Date: Mon, 1 Jul 2024 09:41:01 +0200
From: Neil Armstrong <neil.armstrong@...aro.org>
To: Jerome Brunet <jbrunet@...libre.com>
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 25/06/2024 15:51, Jerome Brunet wrote:
> 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.

I still don't see why it could add so much boilerplate, all the tables and
calculation fonction would be shared, only the debugfs clk_msr_show() and
clk_msr_summary_show() would be kept, all the rest would be common.

I insist, please keep the debugfs interface for debug purposes. You don't
want to mess with IIO when you bring up new platforms with bare minimum
kernels.

> 
>> - 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.

It's a problem for new platforms, you can introduce the split only for the
new ones, the impact on code won't high enough to justify new bindings.

Neil

> 
>> - 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
>>>
> 

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