linux-kernel - Re: [PATCH v2] platform: x86: Add ACPI driver for ChromeOS

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Date:   Tue, 24 Mar 2020 18:16:49 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Enric Balletbo i Serra <enric.balletbo@...labora.com>
Cc:     linux-kernel@...r.kernel.org, vbendeb@...omium.org,
        groeck@...omium.org, bleung@...omium.org, dtor@...omium.org,
        gwendal@...omium.org, andy@...radead.org,
        Collabora Kernel ML <kernel@...labora.com>,
        Ayman Bagabas <ayman.bagabas@...il.com>,
        Darren Hart <dvhart@...radead.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.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>,
        Yauhen Kharuzhy <jekhor@...il.com>,
        platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH v2] platform: x86: Add ACPI driver for ChromeOS

On Tue, Mar 24, 2020 at 06:08:03PM +0100, Enric Balletbo i Serra wrote:
> Hi Greg,
> 
> On 24/3/20 17:49, Greg Kroah-Hartman wrote:
> > On Tue, Mar 24, 2020 at 05:31:10PM +0100, Enric Balletbo i Serra wrote:
> >> Hi Greg,
> >>
> >> Many thanks for your quick answer, some comments below.
> >>
> >> On 22/3/20 12:10, Greg Kroah-Hartman wrote:
> >>> On Sun, Mar 22, 2020 at 10:43:34AM +0100, Enric Balletbo i Serra wrote:
> >>>> This driver attaches to the ChromeOS ACPI device and then exports the values
> >>>> reported by the ACPI in a sysfs directory. 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 sysfs
> >>>> directory tree originating in /sys/devices/platform/chromeos_acpi.
> >>>>
> >>>> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
> >>>
> >>> What is wrong with the "default" ACPI sysfs access?  Why do you need a
> >>> special driver just for this specific ACPI firmware?
> >>>
> >>
> >> Please correct me if I am wrong, as I'm not an ACPI expert and I probably have
> >> some ACPI leaks and misunderstandings.
> >>
> >> What is exporting this driver is the attributes for the non-default Chromebook
> >> specific MLST ACPI method. Hence, I assumed we needed a special driver to expose
> >> these values that can't be done using "default" ACPI sysfs. Note that these
> >> attributes are dynamically created and are different between Chromebooks so need
> >> some parsing.
> >>
> >> I didn't find a "standard" way to expose these attributes to userspace, so,
> >> please kindly point me to one if there is one.
> > 
> > Are you sure they aren't already there under /sys/firmware/acpi/?  I
> > thought all tables and methods were exported there with no need to do
> > anything special.
> > 
> 
> That's the first I did when I started to forward port this patch from chromeos
> kernel to mainline.
> 
> On my system I get:
> 
> /sys/firmware/acpi/tables#
> APIC  DSDT  FACP  FACS  HPET  MCFG  SSDT  data  dynamic
> 
> (data and dynamic are empty directories)
> 
> I quickly concluded (maybe wrong) that as there is no a MLST entry it was not
> exported, but maybe one of those already contains the info? Or, should I expect
> a MLST entry here?
> 
> > What makes these attributes "special" from any other ACPI method?
> > 
> 
> I can't answer this question right now. I need to investigate more I guess ;-)

You can always ask the acpi developers as well, you need to get their
review for your driver anyway :)

good luck!

greg k-h