Messages in this thread | | | Subject | Re: [PATCH v4 07/13] firmware: arm_scmi: Add notification dispatch and delivery | From | Lukasz Luba <> | 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
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