linux-kernel - Re: [PATCH v4 07/13] firmware: arm_scmi: Add notification dispatch and delivery

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Message-ID: <cb4e38be-e8f7-483f-feda-94a19cad1ab1@arm.com>
Date:   Thu, 12 Mar 2020 20:57:28 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Cristian Marussi <cristian.marussi@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc:     sudeep.holla@....com, james.quinlan@...adcom.com,
        Jonathan.Cameron@...wei.com
Subject: Re: [PATCH v4 07/13] firmware: arm_scmi: Add notification dispatch
 and delivery



On 3/12/20 7:24 PM, Cristian Marussi wrote:
> On 12/03/2020 14:06, Lukasz Luba wrote:
>>
>>
>> On 3/12/20 1:51 PM, Lukasz Luba wrote:
>>> Hi Cristian,
>>>
> 
> Hi Lukasz
> 
>>> just one comment below...
>>>
>>> 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@....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?
>>
>> and also from the kfifo_in
>>
> 
> kfifo_in returns the number of effectively written bytes (using __kfifo_in),
> possibly capped to the effectively maximum available space in the fifo, BUT since I
> absolutely cannot afford to write an incomplete/truncated event into the queue, I check
> that in advance and backout on queue full:
> 
> if (unlikely(kfifo_avail(&r_evt->proto->equeue.kfifo) < sizeof(eh) + len)) {
> 	return -ENOMEM;
> 
> and given that the ISR scmi_notify() is the only possible writer on this queue

Yes, your are right, no other IRQ will show up for this channel till
we exit mailbox rx callback and clean the bits.

> I can be sure that the kfifo_in() will succeed in writing the required number of
> bytes after the above check...so I don't need to check the return value.
> 
> Regards
> 
> Cristian
> 
>>>
>>> Regards,
>>> Lukasz
>>>
>>>
>