Hi Cristian,

This looks very robust and general but I'm wondering about the
"notification -> user" latency.  It appears there are at least five or
so memcpy()s of the notification message as it works its way to the
user, and added to that is the work queue deferral latency.  Is there
a concern on your end for low latency notifications, perhaps for
emergency notifications or do you consider this to be quick enough?

Note that we (BrcmSTB) have implemented our own  SCMI notification
system for a proprietary protocol and we designed it to deliver
notifications as quickly as possible.  Our system is more or less a
disposable hack and would not stand up to heavy or general usage.  We
fully intend to move to the approved Linux notification system.  On
that note,  I'm just wondering if you had any comment on the
possibility of  "slimming down" your RFC, e.g. perhaps somehow
collapsing some memcpy()s.

Thanks,
Jim
PS I'm going on vacation so I won't be able to email for a week.


On Fri, Feb 14, 2020 at 10:36 AM Cristian Marussi
[off-list ref] wrote:

Hi all,

this series wants to introduce SCMI Notification Support, built on top of
the standard Kernel notification chain subsystem.

At initialization time each SCMI Protocol takes care to register with the
new SCMI notification core the set of its own events which it intends to
support.

Using a possibly proposed API in include/linux/scmi_protocol.h (not
finalized though, NO EXPORTs_) a Kernel user can register its own
notifier_t callback (via a notifier_block as usual) against any registered
event as identified by the tuple:

                (proto_id, event_id, src_id)

where src_id represents a generic source identifier which is protocol
dependent like domain_id, performance_id, sensor_id and so forth.
(users can anyway do NOT provide any src_id, and subscribe instead to ALL
 the existing (if any) src_id sources for that proto_id/evt_id combination)

Each of the above tuple-specified event will be served on its own dedicated
blocking notification chain.

Upon a notification delivery all the users' registered notifier_t callbacks
will be in turn invoked and fed with the event_id as @action param and a
generated custom per-event struct _report as @data param.
(as in include/linux/scmi_protocol.h)

The final step of notification delivery via users' callback invocation is
instead delegated to a pool of deferred workers (Kernel cmwq): each
SCMI protocol has its own dedicated worker and dedicated queue to push
events from the rx ISR to the worker.

The series is marked as RFC mainly because:

- the API as said is tentative and not EXPORTed; currently consisting of a
  generic interface like:

         scmi_register_event_notifier(proto_id, evt_id, *src_id, *nb)

  as found in scmi_protocol.h, or using the equivalent 'handle' operations
  in scmi_notify_ops if used by an scmi_driver.

  It's open for discussion.

- no Event priorization has been considered: each protocol has its own
  queue and deferred worker instance, so as to avoid that one protocol
  flood can overrun a single queue and influence other protocols'
  notifications' delivery.
  But that's it, all the workers are unbound, low_pri cmwq workers.

  Should we enforce some sort of built-in prio amongst the events ?
  Should this priority instead be compile time configurable ?

  Again, open for discussion.

- no configuration is possible: it can be imagined that on a real platform
  events' priority (if any) and events queues' depth could be something
  somehow compile-time configurable, but this is not addressed by this
  series at all.

Based on scmi-next 5.6 [1], on top of:

commit 5c8a47a5a91d ("firmware: arm_scmi: Make scmi core independent of
                      the transport type")

This series has been tested on JUNO with an experimental firmware only
supporting Perf Notifications.

Any thoughts ?

Thanks

Cristian
----

v1 --> v2:
- dropped anti-tampering patch
- rebased on top of scmi-for-next-5.6, which includes Viresh series that
  make SCMI core independent of transport (5c8a47a5a91d)
- add a few new SCMI transport methods on top of Viresh patch to address
  needs of SCMI Notifications
- reviewed/renamed scmi_handle_xfer_delayed_resp()
- split main SCMI Notification core patch (~1k lines) into three chunks:
  protocol-registration / callbacks-registration / dispatch-and-delivery
- removed awkward usage of IDR maps in favour of pure hashtables
- added enable/disable refcounting in notification core (was broken in v1)
- removed per-protocol candidate API: a single generic API is now proposed
  instead of scmi_register_<proto>_event_notifier(evt_id, *src_id, *nb)
- added handle->notify_ops as an alternative notification API
  for scmi_driver
- moved ALL_SRCIDs enabled handling from protocol code to core code
- reviewed protocol registration/unregistration logic to use devres
- reviewed cleanup phase on shutdown
- fixed  ERROR: reference preceded by free as reported by kbuild test robot

[1] git://git.kernel.org/pub/scm/linux/kernel/git/sudeep.holla/linux.git

Cristian Marussi (10):
  firmware: arm_scmi: Add notifications support in transport layer
  firmware: arm_scmi: Add notification protocol-registration
  firmware: arm_scmi: Add notification callbacks-registration
  firmware: arm_scmi: Add notification dispatch and delivery
  firmware: arm_scmi: Enable notification core
  firmware: arm_scmi: Add Power notifications support
  firmware: arm_scmi: Add Perf notifications support
  firmware: arm_scmi: Add Sensor notifications support
  firmware: arm_scmi: Add Reset notifications support
  firmware: arm_scmi: Add Base notifications support

Sudeep Holla (3):
  firmware: arm_scmi: Add receive buffer support for notifications
  firmware: arm_scmi: Update protocol commands and notification list
  firmware: arm_scmi: Add support for notifications message processing

 drivers/firmware/arm_scmi/Makefile  |    2 +-
 drivers/firmware/arm_scmi/base.c    |  121 +++
 drivers/firmware/arm_scmi/bus.c     |   11 +
 drivers/firmware/arm_scmi/common.h  |   12 +
 drivers/firmware/arm_scmi/driver.c  |  118 ++-
 drivers/firmware/arm_scmi/mailbox.c |   17 +
 drivers/firmware/arm_scmi/notify.c  | 1102 +++++++++++++++++++++++++++
 drivers/firmware/arm_scmi/notify.h  |   78 ++
 drivers/firmware/arm_scmi/perf.c    |  140 +++-
 drivers/firmware/arm_scmi/power.c   |  133 +++-
 drivers/firmware/arm_scmi/reset.c   |  100 ++-
 drivers/firmware/arm_scmi/sensors.c |   77 +-
 drivers/firmware/arm_scmi/shmem.c   |   15 +
 include/linux/scmi_protocol.h       |  114 +++
 14 files changed, 2009 insertions(+), 31 deletions(-)
 create mode 100644 drivers/firmware/arm_scmi/notify.c
 create mode 100644 drivers/firmware/arm_scmi/notify.h

--
2.17.1

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