linux-kernel - Re: [PATCH 0/2] iio: frequency: add iio support for Amlogic clock measure

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Date: Tue, 25 Jun 2024 15:51:51 +0200
From: Jerome Brunet <jbrunet@...libre.com>
To: Neil Armstrong <neil.armstrong@...aro.org>
Cc: Jonathan Cameron <jic23@...nel.org>,  Lars-Peter Clausen
 <lars@...afoo.de>,  Kevin Hilman <khilman@...libre.com>,
  linux-kernel@...r.kernel.org,  linux-amlogic@...ts.infradead.org,
  linux-iio@...r.kernel.org,  Rob Herring <robh@...nel.org>,  Krzysztof
 Kozlowski <krzk+dt@...nel.org>,  Conor Dooley <conor+dt@...nel.org>,
  linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH 0/2] iio: frequency: add iio support for Amlogic clock
 measure

On Tue 25 Jun 2024 at 15:18, Neil Armstrong <neil.armstrong@...aro.org> wrote:

> On 25/06/2024 11:53, Jerome Brunet wrote:
>> On Tue 25 Jun 2024 at 11:38, Neil Armstrong <neil.armstrong@...aro.org> wrote:
>> 
>>> Hi,
>>>
>>> [+cc people from linux-msm]
>>>
>>> On 24/06/2024 19:31, Jerome Brunet wrote:
>>>> Add support for the HW found in most Amlogic SoC dedicated to measure
>>>> system clocks.
>>>> This drivers aims to replace the one found in
>>>> drivers/soc/amlogic/meson-clk-measure.c with following improvements:
>>>> * Access to the measurements through the IIO API:
>>>>     Easier re-use of the results in userspace and other drivers
>>>> * Controllable scale with raw measurements
>>>> * Higher precision with processed measurements
>>>> Jerome Brunet (2):
>>>>     dt-bindings: iio: frequency: add clock measure support
>>>>     iio: frequency: add amlogic clock measure support
>>>>    .../iio/frequency/amlogic,clk-msr-io.yaml     |  50 ++
>>>>    drivers/iio/frequency/Kconfig                 |  15 +
>>>>    drivers/iio/frequency/Makefile                |   1 +
>>>>    drivers/iio/frequency/amlogic-clk-msr-io.c    | 802 ++++++++++++++++++
>>>>    4 files changed, 868 insertions(+)
>>>>    create mode 100644 Documentation/devicetree/bindings/iio/frequency/amlogic,clk-msr-io.yaml
>>>>    create mode 100644 drivers/iio/frequency/amlogic-clk-msr-io.c
>>>>
>>>
>>> While I really appreciate the effort, and the code looks cool, the clkmsr is really
>>> a debug tool, and I'm not sure IIO is the right place for such debug tool ?
>> The reason why I went through the trouble of doing an IIO port is
>> because I need that for other purposes than debug. I need to to be able
>> to check a frequency from another driver. I don't see a reason to invent
>> another API when IIO provide a perfectly good one.
>> The HW does measurements. IIO seems like the best place for it.
>> For the record, I need this for a eARC support.
>> eARC has a PLL that locks on incoming stream. eARC registers show wether
>> the PLL is locked or not, but not at which rate. That information is
>> needed in ASoC. Fortunately the eARC PLL is one of measured clock, which
>> is a life saver in that case.
>
> This is a very interesting use-case, and quite weird nothing is provided
> on the eARC side.

Indeed.

>
> So yes it's definitely a valid use-case, but:
> - we should keep the debugfs interface, perhaps move it in the iio driver ?

I considered this initially but it would add a lot of boiler plate
code to provide over debugfs exactly what iio already provides over
sysfs. As you pointed out, the previous driver only provided debug
information, the debugfs interface it provided is hardly a
critical/stable one.

> - we should keep a single compatible, so simply update the current bindings with iio cells

Using a new compatible allows to split the memory region, making the
interface between DT and driver a lot easier to implement seemlessly
between old and new SoCs. Eventually it may allow to implement the duty
part too.

> - for s4 & c3, it's ok to either add a second reg entry in the bindings

Doing that for s4 and c3 only would still make a mess of offset handling
the region because duty prepend the region on old SoC. The goal is to
have an interface that seemlessly support both old and new SoCs.

>
> Neil
>
>> Everything that was available through the old driver still is, with more
>> precision and more control.
>> 
>>>
>>> There's almost the same interface on qcom SoCs (https://github.com/linux-msm/debugcc) but
>>> they chose to keep it in userspace until we find an appropriate way to expose
>>> this from the kernel the right way.
>>>
>>> If it enabled us to monitor a frequency input for a product use-case, IIO would be
>>> the appropriate interface, but AFAIK it's only internal clocks and thus I'm worried
>>> it's not the best way to expose those clocks.
>>>
>>> Neil
>> 

-- 
Jerome