linux-kernel - Re: [PATCH v20 1/4] usb: Add support for Intel LJCA device

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Message-ID: <9a080d06-586d-686f-997e-674cb8d16099@redhat.com>
Date:   Sat, 14 Oct 2023 12:58:06 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     "Shevchenko, Andriy" <andriy.shevchenko@...el.com>
Cc:     "Wu, Wentong" <wentong.wu@...el.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "oneukum@...e.com" <oneukum@...e.com>,
        "wsa@...nel.org" <wsa@...nel.org>,
        "andi.shyti@...ux.intel.com" <andi.shyti@...ux.intel.com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "bartosz.golaszewski@...aro.org" <bartosz.golaszewski@...aro.org>,
        "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "sakari.ailus@...ux.intel.com" <sakari.ailus@...ux.intel.com>,
        "Wang, Zhifeng" <zhifeng.wang@...el.com>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v20 1/4] usb: Add support for Intel LJCA device

Hi Andy,

On 10/13/23 22:05, Shevchenko, Andriy wrote:
> On Thu, Oct 12, 2023 at 01:14:23PM +0200, Hans de Goede wrote:

<snip>

>> Ah ok, I see. So the code:
>>
>> 1. First tries to find the matching child acpi_device for the auxdev by ADR
>>
>> 2. If 1. fails then falls back to HID + UID matching
>>
>> And there are DSDTs which use either:
>>
>> 1. Only use _ADR to identify which child device is which, like the example
>>    DSDT snippet from the commit msg.
>>
>> 2. Only use _HID + _UID like the 2 example DSDT snippets from me email
>>
>> But there never is a case where both _ADR and _HID are used at
>> the same time (which would be an ACPI spec violation as Andy said).
>>
>> So AFAICT there is no issue here since  _ADR and _HID are never
>> user at the same time and the commit message correctly describes
>> scenario 1. from above, so the commit message is fine too.
>>
>> So I believe that we can continue with this patch series in
>> its current v20 form, which has already been staged for
>> going into -next by Greg.
>>
>> Andy can you confirm that moving ahead with the current
>> version is ok ?
> 
> Yes as we have a few weeks to fix corner cases.
> 
> What I'm worrying is that opening door for _ADR that seems never used is kinda
> an overkill here (resolving non-existing problem).

I assume that there actually some DSDTs using the _ADR approach
and that this support is not there just for fun.

Wentong, can you confirm that the _ADR using codepaths are
actually used on some hardware / with some DSDTs out there ?

> Looking at the design of the
> driver I'm not sure why ACPI HIDs are collected somewhere else than in the
> respective drivers. And looking at the ID lists themselves I am not sure why
> the firmware of the respective hardware platforms are not using _CID.

This is a USB device which has 4 functions:

1. GPIO controller
2. I2C controller 1
3. I2C controller 2
4. SPI controller

The driver for the main USB interface uses
the new auxbus to create 4 child devices. The _ADR
or if that fails _HID + _UID matching is done to
find the correct acpi_device child of the acpi_device
which is the ACPI-companion of the main USB device.

After looking up the correct acpi_device child
this is then set as the fwnode / ACPI-companion
of the auxbus device created for that function.

Having the correct fwnode is important because other
parts of the DSDT reference this fwnode to specify
GPIO / I2C / SPI resources and if the fwnode of
the aux-device is not set correctly then the resources
for other devices referencing it (typically a camera
sensor) can not be found.

As for why the driver for the auxbus devices / children
do not use HID matching, AFAIK the auxbus has no support
for using ACPI (or DT) matching for aux-devices and these
drivers need to be auxiliary_driver's and bind to the
auxbus device and not to a platform_device instantiated for
the acpi_device since they need the auxbus device to access
the USB device.

Regards,

Hans