Hi Chanwoo,

On Thu, Aug 02, 2018 at 10:58:59AM +0900, Chanwoo Choi wrote:

Hi Matthias,

On 2018년 08월 02일 02:08, Matthias Kaehlcke wrote:

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Hi Chanwoo,

On Wed, Aug 01, 2018 at 10:22:16AM +0900, Chanwoo Choi wrote:

quoted

On 2018년 08월 01일 04:39, Matthias Kaehlcke wrote:

quoted

On Mon, Jul 16, 2018 at 10:50:50AM -0700, Matthias Kaehlcke wrote:

quoted

On Thu, Jul 12, 2018 at 05:44:33PM +0900, Chanwoo Choi wrote:

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Hi Matthias,

On 2018년 07월 07일 02:53, Matthias Kaehlcke wrote:

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Hi Chanwoo,

On Wed, Jul 04, 2018 at 03:41:46PM +0900, Chanwoo Choi wrote:

quoted

Firstly,
I'm not sure why devfreq needs the devfreq_verify_within_limits() function.

devfreq already used the OPP interface as default. It means that
the outside of 'drivers/devfreq' can disable/enable the frequency
such as drivers/thermal/devfreq_cooling.c. Also, when some device
drivers disable/enable the specific frequency, the devfreq core
consider them.

So, devfreq doesn't need to devfreq_verify_within_limits() because
already support some interface to change the minimum/maximum frequency
of devfreq device. 

In case of cpufreq subsystem, cpufreq only provides 'cpufreq_verify_with_limits()'
to change the minimum/maximum frequency of cpu. some device driver cannot
change the minimum/maximum frequency through OPP interface.

But, in case of devfreq subsystem, as I explained already, devfreq support
the OPP interface as default way. devfreq subsystem doesn't need to add
other way to change the minimum/maximum frequency.

Using the OPP interface exclusively works as long as a
enabling/disabling of OPPs is limited to a single driver
(drivers/thermal/devfreq_cooling.c). When multiple drivers are
involved you need a way to resolve conflicts, that's the purpose of
devfreq_verify_within_limits(). Please let me know if there are
existing mechanisms for conflict resolution that I overlooked.

Possibly drivers/thermal/devfreq_cooling.c could be migrated to use
devfreq_verify_within_limits() instead of the OPP interface if
desired, however this seems beyond the scope of this series.

Actually, if we uses this approach, it doesn't support the multiple drivers too.
If non throttler drivers uses devfreq_verify_within_limits(), the conflict
happen.

As long as drivers limit the max freq there is no conflict, the lowest
max freq wins. I expect this to be the usual case, apparently it
worked for cpufreq for 10+ years.

However it is correct that there would be a conflict if a driver
requests a min freq that is higher than the max freq requested by
another. In this case devfreq_verify_within_limits() resolves the
conflict by raising p->max to the min freq. Not sure if this is
something that would ever occur in practice though.

If we are really concerned about this case it would also be an option
to limit the adjustment to the max frequency.

quoted

To resolve the conflict for multiple device driver, maybe OPP interface
have to support 'usage_count' such as clk_enable/disable().

This would require supporting negative usage count values, since a OPP
should not be enabled if e.g. thermal enables it but the throttler
disabled it or viceversa.

Theoretically there could also be conflicts, like one driver disabling
the higher OPPs and another the lower ones, with the outcome of all
OPPs being disabled, which would be a more drastic conflict resolution
than that of devfreq_verify_within_limits().

Viresh, what do you think about an OPP usage count?

Ping, can we try to reach a conclusion on this or at least keep the
discussion going?

Not that it matters, but my preferred solution continues to be
devfreq_verify_within_limits(). It solves conflicts in some way (which
could be adjusted if needed) and has proven to work in practice for
10+ years in a very similar sub-system.

It is not true. Current cpufreq subsystem doesn't support external OPP
control to enable/disable the OPP entry. If some device driver
controls the OPP entry of cpufreq driver with opp_disable/enable(),
the operation is not working. Because cpufreq considers the limit
through 'cpufreq_verify_with_limits()' only.

Ok, we can probably agree that using cpufreq_verify_with_limits()
exclusively seems to have worked well for cpufreq, and that in their
overall purpose cpufreq and devfreq are similar subsystems.

