Re: [PATCH v3 1/2] PM / devfreq: Generic CPU frequency to device frequency mapping governor

[Sudeep Holla <sudeep.holla@arm.com>]

On Tue, Aug 07, 2018 at 12:37:07PM -0700, skannan@codeaurora.org wrote:
> On 2018-08-07 09:41, Rob Herring wrote:
> >On Wed, Aug 01, 2018 at 05:57:41PM -0700, Saravana Kannan wrote:
> >>Many CPU architectures have caches that can scale independent of the
> >>CPUs.
> >>Frequency scaling of the caches is necessary to make sure the cache is
> >>not
> >>a performance bottleneck that leads to poor performance and power. The
> >>same
> >>idea applies for RAM/DDR.
> >>
> >>To achieve this, this patch adds a generic devfreq governor that takes
> >>the
> >>current frequency of each CPU frequency domain and then adjusts the
> >>frequency of the cache (or any devfreq device) based on the frequency of
> >>the CPUs. It listens to CPU frequency transition notifiers to keep
> >>itself
> >>up to date on the current CPU frequency.
> >>
> >>To decide the frequency of the device, the governor does one of the
> >>following:
> >>
> >>* Uses a CPU frequency to device frequency mapping table
> >>  - Either one mapping table used for all CPU freq policies (typically
> >>used
> >>    for system with homogeneous cores/clusters that have the same OPPs).
> >>  - One mapping table per CPU freq policy (typically used for ASMP
> >>systems
> >>    with heterogeneous CPUs with different OPPs)
> >>
> >>OR
> >>
> >>* Scales the device frequency in proportion to the CPU frequency. So, if
> >>  the CPUs are running at their max frequency, the device runs at its
> >>max
> >>  frequency.  If the CPUs are running at their min frequency, the device
> >>  runs at its min frequency. And interpolated for frequencies in
> >>between.
> >>
> >>Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
> >>---
> >> .../bindings/devfreq/devfreq-cpufreq-map.txt       |  53 ++
> >
> >Bindings should be a separate patch.
> >
> >> drivers/devfreq/Kconfig                            |   8 +
> >> drivers/devfreq/Makefile                           |   1 +
> >> drivers/devfreq/governor_cpufreq_map.c             | 583
> >>+++++++++++++++++++++
> >> 4 files changed, 645 insertions(+)
> >> create mode 100644
> >>Documentation/devicetree/bindings/devfreq/devfreq-cpufreq-map.txt
> >> create mode 100644 drivers/devfreq/governor_cpufreq_map.c
> >>
> >>diff --git
> >>a/Documentation/devicetree/bindings/devfreq/devfreq-cpufreq-map.txt
> >>b/Documentation/devicetree/bindings/devfreq/devfreq-cpufreq-map.txt
> >>new file mode 100644
> >>index 0000000..982a30b
> >>--- /dev/null
> >>+++ b/Documentation/devicetree/bindings/devfreq/devfreq-cpufreq-map.txt
> >>@@ -0,0 +1,53 @@
> >>+Devfreq CPUfreq governor
> >>+
> >>+devfreq-cpufreq-map is a parent device that contains one or more child
> >>devices.
> >>+Each child device provides CPU frequency to device frequency mapping
> >>for a
> >>+specific device. Examples of devices that could use this are: DDR,
> >>cache and
> >>+CCI.
> >>+
> >>+Parent device name shall be "devfreq-cpufreq-map".
> >>+
> >>+Required child device properties:
> >>+- cpu-to-dev-map, or cpu-to-dev-map-<X>:
> >>+			A list of tuples where each tuple consists of a
> >>+			CPU frequency (KHz) and the corresponding device
> >>+			frequency. CPU frequencies not listed in the table
> >>+			will use the device frequency that corresponds to the
> >>+			next rounded up CPU frequency.
> >>+			Use "cpu-to-dev-map" if all CPUs in the system should
> >>+			share same mapping.
> >>+			Use cpu-to-dev-map-<cpuid> to describe different
> >>+			mappings for different CPUs. The property should be
> >>+			listed only for the first CPU if multiple CPUs are
> >>+			synchronous.
> >>+- target-dev:		Phandle to device that this mapping applies to.
> >>+
> >>+Example:
> >>+	devfreq-cpufreq-map {
> >>+		cpubw-cpufreq {
> >>+			target-dev = <&cpubw>;
> >>+			cpu-to-dev-map =
> >>+				<  300000  1144000 >,
> >>+				<  422400  2288000 >,
> >>+				<  652800  3051000 >,
> >>+				<  883200  5996000 >,
> >>+				< 1190400  8056000 >,
> >>+				< 1497600 10101000 >,
> >>+				< 1728000 12145000 >,
> >>+				< 2649600 16250000 >;
> >
> >Now we have frequencies listed in multiple places, the OPP tables and
> >here? Perhaps it is grouping OPPs that should be done.
> 
> This doesn't list all OPPs (it can if necessary). This is listing the
> minimum frequency needed to give good performance/power for a
> device/product.
>

Shouldn't the "status" property be used to disable OPPs you don't need
on a particular platform ?

Duplicating values is highly prone to errors and should be avoided.

> AFAIK, OPP grouping isn't something that's supported in OPP framework or in
> DT. Is there something specific you had in mind? Also, I'd like for this to
> work even with devices that don't have OPPs listed in DT.
>
Also what's the solution you have for platforms with new *QCom FW Cpufreq* ?
IIUC the frequency is obtained from the firmware. TBH this should ideally
be handled in firmware if cpufreq is also handled by the firmware. I guess
this platform doesn't have that ?

--
Regards,
Sudeep