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Subject: Re: [PATCH v2] iio: accel: bmc150: Check for a second ACPI device for
 BOSC0200
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Hans de Goede <hdegoede@redhat.com>,
        Hartmut Knaack <knaack.h@gmx.de>,
        Jeremy Cline <jeremy@jcline.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Jonathan Cameron <jic23@jic23.retrosnub.co.uk>,
        Lars Kellogg-Stedman <lars@oddbit.com>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
        Wolfram Sang <wsa@the-dreams.de>, linux-iio@vger.kernel.org
References: <010001602cf53153-39ad69f1-1b39-4e6d-a748-9455a16c2fbd-000000@email.amazonses.com>
 <20171210182152.70ad8fbf@archlinux>
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 <20180114104330.2aa7c1fd@archlinux> <20180128094021.572ab366@archlinux>
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 <20180130160107.000006df@huawei.com>
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 <f42f6acc-cb30-e36d-0c98-42b7e3d40457@pressers.name>
 <CAHp75VfmH5aNPzBLvm-GrQ-49uyJNNwdjbte3uh_kmaC91bz6A@mail.gmail.com>
From:   Steven Presser <steve@pressers.name>
Message-ID: <197f448e-322f-dae4-aa4b-3cc2bd5b16bb@pressers.name>
Date:   Tue, 30 Jan 2018 14:27:37 -0500
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On 01/30/2018 02:05 PM, Andy Shevchenko wrote:
> On Tue, Jan 30, 2018 at 8:34 PM, Steven Presser <steve@pressers.name> w=
rote:
>> Andy,
>>
>> I apologize for the long response, but there's several issues to addre=
ss
>> here.
> NP, it it a good explanation why. That's what commit message missed app=
arently.
Probably my fault anyway - I don't recall discussing with Jeremy exactly =

what chip was inside this little Frankenstein.
>
>> First, I believe the "bmc150" in the subject line is in some way a mis=
nomer.
>> You'd have to ask Jeremy for more details on what he intended it to re=
fer
>> to.  However, I believe the device in question is actually the bma250[=
1],
>> which does not have a magnetometer component.  I'm unfortunately away =
from
>> my notes, but I can check later if you need me to verify the exact chi=
p.
> Please do, I would really be on the safe side here.
Will do.=C2=A0 My digital notes indicate I worked from what was exposed b=
ack=20
to what chip matched.=C2=A0 If you can give me through Friday evening, I'=
ll=20
crack it and do a visual verification.=C2=A0 (Alas, I'm traveling and won=
't=20
be back to it until then).
>
>> Second, we're seeing a difference between what's in the data sheet and=

>> what's exposed in the wild via ACPI.  I own the laptop that started th=
e
>> process of building this patch and I did the original ACPI-tables
>> investigation.
>>
>> The device in question (BOSC0200) appears in the Lenovo Yoga 11e (and
>> possibly other laptops - this happens to be the one I own). These lapt=
ops
>> have a 360-degree hinge between the screen and the keyboard, letting t=
hem
>> convert into tablets, if the user desires. The 11e implements this
>> mode-switching by placing an accelerometer in each of the screen and
>> keyboard, then doing math with the resulting vectors to figure out the=
 angle
>> between the two.
> This makes a lot of sense.
>
>>   For whatever reason, Lenovo chose to expose these two
>> (physically separate) accelerometers via a single ACPI device which pr=
esents
>> two i2c devices at sequential addresses.
>
>> As part of my original investigation of the Yoga 11e, I wrote a
>> proof-of-concept of pulling accelerometer data from the two devices ex=
posed
>> under the BOSC0200 ID and using that to calculate the position of the =
screen
>> relative to the keyboard.  So based on my empirical experience, I can =
tell
>> you the BOSC0200 device ID can expose two accelerometers at sequential=

>> addresses in the wild.
>>
>> I don't understand why Lenovo has reused the BOSC0200 ACPI device ID f=
or a
>> device that is fundamentally different from the base device. The ID do=
esn't
>> belong to them and we're (apparently) now stuck in this situation wher=
e this
>> ACPI device ID could represent two different device layouts.
> Bad, bad Lenovo. (DMI strings might help here)
What particular DMI strings would be helpful?=C2=A0 All of them?
>
>> Finally - Andy, I apologize if I came across as challenging you in my
>> initial mail.  I was trying to strike a balance between brevity/respec=
ting
>> your time and asking a question.  Evidently I struck the wrong balance=
 and
>> should have given you more background on why I was doubting what you s=
aw.
>> This is my fault and you have my sincerest apologies for any offense I=
 have
>> caused.
> No need, the root cause is lack of description in the commit message.
>
> Nevertheless, the approach chosen I don't like. It looks like an ugly h=
ack.
>
> What we can do here is:
> - do not contaminate core part with I2C/SPI/etc
> - do not create another driver via board_info, we already in *the same*=
 driver,
> so, the better approach here AFAICS is to add DMI quirk into i2c-core-a=
cpi
>
>
>
>> Steve
>>
>> [1]
>> https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BMA=
250E-DS004-06.pdf


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