linux-kernel - Re: [PATCH 18/18] ipu3: Add driver for dummy INT3472 ACPI device

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Date:   Wed, 2 Dec 2020 17:15:07 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        Dan Scally <djrscally@...il.com>, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-i2c@...r.kernel.org, linux-media@...r.kernel.org,
        devel@...ica.org, rjw@...ysocki.net, lenb@...nel.org,
        gregkh@...uxfoundation.org, mika.westerberg@...ux.intel.com,
        linus.walleij@...aro.org, bgolaszewski@...libre.com,
        wsa@...nel.org, yong.zhi@...el.com, sakari.ailus@...ux.intel.com,
        bingbu.cao@...el.com, tian.shu.qiu@...el.com, mchehab@...nel.org,
        robert.moore@...el.com, erik.kaneda@...el.com, pmladek@...e.com,
        rostedt@...dmis.org, sergey.senozhatsky@...il.com,
        linux@...musvillemoes.dk, kieran.bingham+renesas@...asonboard.com,
        jacopo+renesas@...ndi.org,
        laurent.pinchart+renesas@...asonboard.com,
        jorhand@...ux.microsoft.com, kitakar@...il.com,
        heikki.krogerus@...ux.intel.com
Subject: Re: [PATCH 18/18] ipu3: Add driver for dummy INT3472 ACPI device

On Wed, Dec 02, 2020 at 02:48:47PM +0200, Laurent Pinchart wrote:
> On Tue, Dec 01, 2020 at 09:34:58PM +0100, Hans de Goede wrote:
> > On 12/1/20 8:21 PM, Andy Shevchenko wrote:
> > > On Tue, Dec 01, 2020 at 09:06:38PM +0200, Laurent Pinchart wrote:

...

> > > I would rather ask Hans' opinion since he has quite an expertise with DMI for
> > > good and bad.
> > 
> > So generally there are 2 ways how things like this can go:
> > 
> > 1) There is sufficient information in the ACPI table and we use data from the
> > ACPI tables
> > 
> > 2) There is unsufficient info in the ACPI tables (or we don't know how to
> > get / interpret the data) and we use DMI quirks
> 
> And this specific case I believe there is sufficient data in the ACPI
> tables, as I don't believe the Windows driver uses DMI quirks, or comes
> in the form of machine-specific binaries. We however don't know how to
> interpret all the data, but that should hopefully get better over time
> (especially as we'll get more data points, with ACPI dumps from machines
> whose schematics have leaked).

I think you are too optimistic about this part of Windows drivers.
I would rather think about hardware stuck with the same frequencies which
simply are hard coded in the Windows driver.

I have description of ASL for this camera, but I don't see anything like this
you are describing.

> > Although we do often also use a combination, getting what we can from ACPI,
> > combined with a set of defaults for what we cannot get from ACPI
> > based on what reference designs use (IOW what most devices seem to have
> > copy and pasted). Combined with DMI quirks for when the defaults do not
> > work (which is quite often).
> > 
> > Depending on if "not working because of wrong defaults" has bad side effects,
> > another option is also to only allow the driver to load on devices which
> > have the necessary info provided through a DMI match.
> 
> Right now there shouldn't be bad side effects, but in the future we'll
> need to setup a PMIC whose output voltages can be controlled, and
> getting it wrong would be very bad. For that I'll definitely vote for
> DMI match to start with, but I don't think that precludes using data
> from ACPI. We could just prevent the driver from loading if the machine
> isn't whitelisted in DMI matches, and still use ACPI data.

I also think about DMI as a narrowing scope of supported platforms.

-- 
With Best Regards,
Andy Shevchenko