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Subject: Re: [RFC v3 09/12] gpiolib: cdev: Add hardware timestamp clock type
From:   Dipen Patel <dipenp@nvidia.com>
To:     Kent Gibson <warthog618@gmail.com>
CC:     <thierry.reding@gmail.com>, <jonathanh@nvidia.com>,
        <linux-kernel@vger.kernel.org>, <linux-tegra@vger.kernel.org>,
        <linux-gpio@vger.kernel.org>, <linus.walleij@linaro.org>,
        <brgl@bgdev.pl>, <devicetree@vger.kernel.org>,
        <linux-doc@vger.kernel.org>, <robh+dt@kernel.org>
References: <20211123193039.25154-1-dipenp@nvidia.com>
 <20211123193039.25154-10-dipenp@nvidia.com> <20211126013137.GC10380@sol>
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Date:   Wed, 1 Dec 2021 09:18:09 -0800
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On 11/30/21 7:29 PM, Dipen Patel wrote:
> Hi,
>
> On 11/25/21 5:31 PM, Kent Gibson wrote:
>> On Tue, Nov 23, 2021 at 11:30:36AM -0800, Dipen Patel wrote:
>>> This patch adds new clock type for the GPIO controller which can
>>> timestamp gpio lines in realtime using hardware means. To expose such
>>> functionalities to the userspace, code has been added in this patch
>>> where during line create call, it checks for new clock type and if
>>> requested, calls hardware timestamp related API from gpiolib.c.
>>> During line change event, the HTE subsystem pushes timestamp data
>>> through callbacks.
>>>
>>> Signed-off-by: Dipen Patel <dipenp@nvidia.com>
>>> Acked-by: Linus Walleij <linus.walleij@linaro.org>
>>> ---
>>> Changes in v2:
>>> - Added hte_dir and static structure hte_ts_desc.
>>> - Added callbacks which get invoked by HTE when new data is available.
>>> - Better use of hte_dir and seq from hte_ts_desc.
>>> - Modified sw debounce function to accommodate hardware timestamping.
>>>
>>>  drivers/gpio/gpiolib-cdev.c | 161 ++++++++++++++++++++++++++++++++++--
>>>  include/uapi/linux/gpio.h   |   1 +
>>>  2 files changed, 153 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
>>> index c7b5446d01fd..1736ad54e3ec 100644
>>> --- a/drivers/gpio/gpiolib-cdev.c
>>> +++ b/drivers/gpio/gpiolib-cdev.c
>>> @@ -464,6 +464,12 @@ struct line {
>>>  	 * stale value.
>>>  	 */
>>>  	unsigned int level;
>>> +	/*
>>> +	 * dir will be touched in HTE callbacks hte_ts_cb_t and
>>> +	 * hte_ts_threaded_cb_t and they are mutually exclusive. This will be
>>> +	 * unused when HTE is not supported/disabled.
>>> +	 */
>>> +	enum hte_dir dir;
>>>  };
>>>  
>> Documentation should be in present tense, so 
>>
>> s/will be/is/g
>>
>> Same applies to other patches.
>>
>> Also
>>
>> s/touched/accessed/
>>
>> dir is a poor name for the field.  It is the hte edge direction and
>> effectively the line level, so call it hte_edge_dirn or
>> hte_edge_direction or hte_level.
>>
>> And it is placed in a section of the struct documented as "debouncer specific
>> fields", but it is not specfic to the debouncer.  Add a "hte specific
>> fields" section if nothing else is suitable.
>>
>>>  /**
>>> @@ -518,6 +524,7 @@ struct linereq {
>>>  	 GPIO_V2_LINE_DRIVE_FLAGS | \
>>>  	 GPIO_V2_LINE_EDGE_FLAGS | \
>>>  	 GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME | \
>>> +	 GPIO_V2_LINE_FLAG_EVENT_CLOCK_HARDWARE | \
>>>  	 GPIO_V2_LINE_BIAS_FLAGS)
>>>  
>>>  static void linereq_put_event(struct linereq *lr,
>>> @@ -546,6 +553,94 @@ static u64 line_event_timestamp(struct line *line)
>>>  	return ktime_get_ns();
>>>  }
>>>  
>>> +static hte_return_t process_hw_ts_thread(void *p)
>>> +{
>>> +	struct line *line = p;
>>> +	struct linereq *lr = line->req;
>>> +	struct gpio_v2_line_event le;
>>> +	u64 eflags;
>>> +
>>> +	memset(&le, 0, sizeof(le));
>>> +
>>> +	le.timestamp_ns = line->timestamp_ns;
>>> +	line->timestamp_ns = 0;
>>> +
>> What is the purpose of this zeroing?
