase I do not see a real issue.

Francesco

[1] https://docs.toradex.com/110977-verdin_imx8m_plus_v1.1_datasheet.pdf