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 <preid@electromag.com.au>]

G'day Martin / Jonathan,

On 12/08/2019 18:37, Martin Kaiser wrote:
> Hi Jonathan,
> 
> Thus wrote Jonathan Cameron (jic23@kernel.org):
> 
>> 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...
> 
> 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.

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.

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.


-- 
Regards
Phil Reid