Re: [PATCH 0/4] Introduce OPP bandwidth bindings

[Rob Herring <robh@kernel.org>]

On Sat, Mar 16, 2019 at 12:32:49AM +0530, Sibi Sankar wrote:
> 
> 
> On 3/13/19 2:30 PM, Georgi Djakov wrote:
> > Here is a proposal to extend the OPP bindings with bandwidth based on
> > a previous discussion [1].
> > 
> > Every functional block on a SoC can contribute to the system power
> > efficiency by expressing its own bandwidth needs (to memory or other SoC
> > modules). This will allow the system to save power when high throughput
> > is not required (and also provide maximum throughput when needed).
> > 
> > There are at least three ways for a device to determine its bandwidth
> > needs:
> > 	1. The device can dynamically calculate the needed bandwidth
> > based on some known variable. For example: UART (baud rate), I2C (fast
> > mode, high-speed mode, etc), USB (specification version, data transfer
> > type), SDHC (SD standard, clock rate, bus-width), Video Encoder/Decoder
> > (video format, resolution, frame-rate)
> > 
> > 	2. There is a hardware specific value. For example: hardware
> > specific constant value (e.g. for PRNG) or use-case specific value that
> > is hard-coded.
> > 
> > 	3. Predefined SoC/board specific bandwidth values. For example:
> > CPU or GPU bandwidth is related to the current core frequency and both
> > bandwidth and frequency are scaled together.
> > 
> > This patchset is trying to address point 3 above by extending the OPP
> > bindings to support predefined SoC/board bandwidth values and adds
> > support in cpufreq-dt to scale the interconnect between the CPU and the
> > DDR together with frequency and voltage.
> 
> Hey Georgi,
> Having opp-bw-MBps as a part of cpu opp does greatly simplify the
> problem of scaling multiple interconnect devices with change in cpu
> frequency. But there is still a need to scale other devices (non
> interconnect based) according to cpu frequency. Having a devfreq
> governor for the same would help to have the same generic solution
> across SoCs (msm8916/8996/qcs405/sdm845). The devfreq maintainer did
> like the idea but wanted it incorporated into the passive governor.
> 
> * https://lore.kernel.org/lkml/20180528060014epcms1p87ec68a4d44f9447b06f979a87e545b7d@epcms1p8/
> 
> * https://lore.kernel.org/lkml/20180802095608epcms1p33fb061543efc9ceb3ec12d5567ceffbc@epcms1p3/
> 
> I have a RFC series implementing ddr scaling with passive governor for
> sdm845 with the following bindings, will post it early next week.
> 
> cpus {
> 	...
> 
> 	CPU0: cpu@0 {
> 		...
> 		operating-points-v2 = <&cpu0_opp_table>;
> 		...
> 	};
>         ....
> 
> 	CPU4: cpu@400 {
> 		...
> 		operating-points-v2 = <&cpu4_opp_table>;
> 		...
> 	};
>         ...
> };
> 
> cpu0_opp_table: cpu0_opp_table {
> 	compatible = "operating-points-v2";
> 	opp-shared;
> 
> 	cpu0_opp1: opp-300000000 {
> 		opp-hz = /bits/ 64 <300000000>;
> 	};
> 
> 	...
> 
> 	cpu0_opp16: opp-1612800000 {
> 		opp-hz = /bits/ 64 <1612800000>;
> 	};
> 
> 	...
> };
> 
> cpu4_opp_table: cpu4_opp_table {
> 	compatible = "operating-points-v2";
> 	opp-shared;
> 
> 	...
> 
> 	cpu4_opp4: opp-1056000000 {
> 		opp-hz = /bits/ 64 <1056000000>;
> 	};
> 
> 	cpu4_opp5: opp-1209600000 {
> 		opp-hz = /bits/ 64 <1209600000>;
> 	};
> 
> 	...
> };
> 
> bw_opp_table: bw-opp-table {
> 	compatible = "operating-points-v2";
> 
> 	opp-200  {
> 		opp-hz = /bits/ 64 < 200000000 >; /* 200 MHz */
> 		required-opps = <&cpu0_opp1>;
> 		/* 0 MB/s average and 762 MB/s peak bandwidth */
> 		opp-bw-MBs = <0 762>;
> 	};
> 
> 	opp-300 {
> 		opp-hz = /bits/ 64 < 300000000 >; /* 300 MHz */
> 		/* 0 MB/s average and 1144 MB/s peak bandwidth */
> 		opp-bw-MBs = <0 1144>;
> 	};
> 
> 	...
> 
> 	opp-768 {
> 		opp-hz = /bits/ 64 < 768000000 >; /* 768 MHz */
> 		/* 0 MB/s average and 2929 MB/s peak bandwidth */
> 		opp-bw-MBs = <0 2929>;
> 		required-opps = <&cpu4_opp4>;
> 	};
> 
> 	opp-1017 {
> 		opp-hz = /bits/ 64 < 1017000000 >; /* 1017 MHz */
> 		/* 0 MB/s average and 3879 MB/s peak bandwidth */
> 		opp-bw-MBs = <0 3879>;
> 		required-opps = <&cpu0_opp16>, <&cpu4_opp5>;
> 	};
> };
> 
> cpubw {
> 	compatible = "devfreq-icbw";

Most certainly not a h/w device, so it doesn't go in DT.

> 	interconnects = <&snoc MASTER_APSS_1 &bimc SLAVE_EBI_CH0>;
> 	operating-points-v2 = <&bw_opp_table>;
> };
> 
> 
> > > [1] https://patchwork.kernel.org/patch/10577315/
> > 
> > Georgi Djakov (4):
> >    dt-bindings: opp: Introduce opp-bw-MBs bindings
> >    OPP: Add support for parsing the interconnect bandwidth
> >    OPP: Update the bandwidth on OPP frequency changes
> >    cpufreq: dt: Add support for interconnect bandwidth scaling
> > 
> >   Documentation/devicetree/bindings/opp/opp.txt | 45 ++++++++++++
> >   drivers/cpufreq/cpufreq-dt.c                  | 27 ++++++-
> >   drivers/opp/core.c                            | 71 +++++++++++++++++++
> >   drivers/opp/of.c                              | 44 ++++++++++++
> >   drivers/opp/opp.h                             |  6 ++
> >   include/linux/pm_opp.h                        | 14 ++++
> >   6 files changed, 206 insertions(+), 1 deletion(-)
> > 
> 
> -- 
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