linux-kernel - Re: [RFC PATCH 3/7] i2c: allow DT nodes without 'compatible'

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Message-ID: <20200312114432.GA3384@piout.net>
Date:   Thu, 12 Mar 2020 12:44:32 +0100
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Wolfram Sang <wsa@...-dreams.de>
Cc:     Luca Ceresoli <luca@...aceresoli.net>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        linux-i3c@...ts.infradead.org,
        Kieran Bingham <kieran@...uared.org.uk>,
        Niklas Söderlund 
        <niklas.soderlund@...natech.se>, Jacopo Mondi <jacopo@...ndi.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Vladimir Zapolskiy <vz@...ia.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [RFC PATCH 3/7] i2c: allow DT nodes without 'compatible'

On 12/03/2020 12:19:51+0100, Wolfram Sang wrote:
> > Clearly this does not fit the case reported by Alexandre: a device
> > having a driver which is known to be badly buggy, so we don't want to
> > instantiate it. But again, this should not affect DT as it is not
> > describing the HW, but only an implementation detail. Probably disabling
> > or blacklisting the driver would be a better option there?
> 
> "Fixing the driver" is the first thing coming to my mind ;) But yeah,
> blacklisting would be another good solution. With only the information
> above, DT is not the right place to fix a broken driver.
> 

To be clear, the driver is working properly but the HW isn't. It is a
PMIC and we need to avoid linux talking to it so the PMIC doesn't end up
killing the bus.

We end up with a node properly described in the device tree but with
status = "disabled". The relevance to the discussion was that you know
what is there and you want to avoid using its address.

See the pmic node on i2c1 in arch/arm/boot/dts/at91-sama5d3_xplained.dts
for what I'm referring to.

> > My apologies to Wolfram, I appreciate a lot the effort you are doing,
> > but before reviewing this patch I have never realized what I tried to
> > explain above.
> 
> All good, Luca! Talking over code usually brings in viewpoints which
> have been missed so far. This is expected. Actually, I am very happy to
> have this discussion!
> 
> All the best,
> 
>    Wolfram
> 



-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com