linux-kernel - Re: [PATCH v4] platform: x86: Add ACPI driver for ChromeOS

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Date:   Tue, 14 Apr 2020 16:35:38 +0200
From:   Enric Balletbo i Serra <enric.balletbo@...labora.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Collabora Kernel ML <kernel@...labora.com>,
        Guenter Roeck <groeck@...omium.org>,
        Benson Leung <bleung@...omium.org>,
        Dmitry Torokhov <dtor@...omium.org>,
        Gwendal Grignou <gwendal@...omium.org>, vbendeb@...omium.org,
        Andy Shevchenko <andy@...radead.org>,
        Ayman Bagabas <ayman.bagabas@...il.com>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Blaž Hrastnik <blaz@...n.io>,
        Darren Hart <dvhart@...radead.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Jeremy Soller <jeremy@...tem76.com>,
        Mattias Jacobsson <2pi@....nu>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Rajat Jain <rajatja@...gle.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Platform Driver <"platform- driver-x86"@vger.kernel.org>
Subject: Re: [PATCH v4] platform: x86: Add ACPI driver for ChromeOS

Hi Rafael,

On 13/4/20 22:41, Rafael J. Wysocki wrote:
> On Mon, Apr 13, 2020 at 3:46 PM Enric Balletbo i Serra
> <enric.balletbo@...labora.com> wrote:
>>
>> This driver attaches to the ChromeOS ACPI device and then exports the values
>> reported by the ACPI in a sysfs directory. These values are not exported
>> via the standard ACPI tables, hence a specific driver is needed to do
>> it.
> 
> So how exactly are they exported?
> 

They are exported through sysfs.

>> The ACPI values are presented in the string form (numbers as decimal
>> values) or binary blobs, and can be accessed as the contents of the
>> appropriate read only files in the standard ACPI devices sysfs directory tree.
> 
> My understanding based on a cursory look at the patch is that there is
> an ACPI device with _HID equal to "GGL0001"  and one or more special
> methods under it that return values which you want to export over
> sysfs as binary attributes.  They appear to be read-only.
> 

Exactly, there is an ACPI device equal to "GGL0001" and one special method
called MLST that returns a list of the other control methods supported by the
Chrome OS hardware device. The driver calls the special MLST method and goes
through the list.

> I guess that these data are to be consubed by user space?
> 

Yes, this is used by user space, to be more specific ChromeOS userspace uses it.