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Subject: Re: [PATCH v4 07/13] firmware: arm_scmi: Add notification dispatch
 and delivery
To:     Cristian Marussi <cristian.marussi@arm.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Cc:     sudeep.holla@arm.com, james.quinlan@broadcom.com,
        Jonathan.Cameron@Huawei.com
References: <20200304162558.48836-1-cristian.marussi@arm.com>
 <20200304162558.48836-8-cristian.marussi@arm.com>
 <45d4aee9-57df-6be9-c176-cf0d03940c21@arm.com>
 <363cb1ba-76b5-cc1e-af45-454837fae788@arm.com>
From:   Lukasz Luba <lukasz.luba@arm.com>
Message-ID: <484214b4-a71d-9c63-86fc-2e469cb1809b@arm.com>
Date:   Thu, 12 Mar 2020 21:43:31 +0000
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On 3/12/20 6:34 PM, Cristian Marussi wrote:
> On 12/03/2020 13:51, Lukasz Luba wrote:
>> Hi Cristian,
>>
>> just one comment below...
> 
> Hi Lukasz
> 
> Thanks for the review
> 
>>
>> On 3/4/20 4:25 PM, Cristian Marussi wrote:
>>> Add core SCMI Notifications dispatch and delivery support logic which is
>>> able, at first, to dispatch well-known received events from the RX ISR to
>>> the dedicated deferred worker, and then, from there, to final deliver the
>>> events to the registered users' callbacks.
>>>
>>> Dispatch and delivery is just added here, still not enabled.
>>>
>>> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
>>> ---
>>> V3 --> V4
>>> - dispatcher now handles dequeuing of events in chunks (header+payload):
>>>     handling of these in_flight events let us remove one unneeded memcpy
>>>     on RX interrupt path (scmi_notify)
>>> - deferred dispatcher now access their own per-protocol handlers' table
>>>     reducing locking contention on the RX path
>>> V2 --> V3
>>> - exposing wq in sysfs via WQ_SYSFS
>>> V1 --> V2
>>> - splitted out of V1 patch 04
>>> - moved from IDR maps to real HashTables to store event_handlers
>>> - simplified delivery logic
>>> ---
>>>    drivers/firmware/arm_scmi/notify.c | 334 ++++++++++++++++++++++++++++-
>>>    drivers/firmware/arm_scmi/notify.h |   9 +
>>>    2 files changed, 342 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/firmware/arm_scmi/notify.c b/drivers/firmware/arm_scmi/notify.c
>>
>> [snip]
>>
>>> +
>>> +/**
>>> + * scmi_notify  - Queues a notification for further deferred processing
>>> + *
>>> + * This is called in interrupt context to queue a received event for
>>> + * deferred processing.
>>> + *
>>> + * @handle: The handle identifying the platform instance from which the
>>> + *	    dispatched event is generated
>>> + * @proto_id: Protocol ID
>>> + * @evt_id: Event ID (msgID)
>>> + * @buf: Event Message Payload (without the header)
>>> + * @len: Event Message Payload size
>>> + * @ts: RX Timestamp in nanoseconds (boottime)
>>> + *
>>> + * Return: 0 on Success
>>> + */
>>> +int scmi_notify(const struct scmi_handle *handle, u8 proto_id, u8 evt_id,
>>> +		const void *buf, size_t len, u64 ts)
>>> +{
>>> +	struct scmi_registered_event *r_evt;
>>> +	struct scmi_event_header eh;
>>> +	struct scmi_notify_instance *ni = handle->notify_priv;
>>> +
>>> +	/* Ensure atomic value is updated */
>>> +	smp_mb__before_atomic();
>>> +	if (unlikely(!atomic_read(&ni->enabled)))
>>> +		return 0;
>>> +
>>> +	r_evt = SCMI_GET_REVT(ni, proto_id, evt_id);
>>> +	if (unlikely(!r_evt))
>>> +		return -EINVAL;
>>> +
>>> +	if (unlikely(len > r_evt->evt->max_payld_sz)) {
>>> +		pr_err("SCMI Notifications: discard badly sized message\n");
>>> +		return -EINVAL;
>>> +	}
>>> +	if (unlikely(kfifo_avail(&r_evt->proto->equeue.kfifo) <
>>> +		     sizeof(eh) + len)) {
>>> +		pr_warn("SCMI Notifications: queue full dropping proto_id:%d  evt_id:%d  ts:%lld\n",
>>> +			proto_id, evt_id, ts);
>>> +		return -ENOMEM;
>>> +	}
>>> +
>>> +	eh.timestamp = ts;
>>> +	eh.evt_id = evt_id;
>>> +	eh.payld_sz = len;
>>> +	kfifo_in(&r_evt->proto->equeue.kfifo, &eh, sizeof(eh));
>>> +	kfifo_in(&r_evt->proto->equeue.kfifo, buf, len);
>>> +	queue_work(r_evt->proto->equeue.wq,
>>> +		   &r_evt->proto->equeue.notify_work);
>>
>> Is it safe to ignore the return value from the queue_work here?
>>
> 
> In fact yes, we do not want to care: it returns true or false depending on the
> fact that the specific work was or not already queued, and we just rely on
> this behavior to keep kicking the worker only when needed but never kick
> more than one instance of it per-queue (so that there's only one reader
> wq and one writer here in the scmi_notify)...explaining better:
> 
> 1. we push an event (hdr+payld) to the protocol queue if we found that there was
> enough space on the queue
> 
> 2a. if at the time of the kfifo_in( ) the worker was already running
> (queue not empty) it will process our new event sooner or later and here
> the queue_work will return false, but we do not care in fact ... we
> tried to kick it just in case
> 
> 2b. if instead at the time of the kfifo_in() the queue was empty the worker would
> have probably already gone to the sleep and this queue_work() will return true and
> so this time it will effectively wake up the worker to process our items
> 
> The important thing here is that we are sure to wakeup the worker when needed
> but we are equally sure we are never causing the scheduling of more than one worker
> thread consuming from the same queue (because that would break the one reader/one writer
> assumption which let us use the fifo in a lockless manner): this is possible because
> queue_work checks if the required work item is already pending and in such a case backs
> out returning false and we have one work_item (notify_work) defined per-protocol and
> so per-queue.

I see. That's a good assumption: one work_item per protocol and simplify
the locking. What if there would be an edge case scenario when the
consumer (work_item) has handled the last item (there was NULL from 
scmi_process_event_header()), while in meantime scmi_notify put into
the fifo new event but couldn't kick the queue_work. Would it stay there
till the next IRQ which triggers queue_work to consume two events (one
potentially a bit old)? Or we can ignore such race situation assuming
that cleaning of work item is instant and kfifo_in is slow?

> 
> Now probably I wrote too much of an explanation and confuse stuff more ... :D

No, thank you for the detailed explanation. I will continue my review.

Regards,
Lukasz