linux-kernel - Re: [PATCH v4 3/3] counter: capture-tiecap: capture driver support for ECAP

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Message-ID: <Yvuz9NUWXPhXqzeU@fedora>
Date:   Tue, 16 Aug 2022 11:12:52 -0400
From:   William Breathitt Gray <william.gray@...aro.org>
To:     Julien Panis <jpanis@...libre.com>
Cc:     vilhelm.gray@...il.com, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        mranostay@...com
Subject: Re: [PATCH v4 3/3] counter: capture-tiecap: capture driver support
 for ECAP

On Tue, Aug 16, 2022 at 09:58:10AM +0200, Julien Panis wrote:
> On 14/08/2022 19:03, William Breathitt Gray wrote:
> > On Wed, Aug 10, 2022 at 04:07:24PM +0200, Julien Panis wrote:
> > > +static int ecap_cnt_function_read(struct counter_device *counter,
> > > +				  struct counter_count *count,
> > > +				  enum counter_function *function)
> > > +{
> > > +	*function = COUNTER_FUNCTION_INCREASE;
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int ecap_cnt_action_read(struct counter_device *counter,
> > > +				struct counter_count *count,
> > > +				struct counter_synapse *synapse,
> > > +				enum counter_synapse_action *action)
> > > +{
> > > +	*action = COUNTER_SYNAPSE_ACTION_BOTH_EDGES;
> > > +
> > > +	return 0;
> > > +}
> > Right now you have a Signal defined for the ECAPSIG line, but there is
> > at least one more relevant Signal to define: the clock updating ECAPCNT.
> > The Synapse action of COUNTER_SYNAPSE_ACTION_BOTH_EDGES is for that
> > clock Signal, but for the ECAPSIG Signal you will need to report a
> > Synapse action based on the polarity of the next capture (i.e. whether
> > high or low).
> 
> Just to be sure : by using the word ECAPCNT, I guess that you speak about
> the
> Mod4 counter (0->1->2->3->0...), don't you ? (2 bits)
> Or do you speak about ECAP_TSCNT_REG register content ? (32 bits)

Sorry for the confusion, I'm talking about ECAP_TSCNT_REG (32-bit) here.
You should rename this Count in your ecap_cnt_counts array from
"ECAPCNT" to "Time-Stamp Counter" to make it clearer to users as well;
it would be prudent to rename "ECAPSIG" too.

I didn't know that there was a register exposing the Mod4 counter value.
If that's true, then define a Count for the Mod4 counter in your
ecap_cnt_counts array.

> > > +static struct counter_comp ecap_cnt_count_ext[] = {
> > > +	COUNTER_COMP_COUNT_U64("capture1", ecap_cnt_cap1_read, NULL),
> > > +	COUNTER_COMP_COUNT_U64("capture2", ecap_cnt_cap2_read, NULL),
> > > +	COUNTER_COMP_COUNT_U64("capture3", ecap_cnt_cap3_read, NULL),
> > > +	COUNTER_COMP_COUNT_U64("capture4", ecap_cnt_cap4_read, NULL),
> > > +	COUNTER_COMP_ENABLE(ecap_cnt_enable_read, ecap_cnt_enable_write),
> > I just want to verify: this enable extension should disable the ECAPCNT
> > count itself (i.e. no more increasing count value). Is that what's
> > happening here, or is this meant to disable just the captures?
> 
> Yes, it is what's happening here : no more increasing count value.

Okay that's good. By the way, COUNTER_COMP_ENABLE ensures the enable
value passed to ecap_cnt_enable_write() is either 0 or 1, so you don't
need the enable > 1 check in your callback.

> > > +static irqreturn_t ecap_cnt_isr(int irq, void *dev_id)
> > > +{
> > > +	struct counter_device *counter_dev = dev_id;
> > > +	struct ecap_cnt_dev *ecap_dev = counter_priv(counter_dev);
> > > +	unsigned int clr = 0;
> > > +	unsigned int flg;
> > > +	int i;
> > > +	unsigned long flags;
> > > +
> > > +	regmap_read(ecap_dev->regmap, ECAP_ECINT_EN_FLG_REG, &flg);
> > > +
> > > +	for (i = ECAP_NB_CAP - 1 ; i < ECAP_NB_CEVT ; i++) {
> > Would you walk me through the logic for this loop. Is this for-loop
> > intended to loop through all four capture indices? ECAP_NB_CAP and
> > ECAP_NB_CEVT are defines, so your for-loop has i=3 and i<5; is this what
> > you want?
> 
> In previous versions (IIO subsys), this for-loop was intended to loop
> through all 4 capture indices
> and overflow flag.
> In this version, it has been modified to loop only for the last capture
> indice (the 4th)
> and overflow flag : yes, this is intentional. Only 1 event has to be pushed
> so that the user
> gets all 4 captured timestamps in a single-reading (using 4 watches).
> But if I understand well your previous suggestion, you would like tracking
> Mod4 counter value,
> don't you ? So, I will change back this for-loop, so that it loops for all
> capture indices (and
> overflow flag) -> For all 4 capture indices, Mod4 count will be updated. And
> event will still be
> pushed only for the 4th capture indice.

Instead of limiting the event push to just the 4th capture, I'd push
COUNTER_EVENT_CAPTURE on every capture but delegate them to their own
channels::

    counter_push_event(counter_dev, COUNTER_EVENT_CAPTURE, i);

The captures operate as a circular buffer, so the user can determine the
current capture index based on the watch channel reported and perform a
modulo to read the buffers in right sequence. Alternatively, they can
watch just channel 3 if they want to process only four captures at a
time.

William Breathitt Gray

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