linux-kernel - Re: [PATCH v2 0/2] iio: humidity: si7020: Check device property for skipping reset in probe

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Date:   Thu, 12 May 2022 12:08:07 -0500
From:   Eddie James <eajames@...ux.ibm.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, krzysztof.kozlowski+dt@...aro.org,
        robh+dt@...nel.org, lars@...afoo.de, jic23@...nel.org,
        miltonm@...ibm.com
Subject: Re: [PATCH v2 0/2] iio: humidity: si7020: Check device property for
 skipping reset in probe


On 5/12/22 11:48, Jonathan Cameron wrote:
> On Thu, 12 May 2022 11:20:18 -0500
> Eddie James <eajames@...ux.ibm.com> wrote:
>
>> I2C commands issued after the SI7020 is starting up or after reset
>> can potentially upset the startup sequence. Therefore, the host
>> needs to wait for the startup sequence to finish before issuing
>> further i2c commands. This is impractical in cases where the SI7020
>> is on a shared bus or behind a mux, which may switch channels at
>> any time (generating I2C traffic). Therefore, check for a device
>> property that indicates that the driver should skip resetting the
>> device when probing.
> Why not lock the bus?  It's not ideal, but then not resetting and hence
> potentially ending up in an unknown state isn't great either.


Agreed, but locking the bus doesn't work in the case where the chip is 
behind a mux. The mux core driver deselects the mux immediately after 
the transfer to reset the si7020, causing some i2c traffic, breaking the 
si7020. So it would also be a requirement to configure the mux to idle 
as-is... That's why I went with the optional skipping of the reset. 
Maybe I should add the bus lock too?


Thanks,

Eddie


>
> Jonathan
>
>> Changes since v1:
>>   - Fix dt binding document
>>
>> Eddie James (2):
>>    dt-bindings: iio: humidity: Add si7020 bindings
>>    iio: humidity: si7020: Check device property for skipping reset in probe
>>
>>   .../bindings/iio/humidity/silabs,si7020.yaml  | 47 +++++++++++++++++++
>>   .../devicetree/bindings/trivial-devices.yaml  |  2 -
>>   drivers/iio/humidity/si7020.c                 | 14 +++---
>>   3 files changed, 55 insertions(+), 8 deletions(-)
>>   create mode 100644 Documentation/devicetree/bindings/iio/humidity/silabs,si7020.yaml
>>