Re: [PATCHv2 8/8] devfreq: exynos4: Add busfreq driver for exynos4210/exynos4x12

[Tomasz Figa]

Hi Chanwoo, Mark,

On 14.03.2014 11:56, Chanwoo Choi wrote:
> Hi Mark,
>
> On 03/14/2014 07:35 PM, Mark Rutland wrote:
> > On Fri, Mar 14, 2014 at 07:14:37AM +0000, Chanwoo Choi wrote:
> > > Hi Mark,
> > >
> > > On 03/14/2014 02:53 AM, Mark Rutland wrote:
> > > > On Thu, Mar 13, 2014 at 08:17:29AM +0000, Chanwoo Choi wrote:
> > > > > This patch add busfreq driver for Exynos4210/Exynos4x12 memory interface
> > > > > and bus to support DVFS(Dynamic Voltage Frequency Scaling) according to PPMU
> > > > > counters. PPMU (Performance Profiling Monitorings Units) of Exynos4 SoC provides
> > > > > PPMU counters for DMC(Dynamic Memory Controller) to check memory bus utilization
> > > > > and then busfreq driver adjusts dynamically the operating frequency/voltage
> > > > > by using DEVFREQ Subsystem.
> > > > >
> > > > > Signed-off-by: Chanwoo Choi <cw00.choi@xxxxxxxxxxx>
> > > > > ---
> > > > >   .../devicetree/bindings/devfreq/exynos4_bus.txt    | 49 ++++++++++++++++++++++
> > > > >   1 file changed, 49 insertions(+)
> > > > >   create mode 100644 Documentation/devicetree/bindings/devfreq/exynos4_bus.txt
> > > > >
> > > > > diff --git a/Documentation/devicetree/bindings/devfreq/exynos4_bus.txt b/Documentation/devicetree/bindings/devfreq/exynos4_bus.txt
> > > > > new file mode 100644
> > > > > index 0000000..2a83fcc
> > > > > --- /dev/null
> > > > > +++ b/Documentation/devicetree/bindings/devfreq/exynos4_bus.txt
> > > > > @@ -0,0 +1,49 @@
> > > > > +
> > > > > +Exynos4210/4x12 busfreq driver
> > > > > +-----------------------------
> > > > > +
> > > > > +Exynos4210/4x12 Soc busfreq driver with devfreq for Memory bus frequency/voltage
> > > > > +scaling according to PPMU counters of memory controllers
> > > > > +
> > > > > +Required properties:
> > > > > +- compatible	: should contain Exynos4 SoC type as follwoing:
> > > > > +		  - "samsung,exynos4x12-busfreq" for Exynos4x12
> > > > > +		  - "samsung,exynos4210-busfreq" for Exynos4210
> > > >
> > > > Is there a device called "busfreq"? What device does this binding
> > > > describe?
> > >
> > > I'll add detailed description of busfreq as following:
> > >
> > > "busfreq(bus frequendcy)" driver means that busfreq driver control dynamically
> > > memory bus frequency/voltage by checking memory bus utilization to optimize
> > > power-consumption. When checking memeory bus utilization, exynos4_busfreq driver
> > > would use PPMU(Performance Profiling Monitoring Units).
> >
> > This still sounds like a description of the _driver_, not the _device_.
> > The binding should describe the hardware, now the high level abstraction
> > that software is going to build atop of it.
> >
> > It sounds like this is a binding for the DMC PPMU?
> >
> > Is the PPMU a component of the DMC, or is it bolted on the side?
>
> PPMU(Performance Profiling Monitoring Unit) is to profile performance event of
> various IP on Exynos4. Each PPMU provide perforamnce event for each IP.
> We can check various PPMU as following:
>
> PPMU_3D
> PPMU_ACP
> PPMU_CAMIF
> PPMU_CPU
> PPMU_DMC0
> PPMU_DMC1
> PPMU_FSYS
> PPMU_IMAGE
> PPMU_LCD0
> PPMU_LCD1
> PPMU_MFC_L
> PPMU_MFC_R
> PPMU_TV
> PPMU_LEFT_BUS
> PPMU_RIGHT_BUS
>
> DMC (Dynamic Memory Controller) control the operation of DRAM in Exynos4 SoC.
> If we need to get memory bust utilization of DMC, we can get memory bus utilization
> from PPMU_DMC0/PPMU_DMC1.
>
> So, Exynos4's busfreq used two(PPMU_DMC0/PPMU_DMC1) among upper various PPMU list.

Well, PPMUs and DMCs are separate hardware blocks found inside Exynos 
SoCs. Busfreq/devfreq is just a Linux-specific abstraction responsible 
for collecting data using PPMUs and controlling frequencies and voltages 
of appropriate power planes, vdd_int responsible for powering DMC0 and 
DMC1 blocks in this case.

I'm afraid that the binding you're proposing is unfortunately incorrect, 
because it represents the software abstraction, not the real hardware.

Instead, this should be separated into several independent bindings:

 - PPMU bindings to list all the PPMU instances present in the SoC and 
resources they need,

 - power plane bindings, which define a power plane in which multiple 
IP blocks might reside, can be monitored by one or more PPMU units and 
frequency and voltage of which can be configured according to determined 
performance level. Needed resources will be clocks and regulators to 
scale and probably also operating points.

Then, exynos-busfreq driver should bind to such power planes, parse 
necessary data from DT (list of PPMUs and IP blocks, clocks, regulators 
and operating points) and register a devfreq entity.

Best regards,
Tomasz
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