Re: iio: Is storing output values to non volatile registers something we should do automatically or leave to userspace action. [was Re: [PATCH] iio: potentiometer: max5432: update the non-volatile position]

[Phil Reid]

On 19/08/2019 03:32, Jonathan Cameron wrote:
> On Mon, 12 Aug 2019 19:08:12 +0800
> Phil Reid <preid@xxxxxxxxxxxxxxxxx> wrote:
>
> > G'day Martin / Jonathan,
> >
> > On 12/08/2019 18:37, Martin Kaiser wrote:
> > > Hi Jonathan,
> > >
> > > Thus wrote Jonathan Cameron (jic23@xxxxxxxxxx):
> > >    
> > > > The patch is fine, but I'm wondering about whether we need some element
> > > > of policy control on this restore to current value.
> > >    
> > > > A few things to take into account.
> > >    
> > > > * Some devices don't have a non volatile store.  So userspace will be
> > > >     responsible for doing the restore on reboot.
> > > > * This may be one of several related devices, and it may make no sense
> > > >     to restore this one if the others aren't going to be in the same
> > > >     state as before the reboot.
> > > > * Some devices only have non volatile registers for this sort of value
> > > >     (or save to non volatile on removal of power). Hence any policy to
> > > >     not store the value can't apply to this class of device.
> > >
> > > I see your point. You'd like a consistent bahaviour across all
> > > potentiometer drivers. Or at least a way for user space to figure out
> > > whether a chip uses non-volatile storage or not.
> > > This property doesn't quite fit into the channel info that are defined
> > > in the arrays in drivers/iio/industrialio-core.c. Is there any other way
> > > to set such a property?
> > >    
> > > > My initial thought is that these probably don't matter that much and
> > > > we should apply this, but I would like to seek input from others!
> > >    
> > > > I thought there were some other drivers doing similar store to no
> > > > volatile but I can't find one.
> > >
> > > drivers/iio/potentiometer/max5481.c and max5487.c do something similar.
> > >
> > > They use a command to transfer the setting from volatile to non-volatile
> > > register in the spi remove function. I guess that the intention is to
> > > save the current setting when the system is rebooted. However, my
> > > understanding is that the remove function is called only when a module
> > > is unloaded or when user space does explicitly unbind the device from
> > > the bus via sysfs. That's why I tried using the shutdown function
> > > instead.
> > >
> > > Still, this approach has some disadvantages. For many systems, there's a
> > > soft reboot (shutdown -r) where the driver's shutdown function is called
> > > and a "hard reboot" where the power from the CPU and the potentiometer
> > > chip is removed and reapplied. In this case, the current value would not
> > > be transfered to the non-volatile register.
> > >
> > > At least for the max5432 family, there's no way to read the current
> > > setting. The only option for user space to have a well-defined setting
> > > is to set the wiper position explicitly at startup.
> > >
> > > I guess the only sensible way to use a non-volatile register would be a
> > > write operation where user space gets a response about successful
> > > completion.
> > >
> > > We could have two channels to write to the volatile or to non-volatile
> > > register. Or we stick to one channel and update both volatile and
> > > non-volatile registers when user space changes the value. This assumes
> > > that the setting does not change frequently, which is prabably not true
> > > for all applications...
>
> I'm not keen on multiple channels as that is a fairly non obvious interface.
> Definitely want to avoid writing all the time.
>
> > > Whatever we come up with, we should at least make the max* chips behave
> > > the same way.
> > >    
> > The AD5272/AD5274 Digital Rheostat has a 50-times limit for programming the NV register.
> > So you want to be real sure that you want to set it.
>
> Ouch, I new some were limited to a few thousand cycles, but 50 is rather nasty!
>
> > I'd rather my system default to a known "safe" value for next boot than
> > set to whatever the last write was. So some kind of policy on setting this would
> > be nice. I personally think it's something that userspace should initiate via an explicit
> > command.
> Agreed. I think we should look at an explicit write.
>
> Perhaps an extra attribute on the channels?  Hence a shared_by_all version
> could be used when there is only one write command.

Yes, now the only question is what should it be called.

> > Writing the NV for the AD5272 is something I planned to add at some stage.
> > But so far the default factory values have worked ok.
> > It'd be nice for cross device consistency for any interface for this.
> Agreed. This is an area that crept up on me, so we haven't enforced any
> consistency on it yet.  However, we definitely should!
>
> Thanks,
>
> Jonathan


--
Regards
Phil Reid