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]

From: Phil Reid
Date: Thu Aug 22 2019 - 09:44:43 EST

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

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

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
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!



Phil Reid