Re: [PATCH 4/5] hwmon: (pmbus/core) improve handling of write protected regulators

[Jerome Brunet <jbrunet@baylibre.com>]

On Mon 23 Sep 2024 at 14:21, Mark Brown <broonie@kernel.org> wrote:

> On Sat, Sep 21, 2024 at 06:49:34PM +0200, Jerome Brunet wrote:
>> On Sat 21 Sep 2024 at 08:22, Guenter Roeck <linux@roeck-us.net> wrote:
>
>> > In other words, if always-on is _not_ set in
>> > regulator constraints, I'd see that as request to override write-protect
>> > in the driver if there is a change request from regulator code.
>
>> That's very much different from what we initially discussed. It can
>> certainly be done, what is proposed here already does 90% of the job in
>> that direction. However, I'm not sure that is what people intended when
>> they did not put anything. A chip that was previously locked, would be
>> unlocked following such change. It's an important behaviour change.
>
> The general approach we take for regulators is to not touch the hardware
> state unless we were explicitly asked to do something by firmware
> configuration.  The theory is that this avoids us doing anything that
> causes physical damage by mistake.
>
>> >> This is something that might get fix with this change [1]. Even with that
>> >> fixed, passing init_data systematically would be convenient only if you
>> >> plan on skipping DT provided constraints (there are lot of those), or
>> >> redo the parsing in PMBus.
>
>> > I disagree. I am perfectly fine with DT overriding constraints provided
>> > by the driver. The driver can provide its own constraints, and if dt
>> > overrides them, so be it.
>
>> That's not what the regulator framework does. At the moment, it is DT
>> and nothing else. After the linked change, it would be DT if no
>> init_data is passed - otherwise, the init_data.
>
>> If a something in between, whichever the one you want to give priority
>> to, that will have to re-implemented on the caller side.
>> This is what I meant by redo the parsing on pmbus side.
>
> Right, and I've got a feeling that any attempt to combine constraints is
> going to need to be done in a case by case manner since what's tasteful
> is going to vary depending on how much we trust the various sources of
> information.
>
>> It goes way beyond what I'm proposing.
>> The only thing done here is something you simply cannot put in DT
>> because DT is static. Following init, if the chip write protected,
>> REGULATOR_CHANGE_STATUS should not be set, regardless of what is in DT.
>> If it is not set, I'm not adding it.
>
>> Also, what I'm proposing does not get in the way of DT, or anything
>> else, providing constraints. What I propose allow to make adjustement in
>> the HW based on the constraint, if this is what you want to do. It also
>> allows to update the constaints based on what the HW actually is.
>> If the chip cannot be written, regulator needs to know.
>
> So, I know we talked about this a bit on IRC but I didn't register the
> specific use case.  Now I see that it's coming down to the fact that the
> chip is simply write protected I'm wondering if it might not be simpler
> to handle this at the ops level rather than the constraints level.  When
> registering the driver could check for write protection and then instead
> of registering the normal operations register an alternative set with
> the relevant write operations removed.  That would have the same effect
> that you're going for AFAICT.  Sorry for not thinking of it earlier.

Actually I thaught about it, that was my first idea.

There is tiny difference between the 2 approach:
* A regulator that does not provide enable()/disable() is considered
always-on, so it is not really checked if it is enabled or not
* A regulator that does provide enable()/disable() but disallow status
change will be checked with is_enabled()

In the second case, we will pick up on regulator that is disabled and we
can't enable. In the first case, I don't think we do. I don't know if it
is a bug of not but that makes the 2nd case more correct for what we do
with pmbus regs I think

The other thing, although is more a pmbus implemantion consideration,
is that the type regulator is opaque in
pmbus_regulator_register. Different types could be registered so
manipulating the ops is tricky. That's where a callback is helpful 

If building the ops dynamically is the preferred way, I'll find a way to
make it happen. I'll need to way to identify which one need it, loose
the 'const' qualifier in a lot of place, etc ... that will be a bit
hackish I'm afraid.

>
>> > We should not try to override devicetree constraints.
>
>> I don't think I am. I'm just reading the chip state and adjusting the
>> constraint. Even after implementing what is suggested above, it will
>> still be necessary to readback and adjust the constraint based the
>> read protection. Unlock is not guranteed to succeed, the chip may be
>> permanently lock. Some provide the option to do that.
>
> I'm not familiar with this hardware so I'll defer to the two of you on
> what's tasteful with regard to handling this, based on the above it
> might be a per device thing depending on how reversable the write
> protection is.  It looks like currently we don't change this at runtime
> but I might not be looking properly?

At the moment, it does not. With this patch it might but only a
registration. We've been discussing other modes but it would not impact
regulator I think.

-- 
Jerome