>>
>>> +	if (line->dir >= HTE_DIR_NOSUPP) {
>>> +		eflags = READ_ONCE(line->eflags);
>>> +		if (eflags == GPIO_V2_LINE_FLAG_EDGE_BOTH) {
>>> +			int level = gpiod_get_value_cansleep(line->desc);
>>> +
>>> +			if (level)
>>> +				/* Emit low-to-high event */
>>> +				le.id = GPIO_V2_LINE_EVENT_RISING_EDGE;
>>> +			else
>>> +				/* Emit high-to-low event */
>>> +				le.id = GPIO_V2_LINE_EVENT_FALLING_EDGE;
>>> +		} else if (eflags == GPIO_V2_LINE_FLAG_EDGE_RISING) {
>>> +			/* Emit low-to-high event */
>>> +			le.id = GPIO_V2_LINE_EVENT_RISING_EDGE;
>>> +		} else if (eflags == GPIO_V2_LINE_FLAG_EDGE_FALLING) {
>>> +			/* Emit high-to-low event */
>>> +			le.id = GPIO_V2_LINE_EVENT_FALLING_EDGE;
>>> +		} else {
>>> +			return HTE_CB_ERROR;
>>> +		}
>>> +	} else {
>>> +		if (line->dir == HTE_RISING_EDGE_TS)
>>> +			le.id = GPIO_V2_LINE_EVENT_RISING_EDGE;
>>> +		else
>>> +			le.id = GPIO_V2_LINE_EVENT_FALLING_EDGE;
>>> +	}
>> The mapping from line->dir to le.id needs to take into account the active
>> low setting for the line.
>>
>> And it might be simpler if the hte_ts_data provided the level, equivalent
>> to gpiod_get_raw_value_cansleep(), rather than an edge direction, so you
>> can provide a common helper to determine the edge given the raw level.
> (So from the level determine the edge?) that sound right specially when

                                                                ^^^

                                                                does not*

>
> HTE provider has capability to record the edge in that case why bother
>
> getting the level and determine edge?
>
> Calculating the edge from the level makes sense when hte provider does not
>
> have that feature and that is what if (line->dir >= HTE_DIR_NOSUPP) does.
>
>>> +
>>> +	le.line_seqno = line->line_seqno;
>>> +	le.seqno = (lr->num_lines == 1) ? le.line_seqno : line->req_seqno;
>>> +	le.offset = gpio_chip_hwgpio(line->desc);
>>> +
>>> +	linereq_put_event(lr, &le);
>>> +
>>> +	return HTE_CB_HANDLED;
>>> +}
>>> +
>>> +static hte_return_t process_hw_ts(struct hte_ts_data *ts, void *p)
>>> +{
>>> +	struct line *line = p;
>>> +	struct linereq *lr = line->req;
>>> +
>>> +	if (!ts)
>>> +		return HTE_CB_ERROR;
>>> +
>>> +	line->timestamp_ns = ts->tsc;
>>> +	line->dir = ts->dir;
>>> +
>> The doc for timestamp_ns states:
>>
>> 	 * timestamp_ns and req_seqno are accessed only by
>> 	 * edge_irq_handler() and edge_irq_thread(), which are themselves
>> 	 * mutually exclusive, so no additional protection is necessary.
>>
>> That no longer holds.  It is now also accessed here, and in
>> process_hw_ts_thread(), which wont run concurrently with each other or
>> the edge_irq_* handlers, but also in debounce_work_func() which may run
>> concurrently with the others.
>> So timestamp_ns now requires protection from concurrent access.
>>
>>> +	/*
>>> +	 * It is possible that HTE engine detects spurious edges for the
>>> +	 * lines where software debounce is enabled. This primary callback
>>> +	 * will be called multiple times in that case. It will be better to
>>> +	 * let debounce_work_func handle instead of process_hw_ts_thread.
>>> +	 * The timestamp_ns will be overwritten here which is fine as we are
>>> +	 * interested in the last value anyway. The debounce_work_func will
>>> +	 * then just read whatever last line->timestamp_ns is stored. Because
>>> +	 * this callback can be called multiple times, we are not really
>>> +	 * interested in ts->seq.
>>> +	 */
>> Not sure what this is trying to say.
>> Is this the primary callback? Or debounce_irq_handler()?
> This is primary callback called from HTE when it pushes new TS data per line, it
>
> also says so in the second line.
