Messages in this thread | | | Date | Wed, 18 May 2022 10:28:42 -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 5/14/22 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?
Thanks,
Eddie
> > Me waves hand a bit... > > Cheers, > Peter
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