linux-kernel - Re: [PATCH 4/9] PM / devfreq: exynos-bus: Replace deprecated 'devfreq' property

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Date:   Fri, 27 Dec 2019 09:09:58 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Rob Herring <robh@...nel.org>
Cc:     krzk@...nel.org, mark.rutland@....com, heiko@...ech.de,
        leonard.crestez@....com, lukasz.luba@....com, a.swigon@...sung.com,
        m.szyprowski@...sung.com, kgene@...nel.org,
        myungjoo.ham@...sung.com, kyungmin.park@...sung.com,
        linux-pm@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH 4/9] PM / devfreq: exynos-bus: Replace deprecated
 'devfreq' property

On 12/27/19 6:01 AM, Rob Herring wrote:
> On Tue, Dec 17, 2019 at 02:57:33PM +0900, Chanwoo Choi wrote:
>> In order to remove the deprecated 'devfreq' property, replace with
>> new 'exynos,parent-bus' property in order to get the parent devfreq device
>> in devicetree file instead of 'devfreq' property. But, to guarantee the
>> backward-compatibility, keep the support 'devfreq' property.
>>
>> Signed-off-by: Chanwoo Choi <cw00.choi@...sung.com>
>> ---
>>  .../bindings/devfreq/exynos-bus.txt           | 16 +++++++--------
>>  drivers/devfreq/exynos-bus.c                  | 20 ++++++++++++-------
>>  2 files changed, 21 insertions(+), 15 deletions(-)
>>
>> diff --git a/Documentation/devicetree/bindings/devfreq/exynos-bus.txt b/Documentation/devicetree/bindings/devfreq/exynos-bus.txt
>> index e71f752cc18f..c948cee01124 100644
>> --- a/Documentation/devicetree/bindings/devfreq/exynos-bus.txt
>> +++ b/Documentation/devicetree/bindings/devfreq/exynos-bus.txt
>> @@ -45,7 +45,7 @@ Required properties only for parent bus device:
>>    of buses.
>>  
>>  Required properties only for passive bus device:
>> -- devfreq: the parent bus device.
>> +- exynos,parent-bus: the parent bus device.
> 
> If you are going to do something new, why not use the interconnect 
> binding here?

As I knew, interconnect make the data path among multiple nodes
and set the average and peak bandwidth to the specific data path.

It means that some data will be flowed from node_a to node_d
or the reverse way because each node has the tightly coupled
dependency for data flow.

	node_a <-> node_b <-> node_c <-> node_d


On the other hand, exynos-bus.c driver is not related to 'data path'.
Each bus just need to control the their own frequency and voltage.
But, share the power line (regulator) between exynos-bus device
even if there are no any dependency of data flow.

'exynos,parent-bus' property just indicate the specific
devfreq device(parent bus device) which controls
the shared power line(regulator) in order to prevent
the h/w problem due to the wrong pair of frequency and voltage.

'exynos,parent-bus' property is only used to catch
the change timing of shared power line.


And,
as you commented, there are some data path among the exynos-bus
devices for the display h/w as following:

	bus_display -> bus_leftbus -> bus_dmc

In order to make the data path between bus devices,
interconnect binding is required. This approach[1] was posted.
[1] https://patchwork.kernel.org/cover/11305265/
- [RFC,v3,0/7] PM / devfreq: Simple QoS for exynos-bus using interconnect

-- 
Best Regards,
Chanwoo Choi
Samsung Electronics