Re: [PATCH v2] iio: accel: bmc150: Check for a second ACPI device for BOSC0200

[Steven Presser <steve@pressers.name>]

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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> wrote:
>> Andy,
>>
>> I apologize for the long response, but there's several issues to address
>> here.
> NP, it it a good explanation why. That's what commit message missed apparently.
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 misnomer.
>> You'd have to ask Jeremy for more details on what he intended it to refer
>> 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 chip.
> Please do, I would really be on the safe side here.
Will do.  My digital notes indicate I worked from what was exposed back 
to what chip matched.  If you can give me through Friday evening, I'll 
crack it and do a visual verification.  (Alas, I'm traveling and won't 
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 the
>> 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 laptops
>> have a 360-degree hinge between the screen and the keyboard, letting them
>> 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 presents
>> 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 exposed
>> 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 for a
>> device that is fundamentally different from the base device. The ID doesn't
>> belong to them and we're (apparently) now stuck in this situation where 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?  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/respecting
>> 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 saw.
>> 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 hack.
>
> 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-acpi
>
>
>
>> Steve
>>
>> [1]
>> https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BMA250E-DS004-06.pdf



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