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

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Date:   Tue, 1 Dec 2020 21:34:58 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     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

Hi,

On 12/1/20 8:21 PM, Andy Shevchenko wrote:
> On Tue, Dec 01, 2020 at 09:06:38PM +0200, Laurent Pinchart wrote:
>> On Tue, Dec 01, 2020 at 09:05:23PM +0200, Andy Shevchenko wrote:
>>> On Tue, Dec 01, 2020 at 08:55:48PM +0200, Laurent Pinchart wrote:
>>>> On Tue, Dec 01, 2020 at 08:54:17PM +0200, Andy Shevchenko wrote:
>>>>> On Tue, Dec 01, 2020 at 08:30:03AM +0000, Dan Scally wrote:
>>>>>> On 30/11/2020 20:07, Andy Shevchenko wrote:
>>>
>>> ...
>>>
>>>>>>>> +static struct int3472_sensor_regulator_map int3472_sensor_regulator_maps[] = {
>>>>>>>> +	{ "GNDF140809R", 2, miix_510_ov2680 },
>>>>>>>> +	{ "YHCU", 2, surface_go2_ov5693 },
>>>>>>>> +	{ "MSHW0070", 2, surface_book_ov5693 },
>>>>>>>> +};
>>>>>>>
>>>>>>> Hmm... Usual way is to use DMI for that. I'm not sure above will not give us
>>>>>>> false positive matches.
>>>>>>
>>>>>> I considered DMI too, no problem to switch to that if it's a better choice.
>>>>>
>>>>> I prefer DMI as it's a standard way to describe platform quirks in x86 world.
>>>>
>>>> Do you think the Windows driver would use DMI ?
>>>
>>> Linux is using DMI for quirks.
>>>
>>>> That seems quite
>>>> unlikely to me, given how they would have to release a new driver binary
>>>> for every machine. I'm pretty sure that a different mechanism is used to
>>>> identify camera integration, and I think it would make sense to follow
>>>> the same approach. That would allow us to avoid large tables of DMI
>>>> identifiers that would need to be constently updated, potentially making
>>>> user experience better.
>>>
>>> All Surface family can be matched in a way as Apple machines [1].
>>>
>>> [1]: https://lkml.org/lkml/2020/4/15/1198
>>
>> But not all Surface machines necessarily have the same camera
>> architecture. My point is that there seems to be identifiers reported in
>> ACPI for the exact purpose of identifying the camera architecture. If we
>> used DMI instead, we would have to handle each machine individually.
> 
> With help of DMI we may narrow down the search.
> 
> But again, we are talking about uncertainity. It may be your way (a lot of
> platforms that have different settings), or mine (only a few with more or less
> standard sets of settings).
> 
> DMI is simply standard in Linux (people usually easier can grep for quirks for
> a specific platform).
> 
> 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

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.

I hope this helps.

Regards,

Hans