[PATCH 0/9] Add SCMI Virtio & Clock atomic support

[Cristian Marussi]

Hi,

This small series is the tail-subset of the previous V8 series about atomic
support in SCMI [1], whose 8-patches head-subset has now been queued on
[2]; as such, it is based on [2] on top of tag scmi-updates-5.17:

commit 94d0cd1da14a ("firmware: arm_scmi: Add new parameter to
		     mark_txdone")

Patch [1/9] substitute virtio-scmi ready flag and lock with a reference
counter to keep track of vio channels lifetime while removing the need of
a wide spinlocked section (that would cause issues with introduction of
virtio polling support)

Patch [2/9] adds a few helpers to handle the TX free_list and a dedicated
spinlock to reduce the reliance on the main one.

Patch [3/9] is an RFC patch to the virtio core around virtqueue_poll() API:
the aim is to solve a possible ABA problem while polling on a very busy
virtqueue.

Patch [4/9] adds polling mode to SCMI VirtIO transport in order to support
atomic operations on such transport.

Patches [5,6/9] introduce a new optional SCMI binding, atomic_threshold, to
configure a platform specific time threshold used in the following patches
to select with a finer grain which SCMI resources should be eligible for
atomic operations when requested.

Patch [7/9] exposes new SCMI Clock protocol operations to allow an SCMI
user to request atomic mode on clock enable commands.

Patch [8/9] adds support to SCMI Clock protocol for a new clock attributes
field which advertises typical enable latency for a specific resource.
It is marked as RFC since the SCMI spec including such addition is still
to be finalized.

Finally patch [9/9] add support for atomic operations to the SCMI clock
driver; the underlying logic here is that we register with the Clock
framework atomic-capable clock resources if and only if the backing SCMI
transport is capable of atomic operations AND the specific clock resource
has been advertised by the SCMI platform as having:

	clock_enable_latency <= atomic_threshold

The idea is to avoid costly atomic busy-waiting for resources that have
been advertised as 'slow' to operate upon. (i.e. a PLL vs a gating clock)
This last patch is marked as RFC too since it is dependent on the previous
one (that is RFC-tagged too)

To ease testing the whole series can be find at [3].

Any feedback/testing welcome as usual.

Thanks,
Cristian

[1]: https://lore.kernel.org/linux-arm-kernel/20211220195646.44498-1-cristian.marussi@arm.com/
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/sudeep.holla/linux.git/tag/?h=scmi-updates-5.17
[3]: https://gitlab.arm.com/linux-arm/linux-cm/-/commits/scmi_atomic_clk_virtio_V2/

---
V1 --> V2
 - added vio channel refcount support patch
 - reviewed free_list support and usage
 - added virtio_ring RFC patch
 - shrinked spinlocked section within virtio_poll_done to exclude
   virtqueue_poll call
 - removed poll_lock
 - use vio channel refcount acquire/release logic when polling
 - using new free_list accessors
 - added new dedicated pending_lock to access pending_cmds_list
 - fixed a few comments

Cristian Marussi (9):
  firmware: arm_scmi: Add a virtio channel refcount
  firmware: arm_scmi: Review virtio free_list handling
  [RFC] virtio_ring: Embed a wrap counter in opaque poll index value
  firmware: arm_scmi: Add atomic mode support to virtio transport
  dt-bindings: firmware: arm,scmi: Add atomic_threshold optional
    property
  firmware: arm_scmi: Support optional system wide atomic_threshold
  firmware: arm_scmi: Add atomic support to clock protocol
  [RFC] firmware: arm_scmi: Add support for clock_enable_latency
  [RFC] clk: scmi: Support atomic clock enable/disable API

 .../bindings/firmware/arm,scmi.yaml           |  11 +
 drivers/clk/clk-scmi.c                        |  71 ++-
 drivers/firmware/arm_scmi/Kconfig             |  15 +
 drivers/firmware/arm_scmi/clock.c             |  34 +-
 drivers/firmware/arm_scmi/driver.c            |  36 +-
 drivers/firmware/arm_scmi/virtio.c            | 504 +++++++++++++++---
 drivers/virtio/virtio_ring.c                  |  51 +-
 include/linux/scmi_protocol.h                 |   9 +-
 8 files changed, 622 insertions(+), 109 deletions(-)

-- 
2.17.1