Re: [PATCH v5 3/5] counter: Add character device interface

[David Lechner <david@lechnology.com>]

On 10/14/20 2:05 PM, William Breathitt Gray wrote:
> On Wed, Oct 14, 2020 at 12:43:08PM -0500, David Lechner wrote:
>> On 9/26/20 9:18 PM, William Breathitt Gray wrote:
>>> diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/counter-chrdev.c
>>> new file mode 100644
>>> index 000000000000..2be3846e4105
>>> --- /dev/null
>>> +++ b/drivers/counter/counter-chrdev.c
>>
>>
>>> +/**
>>> + * counter_push_event - queue event for userspace reading
>>> + * @counter:	pointer to Counter structure
>>> + * @event:	triggered event
>>> + * @channel:	event channel
>>> + *
>>> + * Note: If no one is watching for the respective event, it is silently
>>> + * discarded.
>>> + *
>>> + * RETURNS:
>>> + * 0 on success, negative error number on failure.
>>> + */
>>> +int counter_push_event(struct counter_device *const counter, const u8 event,
>>> +		       const u8 channel)
>>> +{
>>> +	struct counter_event ev = {0};
>>> +	unsigned int copied = 0;
>>> +	unsigned long flags;
>>> +	struct counter_event_node *event_node;
>>> +	struct counter_comp_node *comp_node;
>>> +	int err;
>>> +
>>> +	ev.timestamp = ktime_get_ns();
>>> +	ev.watch.event = event;
>>> +	ev.watch.channel = channel;
>>> +
>>> +	raw_spin_lock_irqsave(&counter->events_lock, flags);
>>> +
>>> +	/* Search for event in the list */
>>> +	list_for_each_entry(event_node, &counter->events_list, l)
>>> +		if (event_node->event == event &&
>>> +		    event_node->channel == channel)
>>> +			break;
>>> +
>>> +	/* If event is not in the list */
>>> +	if (&event_node->l == &counter->events_list)
>>> +		goto exit_early;
>>> +
>>> +	/* Read and queue relevant comp for userspace */
>>> +	list_for_each_entry(comp_node, &event_node->comp_list, l) {
>>> +		err = counter_get_data(counter, comp_node, &ev.value_u8);
>>
>> Currently all counter devices are memory mapped devices so calling
>> counter_get_data() here with interrupts disabled is probably OK, but
>> if any counter drivers are added that use I2C/SPI/etc. that will take
>> a long time to read, it would cause problems leaving interrupts
>> disabled here.
>>
>> Brainstorming: Would it make sense to separate the event from the
>> component value being read? As I mentioned in one of my previous
>> reviews, I think there are some cases where we would just want to
>> know when an event happened and not read any additional data anyway.
>> In the case of a slow communication bus, this would also let us
>> queue the event in the kfifo and notify poll right away and then
>> defer the reads in a workqueue for later.
> 
> I don't see any problems with reporting just an event without any
> component value attached (e.g. userspace could handle the component
> reads via sysfs at a later point). We would just need a way to inform
> userspace that the struct counter_component in the struct counter_watch
> returned should be ignored.
> 
> Perhaps we can add an additional member to struct counter_watch
> indicating whether it's an empty watch; or alternatively, add a new
> component scope define to differentiate between an actual component and
> an empty one (e.g. COUNTER_SCOPE_EVENT). What do you think?
> 
> William Breathitt Gray
> 

I made the same suggestion in one of my other replies - except
I called it COUNTER_SCOPE_NONE.