On Sat, 14 May 2022 00:48:51 +0200 Peter Rosin <peda@xxxxxxxxxx> wrote: > Hi! > > 2022-05-13 at 18:45, Jonathan Cameron wrote: > > On Thu, 12 May 2022 12:08:07 -0500 > > Eddie James <eajames@xxxxxxxxxxxxx> wrote: > > > >> On 5/12/22 11:48, Jonathan Cameron wrote: > >>> On Thu, 12 May 2022 11:20:18 -0500 > >>> Eddie James <eajames@xxxxxxxxxxxxx> 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!) > > IIUC, the chip in question cannot handle *any* action on the I2C bus > for 15ms (or so) after a "soft reset", or something bad<tm> happens > (or at least may happen). > > If that's the case, then providing a means of skipping the reset is > insufficient. If you don't lock the bus, you would need to *always* > skip the reset, because you don't know for certain if something else > does I2C xfers. > > So, in order to make the soft reset not be totally dangerous even in > a normal non-muxed environment, the bus must be locked for the 15ms. > > However, Eddie is correct in that the I2C mux code may indeed do its > muxing xfer right after the soft reset command. There is currently > no way to avoid that muxing xfer. However, it should be noted that > there are ways to mux an I2C bus without using xfers on the bus > itself, so it's not problematic for *all* mux variants. > > It can be debated if the problem should be worked around with extra > dt properties like this, or if a capability should be added to delay > a trailing muxing xfer. > > I bet there are other broken chips that have drivers that do in fact > lock the bus to give the chip a break, but then it all stumbles > because of the unexpected noise if there's a (wrong kind of) mux in > the mix. Ok, so for now I think we need the bus lock for the reset + either a work around similar to this series, or additions to the i2c mux code to stop it doing a muxing xfer if the bus is locked? Jonathan > > Cheers, > Peter > > > > > 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 > >>>> > >
