Messages in this thread | | | Subject | 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 | Mon, 12 Aug 2019 19:08:12 +0800 |
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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
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