linux-kernel - Re: [PATCH v3 3/3] counter: Add STM based counter

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Message-ID: <ba28863a-a5fe-498f-92a0-d71c356e7189@linaro.org>
Date: Sun, 28 Dec 2025 18:37:22 +0100
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: William Breathitt Gray <wbg@...nel.org>
Cc: robh@...nel.org, conor+dt@...nel.org, krzk+dt@...nel.org, s32@....com,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-iio@...r.kernel.org, Maxime Coquelin <mcoquelin.stm32@...il.com>,
 Alexandre Torgue <alexandre.torgue@...s.st.com>,
 linux-stm32@...md-mailman.stormreply.com,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 3/3] counter: Add STM based counter


Hi William,

On 12/28/25 07:52, William Breathitt Gray wrote:
> On Wed, Dec 17, 2025 at 08:49:57AM +0100, Daniel Lezcano wrote:
>> The NXP S32G2 automotive platform integrates four Cortex-A53 cores and
>> three Cortex-M7 cores, along with a large number of timers and
>> counters. These hardware blocks can be used as clocksources or
>> clockevents, or as timestamp counters shared across the various
>> subsystems running alongside the Linux kernel, such as firmware
>> components. Their actual usage depends on the overall platform
>> software design.
>>
>> In a Linux-based system, the kernel controls the counter, which is a
>> read-only shared resource for the other subsystems. One of its primary
>> purposes is to act as a common timestamp source for messages or
>> traces, allowing correlation of events occurring in different
>> operating system contexts.
>>
>> These changes introduce a basic counter driver that can start, stop,
>> and reset the counter. It also handles overflow accounting and
>> configures the prescaler value.
>>
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> 
> Hi Daniel,
> 
> It sounds like you're trying to implement a clock for timestamping.

Well no, it is a counter which is used for timestamping. It is an 
automotive design.

> Although the Generic Counter interface is flexible enough to shoehorn a
> a clock into its representation, I don't believe it's the right
> abstraction for this particular device.
 >
> Perhaps reimplementing this
> driver under the Linux common clock framework would be a better approach
> to achieve what you want.

The common clock framework ? Sorry I may have misunderstood the CCF but 
how a counter exported and controlled by the userspace can be managed by 
the CCF. Can you elaborate ?

> Regardless, if you do pursue a Counter driver you'll need to follow the
> Generic Counter paradigm[^1] and define at least three core components:
> a Signal, a Synapse, and a Count. Resetting the Count is typically
> implemented by defining a struct counter_ops counter_write()
> callback[^2], while overflows are typically implemented by pushing
> COUNTER_EVENT_OVERFLOW Counter events[^3] that can be watched by
> userspace.

Yes, I think the Generic counter makes sense here for the goal to be 
achieved. Thanks for the pointers, I'll see how the counter fits with 
the paradigm.

   -- Daniel


> William Breathitt Gray
> 
> [^1] https://docs.kernel.org/driver-api/generic-counter.html#paradigm
> [^2] https://docs.kernel.org/driver-api/generic-counter.html#c.counter_ops
> [^3] https://docs.kernel.org/driver-api/generic-counter.html#counter-events


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