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

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Thu, 12 May 2022 12:08:07 -0500
Eddie James <eajames@linux.ibm.com> wrote:

> On 5/12/22 11:48, Jonathan Cameron wrote:
> > On Thu, 12 May 2022 11:20:18 -0500
> > Eddie James <eajames@linux.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?
> 

+Cc Peter and linux-i2c for advice as we should resolve any potential
issue with the mux side of things rather than hiding it in the driver
(if possible!)

Jonathan




> 
> 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
> >>