[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <42d99cc8-e59b-6c0b-d1e3-5690b8d1fe53@electromag.com.au>
Date: Mon, 12 Aug 2019 19:08:12 +0800
From: Phil Reid <preid@...ctromag.com.au>
To: Martin Kaiser <martin@...ser.cx>,
Jonathan Cameron <jic23@...nel.org>
Cc: Hartmut Knaack <knaack.h@....de>,
Peter Meerwald-Stadler <pmeerw@...erw.net>,
Lars-Peter Clausen <lars@...afoo.de>,
linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
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]
G'day Martin / Jonathan,
On 12/08/2019 18:37, Martin Kaiser wrote:
> Hi Jonathan,
>
> Thus wrote Jonathan Cameron (jic23@...nel.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
Powered by blists - more mailing lists