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Subject: Re: [PATCH] drm/panfrost: Fix regulator_get_optional() misuse
To:     Steven Price <steven.price@arm.com>,
        Mark Brown <broonie@kernel.org>
Cc:     David Airlie <airlied@linux.ie>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Tomeu Vizoso <tomeu.vizoso@collabora.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20190904123032.23263-1-broonie@kernel.org>
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From:   Neil Armstrong <narmstrong@baylibre.com>
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Organization: Baylibre
Message-ID: <677fc044-6c12-6fb4-9d85-1e4d6c4b96ae@baylibre.com>
Date:   Thu, 5 Sep 2019 15:49:19 +0200
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On 05/09/2019 15:02, Steven Price wrote:
> On 05/09/2019 13:40, Mark Brown wrote:
>> On Thu, Sep 05, 2019 at 10:37:53AM +0100, Steven Price wrote:
>>
>>> Ah, I didn't realise that regulator_get() will return a dummy regulator
>>> if none is provided in the DT. In theory that seems like a nicer
>>> solution to my two commits. However there's still a problem - the dummy
>>> regulator returned from regulator_get() reports errors when
>>> regulator_set_voltage() is called. So I get errors like this:
>>
>>> [  299.861165] panfrost e82c0000.mali: Cannot set voltage 1100000 uV
>>> [  299.867294] devfreq devfreq0: dvfs failed with (-22) error
>>
>>> (And therefore the frequency isn't being changed)
>>
>>> Ideally we want a dummy regulator that will silently ignore any
>>> regulator_set_voltage() calls.
>>
>> Is that safe?  You can't rely on being able to change voltages even if
>> there's a physical regulator available, system constraints or the
>> results of sharing the regulator with other users may prevent changes.
> 
> Perhaps I didn't express myself clearly. I mean that in the case of the
> Hikey960 it would be convenient to have a "dummy regulator" that simply
> accepted any change because ultimately Linux doesn't have direct control
> of the voltages but simply requests a particular operating frequency via
> the mailbox.
> 
>> I guess at the minute the code is assuming that if you can't vary the
>> regulator it's fixed at the maximum voltage and that it's safe to run at
>> a lower clock with a higher voltage (some devices don't like doing that).
> 
> No - at the moment if the regulator reports an error then the function
> bails out and doesn't change the frequency.
> 
>> If the device always starts up at full speed I guess that's OK.  It's
>> certainly in general a bad idea to do this in general, we can't tell how
>> important it is to the consumer that they actually get the voltage that
>> they asked for - for some applications like this it's just adding to the
>> power saving it's likely fine but for others it might break things.
> 
> Agreed.
> 
>> If you're happy to change the frequency without the ability to vary the
>> voltage you can query what's supported through the API (the simplest
>> interface is regulator_is_supported_voltage()).  You should do the
>> regulator API queries at initialization time since they can be a bit
>> expensive, the usual pattern would be to go through your OPP table and
>> disable states where you can't support the voltage but you *could* also
>> flag states where you just don't set the voltage.  That seems especially
>> reasonable if no voltages in the range the device supports can be set.
>>
>> I do note that the current code requires exactly specified voltages with
>> no variation which doesn't match the behaviour you say you're OK with
>> here, what you're describing sounds like the driver should be specifying
>> a voltage range from the hardware specified maximum down to whatever the
>> minimum the OPP supports rather than exactly the OPP voltage.  As things
>> are you might also run into voltages that can't be hit exactly (eg, in
>> the Exynos 5433 case in mainline a regulator that only offers steps of
>> 2mV will error out trying to set several of the OPPs).

In case we need a fixed voltage, simply add a new fixed-regulator in the board/soc DT,
use this regulator for panfrost and use the same fixed voltage in the OPP table.

It relies on a correct DT, but isn't is the goal of mainline linux ?

Neil

> 
> Perhaps there's a better way of doing devfreq? Panfrost itself doesn't
> really care must about this - we just need to be able to scaling up/down
> the operating point depending on load.
> 
> On many platforms to set the frequency it's necessary to do the dance to
> set an appropriate voltage before/afterwards, but on the Hikey960
> because this is handled via a mailbox we don't actually have a regulator
> to set the voltage on. My commit[1] supports this by simply not listing
> the regulator in the DT and assuming that nothing is needed when
> switching frequency. I'm happy for some other way of handling this if
> there's a better method.
> 
> At the moment your change from devm_regulator_get_optional() to
> devm_regulator_get() is a regression on this platform because it means
> there is now a dummy regulator which will always fail the
> regulator_set_voltage() calls preventing frequency changes. And I can't
> see anything I can do in the DT to fix that.
> 
> Steve
> 
> [1] e21dd290881b drm/panfrost: Enable devfreq to work without regulator
> (plus bug fix in 52282163dfa6)
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