Re: [PATCH v1 11/30] drm/tegra: dc: Support OPP and SoC core voltage scaling

[Dmitry Osipenko]

13.11.2020 20:28, Mark Brown пишет:
> On Fri, Nov 13, 2020 at 08:13:49PM +0300, Dmitry Osipenko wrote:
>> 13.11.2020 19:15, Mark Brown пишет:
> 
>>> My point here is that the driver shouldn't be checking for a dummy
>>> regulator, the driver should be checking the features that are provided
>>> to it by the regulator and handling those.
> 
>> I understand yours point, but then we need dummy regulator to provide
>> all the features, and currently it does the opposite.
> 
> As could any other regulator?

Yes

>>> It doesn't matter if this is
>>> a dummy regulator or an actual regulator with limited features, the
>>> effect is the same and the handling should be the same.  If the driver
>>> is doing anything to handle dummy regulators explicitly as dummy
>>> regulators it is doing it wrong.
> 
>> It matters because dummy regulator errors out all checks and changes
>> other than enable/disable, instead of accepting them. If we could add an
>> option for dummy regulator to succeed all the checks and accept all the
>> values, then it could become more usable.
> 
> I'm a bit confused here TBH - I'm not sure I see a substantial
> difference between a consumer detecting that it can't set any voltages
> at all and the handling for an optional regulator.  Either way if it's
> going to carry on and assume that whatever voltage is there works for
> everything it boils down to setting a flag saying to skip the set
> voltage operation.  I think you are too focused on the specific
> implementation you currently have here.
> 
> We obviously can't just accept voltage change operations when we've no
> idea what the current voltage of the device is.
> 
>>> To repeat you should *only* be using regulator_get_optional() in the
>>> case where the supply may be physically absent which is not the case
>>> here.
>>
>> Alright, but then we either need to improve regulator core to make dummy
>> regulator a bit more usable, or continue to work around it in drivers.
>> What should we do?
> 
> As I keep saying the consumer driver should be enumerating the voltages
> it can set, if it can't find any and wants to continue then it can just
> skip setting voltages later on.  If only some are unavailable then it
> probably wants to eliminate those specific OPPs instead.

I'm seeing a dummy regulator as a helper for consumer drivers which
removes burden of handling an absent (optional) regulator. Is this a
correct understanding of a dummy regulator?

Older DTBs don't have a regulator and we want to keep them working. This
is equal to a physically absent regulator and in this case it's an
optional regulator, IMO.

Consumer drivers definitely should check voltages, but this should be
done only for a physical regulator.