linux-kernel - Re: [PATCH v6 05/15] regulator: Introduce tps68470-regulator driver

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Date:   Mon, 29 Nov 2021 17:35:20 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Mark Gross <markgross@...nel.org>,
        Andy Shevchenko <andy@...radead.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Daniel Scally <djrscally@...il.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, Len Brown <lenb@...nel.org>,
        linux-acpi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Kate Hsuan <hpa@...hat.com>, linux-media@...r.kernel.org,
        linux-clk@...r.kernel.org,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [PATCH v6 05/15] regulator: Introduce tps68470-regulator driver

Hi Mark,

On Mon, Nov 29, 2021 at 12:08:06PM +0000, Mark Brown wrote:
> On Sun, Nov 28, 2021 at 01:38:34AM +0200, Laurent Pinchart wrote:
> > On Fri, Nov 26, 2021 at 12:22:35PM +0100, Hans de Goede wrote:
> > > On 11/26/21 00:32, Laurent Pinchart wrote:
> > > > On Thu, Nov 25, 2021 at 05:54:02PM +0100, Hans de Goede wrote:
> > > >> The TPS68470 PMIC provides Clocks, GPIOs and Regulators. At present in
> > > >> the kernel the Regulators and Clocks are controlled by an OpRegion
> > > >> driver designed to work with power control methods defined in ACPI, but
> 
> Please delete unneeded context from mails when replying.  Doing this
> makes it much easier to find your reply in the message, helping ensure
> it won't be missed by people scrolling through the irrelevant quoted
> material.

I have mixed feelings about that, someones the context is indeed not
needed, but I've found myself more often than not replying deep in a
mail thread and wishing the context hadn't been deleted, because it
ended up being relevant.

> > > >> + * (1) This regulator must have the same voltage as VIO if S_IO LDO is used to
> > > >> + *     power a sensor/VCM which I2C is daisy chained behind the PMIC.
> > > >> + * (2) If there is no I2C daisy chain it can be set freely.
> > > >> + */
> 
> > > > Do we need safety checks for this ?
> 
> > > There really is no way to deal this condition needs to matches inside the driver,
> > > this should be enforced by setting proper constraints on the 2 regulators where
> > > the PMIC is used with a sensor I2C daisy chained behind it.
> 
> > Right. I tend to be cautious here, as incorrect settings can destroy the
> > hardware. We should err on the side of too many safety checks rather
> > than too few. I was thinking that the cio2-bridge driver could set a
> > daisy-chaining flag, which could trigger additional checks here, but it
> > wouldn't protect against someone experimenting to support a new device
> > and setting different voltages without the daisy-chaining flag.
> 
> > My biggest worry is that someone with an unsupported machine may start
> > by copying and pasting an existing configuration to try it out, and fry
> > their hardware.
> 
> There's really nothing you can do that prevents this, especially in the
> cut'n'paste scenario.  Overrides tend to get copied along with the rest
> of the configuration, or checks hacked out if people think they're
> getting in the way without realising what they're there for.

Maybe a big fat warning comment in the code ? Apart from that, I agree,
I don't think we can do much.

-- 
Regards,

Laurent Pinchart