The current throttler series with devfreq_verify_within_limits() takes
the enabled OPPs into account, the lowest and highest OPP are used as
a starting point for the frequency adjustment and (in theory) the
frequency range should only be narrowed by
devfreq_verify_within_limits().

quoted

As I already commented[1], there is different between cpufreq and devfreq.
[1] https://lkml.org/lkml/2018/7/4/80

Already, subsystem already used OPP interface in order to control
specific OPP entry. I don't want to provide two outside method
to control the frequency of devfreq driver. It might make the confusion.

I understand your point, it would indeed be preferable to have a
single method. However I'm not convinced that the OPP interface is
a suitable solution, as I exposed earlier in this thread (quoted
below).

I would like you to at least consider the possibility of changing
drivers/thermal/devfreq_cooling.c to devfreq_verify_within_limits().
Besides that it's not what is currently used, do you see any technical
concerns that would make devfreq_verify_within_limits() an unsuitable
or inferior solution?

As we already discussed, devfreq_verify_within_limits() doesn't support
the multiple outside controllers (e.g., devfreq-cooling.c).

That's incorrect, its purpose is precisely that.

Are you suggesting that cpufreq with its use of
cpufreq_verify_within_limits() (the inspiration for
devfreq_verify_within_limits()) is broken? It is used by cpu_cooling.c
and other drivers when receiving a CPUFREQ_ADJUST event, essentially
what I am proposing with DEVFREQ_ADJUST.

Could you elaborate why this model wouldn't work for devfreq? "OPP
interface is mandatory for devfreq" isn't really a technical argument,
is it mandatory for any other reason than that it is the interface
that is currently used?

After you are suggesting the throttler core, there are at least two
outside controllers (e.g., devfreq-cooling.c and throttler driver).
As I knew the problem about conflict, I cannot agree the temporary
method. OPP interface is mandatory for devfreq in order to control
the OPP (frequency/voltage). In this situation, we have to try to
find the method through OPP interface.

What do you mean with "temporary method"?

We can try to find a method through the OPP interface, but at this
point I'm not convinced that it is technically necessary or even
preferable.

Another inconvenient of the OPP approach for both devfreq-cooling.c
and the throttler is that they have to bother with disabling all OPPs
above/below the max/min (they don't/shouldn't have to care), instead
of just telling devfreq the max/min.

We can refer to regulator/clock. Multiple device driver can use
the regulator/clock without any problem. I think that usage of OPP
is similiar with regulator/clock. As you mentioned, maybe OPP
would handle the negative count. Although opp_enable/opp_disable()
have to handle the negative count and opp_enable/opp_disable()
can support the multiple usage from device drivers, I think that
this approach is right.

The regulator/clock approach with the typical usage counts seems more
intuitive to me, personally I wouldn't write an interface with
negative usage count if I could reasonably avoid it.

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I want to use only OPP interface to enable/disable frequency
even if we have to modify the OPP interface.

These are the concerns I raised earlier about a solution with OPP
usage counts:

"This would require supporting negative usage count values, since a OPP
should not be enabled if e.g. thermal enables it but the throttler
disabled it or viceversa.

Already replied	about negative usage count. I think that negative usage count
is not problem if this approach could resolve the issue.

quoted

Theoretically there could also be conflicts, like one driver disabling
the higher OPPs and another the lower ones, with the outcome of all
OPPs being disabled, which would be a more drastic conflict resolution
than that of devfreq_verify_within_limits()."

What do you think about these points?

It depends on how to use OPP interface on multiple device driver.
Even if devfreq/opp provides the control method, outside device driver
are misusing them. It is problem of user.

I wouldn't call it misusing if two independent drivers take
contradictory actions on an interface that doesn't provide
arbitration. How can driver A know that it shouldn't disable OPPs a, b
and c because driver B disabled d, e and f? Who is misusing the
interface, driver A or driver B?

Instead, if we use the OPP interface, we can check why OPP entry
is disabled or enabled through usage count.

quoted

The negative usage counts aren't necessarily a dealbreaker in a
technical sense, though I'm not a friend of quirky interfaces that
don't behave like a typical user would expect (e.g. an OPP isn't
necessarily enabled after dev_pm_opp_enable()).

I can sent an RFC with OPP usage counts, though due to the above
concerns I have doubts it will be well received.

Please add me to Cc list.

Will do

Thanks

Matthias