>
>> You say you really aren't interested in ts->seq, but the code immediately
>> uses it.
> That is when sw_debounced is not set and whole paragraph is about when
>
> sw_debounced is set.
>
>> Reword to clarify.
>> And add braces after function names to highlight them, so
>> debounce_work_func().
> Will do.
>>> +	if (!READ_ONCE(line->sw_debounced)) {
>>> +		line->line_seqno = ts->seq;
>>> +
>>> +		/*
>>> +		 * Increment in this callback incase all the lines in linereq
>>> +		 * are enabled for hw timestamping. This will work even if
>>> +		 * subset of lines are enabled for hw timestamping as
>>> +		 * edge_irq_* callbacks will proceed as usual for them.
>>> +		 */
>> s/incase/in case/
>>
>> Not sure what the comment is trying to say. There is no check here that
>> the other lines have HTE enabled.  And that is not relevant anyway.
>> The edge_irq_* handlers will proceed as usual for those lines NOT
>> enabled for hw timestamping.
>>
>> To clarify, the line_seqno indicates where this event lies in the
>> sequence of events for the line.
>> The request seqno indicates where this event lines in the sequence of
>> events for the request.
>> For a single line request these are the same, hence the minor
>> optimisation of not updating lr->seqno below.
>>
>>> +		if (lr->num_lines != 1)
>>> +			line->req_seqno = atomic_inc_return(&lr->seqno);
>>> +
>> The req_seqno should be updated corresponding to the change in the
>> line_reqno.  That always used to be 1, but no longer if hte can discard
>> events, i.e. skip over line_seqnos.
> HTE does not discard any events, it pushes to clients as soon as its
>
> available through primary callback.
>
>> To be consistent, i.e. if events were lost for this line then they were
>> also lost for the requested lines, the lr->seqno should be incremented by
>> the change in line_seqno.  Probably with some sanity checks.
>>
>>> +		return HTE_RUN_THREADED_CB;
>>> +	}
>>> +
>>> +	return HTE_CB_HANDLED;
>>> +}
>>> +
>>>  static irqreturn_t edge_irq_thread(int irq, void *p)
>>>  {
>>>  	struct line *line = p;
>>> @@ -553,6 +648,10 @@ static irqreturn_t edge_irq_thread(int irq, void *p)
>>>  	struct gpio_v2_line_event le;
>>>  	u64 eflags;
>>>  
>>> +	/* Let process_hw_ts_thread handle */
>>> +	if (test_bit(FLAG_EVENT_CLOCK_HARDWARE, &line->desc->flags))
>>> +		return IRQ_HANDLED;
>>> +
>> This adds pointless runtime overhead, and for everyone not just hte users.
>> Don't stub out a handler in the handler - stub it out where it is
>> registered by registering a stub handler.  Or don't request it at all.
>>
>> So why would gpiolib-cdev be requesting the irq, only to stub out
>> the handlers?
>> If that has a side-effect that hte requires then hte should be taking
>> care of it - it is not gpiolib-cdev's problem.
> - Why stop at moving irq and debounce related stuff to hte then?
>
> I mean if there is hte provider which can TS GPIO output/input
>
> does it mean hte is responsible for parsing the GPIO line configs, setting them up
>
> (i.e. input or output) as well? Are we not duplicating logic instead of
>
> leveraging gpio-cdev? Does it make sense for the HTE subsystem which not
>
> only TS the GPIOs but other SoC lines?
>
> - What happens to in kernel GPIO HTE client (for example, hte-tegra194-gpio-test.c)?
>
> some clients do more in their IRQ handler than what edge_irq_handler does in which
>
> case it would make sense to have them request irq in their code than through HTE.
>
>> And speaking as to how the whole hte/gpiolib-cdev interface should work,
>> hte should be an edge event generator alternative to irq.  So lines with
>> hte enabled should work without any irq calls from gpiolib-cdev.
>> That includes the sw debouncer - more on that below.
>>
>>>  	/* Do not leak kernel stack to userspace */
>>>  	memset(&le, 0, sizeof(le));
>>>  
>>> @@ -604,6 +703,10 @@ static irqreturn_t edge_irq_handler(int irq, void *p)
>>>  	struct line *line = p;
>>>  	struct linereq *lr = line->req;
>>>  
>>> +	/* Let HTE supplied callbacks handle */
>>> +	if (test_bit(FLAG_EVENT_CLOCK_HARDWARE, &line->desc->flags))
>>> +		return IRQ_HANDLED;
>>> +
>>>  	/*
>>>  	 * Just store the timestamp in hardirq context so we get it as
>>>  	 * close in time as possible to the actual event.
