Messages in this thread | | | Date | Wed, 18 May 2022 10:51:26 -0500 | Subject | Re: [PATCH v2 0/2] iio: humidity: si7020: Check device property for skipping reset in probe | From | Eddie James <> |
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On 5/18/22 10:28, Eddie James wrote: > > On 5/14/22 10:02, Peter Rosin wrote: >> 2022-05-14 at 15:43, Jonathan Cameron wrote: >>> On Sat, 14 May 2022 00:48:51 +0200 >>> Peter Rosin <peda@axentia.se> wrote: >>> >>>> Hi! >>>> >>>> 2022-05-13 at 18:45, Jonathan Cameron wrote: >>>>> 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!) >>>> 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? >> I think there might be cases where it might be valid to restore the mux >> directly after an xfer even if the mux is externally locked prior to the >> muxed xfer. But I'm not sure? In any case, it will be a bit convoluted >> for the mux code to remember that it might need to restore the mux >> later. And it will get even hairier when multiple levels of muxing is >> considered... >> >> Maybe some kind of hook/callback that could be installed temporarily on >> the I2C adapter that is called right after the "real" xfer, where the >> driver could then make the needed mdelay call? >> >> I.e. >> 1. lock the bus >> 2. install this new hook/callback >> 3. do an unlocked xfer, get notified and call mdelay >> 5. uninstall the hook/callback >> 6. unlock the bus >> >> The hook/callback could be uninstalled automatically on unlock, then >> you would not need to handle multiple notifications. But then again, >> there is probably some existing framework that should be used that >> handles all than neatly and efficiently. > > > Hm, interesting. Sounds a bit complicated, though very flexible. For a > less flexible, but less complex, approch, we could add a i2c_msg flag > that says to do a delay in the core? And then si7020 could just submit > a couple of raw messages rather than smbus... What do you think?
Um, nevermind... that would require changes in all the bus drivers. I'll look into implementing the hook/callback.
Thanks,
Eddie
> > > Thanks, > > Eddie > > > >> >> Me waves hand a bit... >> >> Cheers, >> Peter > >
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