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>]

On 19/08/2019 03:32, Jonathan Cameron wrote:
> On Mon, 12 Aug 2019 19:08:12 +0800
> Phil Reid <preid@electromag.com.au> wrote:
> 
>> 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...
> 
> 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