>>> @@ -682,14 +785,6 @@ static void debounce_work_func(struct work_struct *work)
>>>  	/* Do not leak kernel stack to userspace */
>>>  	memset(&le, 0, sizeof(le));
>>>  
>>> -	lr = line->req;
>>> -	le.timestamp_ns = line_event_timestamp(line);
>>> -	le.offset = gpio_chip_hwgpio(line->desc);
>>> -	line->line_seqno++;
>>> -	le.line_seqno = line->line_seqno;
>>> -	le.seqno = (lr->num_lines == 1) ?
>>> -		le.line_seqno : atomic_inc_return(&lr->seqno);
>>> -
>>>  	if (level)
>>>  		/* Emit low-to-high event */
>>>  		le.id = GPIO_V2_LINE_EVENT_RISING_EDGE;
>>> @@ -697,6 +792,23 @@ static void debounce_work_func(struct work_struct *work)
>>>  		/* Emit high-to-low event */
>>>  		le.id = GPIO_V2_LINE_EVENT_FALLING_EDGE;
>>>  
>>> +	if (test_bit(FLAG_EVENT_CLOCK_HARDWARE, &line->desc->flags)) {
>>> +		le.timestamp_ns = line->timestamp_ns;
>>> +		if (line->dir < HTE_DIR_NOSUPP)
>>> +			le.id = (line->dir == HTE_RISING_EDGE_TS) ?
>>> +				 GPIO_V2_LINE_EVENT_RISING_EDGE :
>>> +				 GPIO_V2_LINE_EVENT_FALLING_EDGE;
>>> +	} else {
>>> +		le.timestamp_ns = line_event_timestamp(line);
>>> +	}
>>> +
>> Move the FLAG_EVENT_CLOCK_HARDWARE check into line_event_timestamp().
>>
>> And the id fudging is necessary because the level returned by
>> gpiod_get_raw_value_cansleep() can disagree with the level from hte?
>> So you are still trying to synchronise events from two streams.
>> And that is still broken.
>> If a hte event occurs between the level being sampled by
>> gpiod_get_raw_value_cansleep() and the line->dir being read then the line
>> will have toggled and you will be reporting the opposite state than the
>> one the debouncer determined was stable.  And maybe the wrong timestamp as
>> well.
>>
>> For lines where hte is enabled, the hte should be the source of level for
>> the debouncer, not the raw value.  And the mod_delayed_work() that
>> drives the debouncer should be called by a hte handler, not an irq handler.
>>
>> There is also a race on reading the hte timestamp (line->timestamp_ns) and
>> the hte level (line->dir), such that you can get the level from one event
>> the timestamp from another.
>>
>>> +	lr = line->req;
>>> +	le.offset = gpio_chip_hwgpio(line->desc);
>>> +	line->line_seqno++;
>>> +	le.line_seqno = line->line_seqno;
>>> +	le.seqno = (lr->num_lines == 1) ?
>>> +		le.line_seqno : atomic_inc_return(&lr->seqno);
>>> +
>> What is the purpose of moving this block of code moved from before the
>> if (level)?
>>
>>
>>>  	linereq_put_event(lr, &le);
>>>  }
>>>  
>>> @@ -891,7 +1003,6 @@ static int gpio_v2_line_flags_validate(u64 flags)
>>>  	/* Return an error if an unknown flag is set */
>>>  	if (flags & ~GPIO_V2_LINE_VALID_FLAGS)
>>>  		return -EINVAL;
>>> -
>> Gratuitous whitespace change.
>>
>>>  	/*
>>>  	 * Do not allow both INPUT and OUTPUT flags to be set as they are
>>>  	 * contradictory.
