linux-kernel - Re: [RFC PATCH v3 9/9] ipu3-cio2: Add functionality allowing software_node connections to sensors on platforms designed for Windows

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Message-ID: <20201029212930.GE15024@pendragon.ideasonboard.com>
Date:   Thu, 29 Oct 2020 23:29:30 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Dan Scally <djrscally@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        prabhakar.mahadev-lad.rj@...renesas.com,
        "Krogerus, Heikki" <heikki.krogerus@...ux.intel.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        laurent.pinchart+renesas@...asonboard.com,
        kieran.bingham+renesas@...asonboard.com,
        Jacopo Mondi <jacopo+renesas@...ndi.org>,
        Rob Herring <robh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tian Shu Qiu <tian.shu.qiu@...el.com>,
        Bingbu Cao <bingbu.cao@...el.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Yong Zhi <yong.zhi@...el.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tsuchiya Yuto <kitakar@...il.com>
Subject: Re: [RFC PATCH v3 9/9] ipu3-cio2: Add functionality allowing
 software_node connections to sensors on platforms designed for Windows

On Thu, Oct 29, 2020 at 10:26:56PM +0200, Andy Shevchenko wrote:
> On Thu, Oct 29, 2020 at 10:21 PM Laurent Pinchart wrote:
> > On Mon, Oct 26, 2020 at 06:10:50PM +0200, Andy Shevchenko wrote:
> > > On Sat, Oct 24, 2020 at 12:37:02PM +0300, Laurent Pinchart wrote:
> > > > On Sat, Oct 24, 2020 at 09:50:07AM +0100, Dan Scally wrote:
> > > > > On 24/10/2020 02:24, Laurent Pinchart wrote:
> > > > > > On Mon, Oct 19, 2020 at 11:59:03PM +0100, Daniel Scally wrote:
> > >
> > > > > >> +              adev = acpi_dev_get_first_match_dev(supported_devices[i], NULL, -1);
> > > > > >
> > > > > > What if there are multiple sensor of the same model ?
> > > > >
> > > > > Hmm, yeah, that would be a bit of a pickle. I guess the newer
> > > > > smartphones have multiple sensors on the back, which I presume are the
> > > > > same model. So that will probably crop up at some point. How about
> > > > > instead I use bus_for_each_dev() and in the applied function check if
> > > > > the _HID is in the supported list?
> > > >
> > > > Sounds good to me.
> > > >
> > > > > >> +              if (!adev)
> > > > > >> +                      continue;
> > >
> > > Please, don't.
> > >
> > > If we have so weird ACPI tables it must be w/a differently. The all, even badly
> > > formed, ACPI tables I have seen so far are using _UID to distinguish instance
> > > of the device (see second parameter to the above function).
> > >
> > > If we meet the very broken table I would like rather to know about, then
> > > silently think ahead what could be best.
> > >
> > > I.o.w. don't change this until we will have a real example of the problematic
> > > firmware.
> >
> > I'm not sure to follow you. Daniel's current code loops over all the
> > supported HID (as stored in the supported_devices table), and then gets
> > the first ACPI device for each of them. If multiple ACPI devices exist
> > with the same HID, we need to handle them all, so enumerating all ACPI
> > devices and checking whether their HID is one we handle seems to be the
> > right option to me.
> 
> Devices with the same HID should be still different by another
> parameter in ACPI. The above mentioned call just uses the rough
> estimation for relaxed conditions. If you expect more than one device
> with the same HID how do you expect to distinguish them? The correct
> way is to use _UID. It may be absent, or set to a value. And this
> value should be unique (as per U letter in UID abbreviation). That
> said, the above is good enough till we find the firmware with the
> above true (several devices with the same HID). Until then the code is
> fine.

I expect those devices with the same _HID to have different _UID values,
yes. On the systems I've seen so far, that assumption is not violated,
and I don't think we need to already plan how we will support systems
where multiple devices would have the same _HID and _UID (within the
same scope). There's no disagreement there.

My point is that supported_devices stores HID values, and doesn't care
about UID. The code loops over supported_devices, and for each entry,
calls acpi_dev_get_first_match_dev() and process the ACPI devices
returned by that call. We thus process at most one ACPI device per HID,
which isn't right.

-- 
Regards,

Laurent Pinchart