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

From: skannan
Date: Wed Aug 08 2018 - 17:18:24 EST

On 2018-08-08 01:47, Sudeep Holla wrote:
On Tue, Aug 07, 2018 at 12:37:07PM -0700, skannan@xxxxxxxxxxxxxx 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@xxxxxxxxxxxxxx>
>>---
>> .../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 ?

But that's not the point here? We aren't trying to disable any OPPs here? Not sure what you mean.

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 ?

All QC platforms would use this.

As a personal (non-Qcom) opinion, I'd rather the kernel control this than have some black magic FW manage this. I've a really bitter taste in my mouth for FW hiding this because of a broken ACPI implementation in one of my x86 motherboards prevented CPUfreq from working (this was well before I worked on CPUfreq). Pushing stuff to FW seems to beat the ideal behind an opensource OS. In a few cases it's elegant or more robust, so maybe in those cases its okay to use a FW. But I'd rather not for simpler stuff like this.

-Saravana