>>> @@ -900,6 +1011,11 @@ static int gpio_v2_line_flags_validate(u64 flags)
>>>  	    (flags & GPIO_V2_LINE_FLAG_OUTPUT))
>>>  		return -EINVAL;
>>>  
>>> +	/* Only allow one event clock source */
>>> +	if ((flags & GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME) &&
>>> +	    (flags & GPIO_V2_LINE_FLAG_EVENT_CLOCK_HARDWARE))
>>> +		return -EINVAL;
>>> +
>>>  	/* Edge detection requires explicit input. */
>>>  	if ((flags & GPIO_V2_LINE_EDGE_FLAGS) &&
>>>  	    !(flags & GPIO_V2_LINE_FLAG_INPUT))
>>> @@ -992,6 +1108,8 @@ static void gpio_v2_line_config_flags_to_desc_flags(u64 flags,
>>>  
>>>  	assign_bit(FLAG_EVENT_CLOCK_REALTIME, flagsp,
>>>  		   flags & GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME);
>>> +	assign_bit(FLAG_EVENT_CLOCK_HARDWARE, flagsp,
>>> +		   flags & GPIO_V2_LINE_FLAG_EVENT_CLOCK_HARDWARE);
>>>  }
>>>  
>>>  static long linereq_get_values(struct linereq *lr, void __user *ip)
>>> @@ -1154,6 +1272,21 @@ static long linereq_set_config_unlocked(struct linereq *lr,
>>>  				return ret;
>>>  		}
>>>  
>>> +		/* Check if new config sets hardware assisted clock */
>>> +		if (flags & GPIO_V2_LINE_FLAG_EVENT_CLOCK_HARDWARE) {
>>> +			ret = gpiod_req_hw_timestamp_ns(desc, process_hw_ts,
>>> +							process_hw_ts_thread,
>>> +							&lr->lines[i]);
>>> +			if (ret)
>>> +				return ret;
>> Note that the line config is the complete line config, not a delta.
>>
>> What happens when a line that already has hte enabled is reconfigured
>> and still has hte enabled?  i.e. what happens when
>> gpiod_req_hw_timestamp_ns() is called for the second time?
> HTE will return without doing anything with error code.
>
>> You provide a comment for the release case below, what of the request
>> case?
>>
>> If you need to check for change then compare the old and new flags, as
>> the polarity_change check does (not visible in the diff here).
>>
>>> +		} else {
>>> +			/*
>>> +			 * HTE subsys will do nothing if there is nothing to
>>> +			 * release.
>>> +			 */
>>> +			gpiod_rel_hw_timestamp_ns(desc);
>>> +		}
>>> +
>> Comment will fit on one line.
>>
>> And it would be better to document that the function is idempotent in the
>> function documentation, not everywhere it is used.
>>
>>>  		blocking_notifier_call_chain(&desc->gdev->notifier,
>>>  					     GPIO_V2_LINE_CHANGED_CONFIG,
>>>  					     desc);
>>> @@ -1409,6 +1542,14 @@ static int linereq_create(struct gpio_device *gdev, void __user *ip)
>>>  					flags & GPIO_V2_LINE_EDGE_FLAGS);
>>>  			if (ret)
>>>  				goto out_free_linereq;
>>> +
>>> +			if (flags & GPIO_V2_LINE_FLAG_EVENT_CLOCK_HARDWARE) {
>>> +				ret = gpiod_req_hw_timestamp_ns(desc, process_hw_ts,
>>> +							process_hw_ts_thread,
>>> +							&lr->lines[i]);
>>> +				if (ret)
>>> +					goto out_free_linereq;
>>> +			}
>>>  		}
>>>  
>>>  		blocking_notifier_call_chain(&desc->gdev->notifier,
>>> @@ -1959,6 +2100,8 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
>>>  
>>>  	if (test_bit(FLAG_EVENT_CLOCK_REALTIME, &desc->flags))
>>>  		info->flags |= GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME;
>>> +	else if (test_bit(FLAG_EVENT_CLOCK_HARDWARE, &desc->flags))
>>> +		info->flags |= GPIO_V2_LINE_FLAG_EVENT_CLOCK_HARDWARE;
>>>  
>>>  	debounce_period_us = READ_ONCE(desc->debounce_period_us);
>>>  	if (debounce_period_us) {
>>> diff --git a/include/uapi/linux/gpio.h b/include/uapi/linux/gpio.h
>>> index eaaea3d8e6b4..d360545b4c21 100644
>>> --- a/include/uapi/linux/gpio.h
>>> +++ b/include/uapi/linux/gpio.h
>>> @@ -80,6 +80,7 @@ enum gpio_v2_line_flag {
>>>  	GPIO_V2_LINE_FLAG_BIAS_PULL_DOWN	= _BITULL(9),
>>>  	GPIO_V2_LINE_FLAG_BIAS_DISABLED		= _BITULL(10),
>>>  	GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME	= _BITULL(11),
>>> +	GPIO_V2_LINE_FLAG_EVENT_CLOCK_HARDWARE	= _BITULL(12),
>>>  };
>>>  
>> I'm now thinking this name, "HARDWARE" is too vague, in case other
>> timestamp source alternatives join the fray, and so should be "HTE".
>>
>> Cheers,
>> Kent.