[PATCH v3 0/2] Add SCMI support for mailbox unidirectional channels

[Cristian Marussi]

Hi,

this series aims to extend SCMI mailbox transport layer to support mailbox
controllers that expose unidirectional channels.

SCMI communications between an agent like Linux and the platform fw server
happens through 2 main communication channels: an 'a2p' bidirectional
channel (called TX in SCMI parlance) used to send synchronous commands
and receive related replies and an optional 'p2a' unidirectional channel
(called RX) used to convey notfications or delayed responses to
asynchronous commands possibly emitted by the platform toward the agent.

The current SCMI mailbox transport support was modelled around mailboxes
that were bidirectional by nature, and, as such, fit well in the above
SCMI communication scheme, allowing us to currently describe the mailbox
transport channels as in the following examples:

 1.  system with a single TX 'a2p' defined over a mailbox bidirectional
     channel:

	mboxes = <&mb 0 0>;
	shmem = <&a2p_mem>;

 2. system with a TX 'a2p' defined over a bidirectional mailbox channel
    AND an optional RX 'p2a' defined over a unidirectional channel:

	mboxes = <&mb 0 0>, <&mb 0 1>,
	shmem = <&a2p_shmem>, <&p2a_shmem>;

This binding, as it is now, does NOT support the usage of mailbox
controllers exposing channels that are unidirectional by nature, like
it is the case with ARM MHUv2 mailboxes as an example, since 2 distinct
unidirectional mailbox channels would be needed to represent just the
SCMI TX bidirectional communication path.

Note that the mboxes property referred in the SCMI nodes to configure the
transport is compliant with (and parsed by) the mailbox common subsystem,
which is the entity that exposes and finally handles the mailbox
controller: as a consequence playing creatively (or dirty :P) with the
syntax of the mboxes property to fit our needs is not an option.

This series extends the SCMI mailbox-related bindings, which defines how
mboxes and shmem properties are interpreted, and the logic inside the
SCMI mailbox transport subsystem to support the usage of these type of
unidirectional mailbox channels, while aiming to maintain backward
compatibility with the original scheme based on bidirectional channels.

With these proposed DT extensions, in addition to the above definitions,
the following descriptions can be crafted for a system using a mailbox
controller exposing unidirectional channels:

 2. system with a single TX 'a2p' defined over a pair of unidirectional
    mailbox channels (similar to 1):

	mboxes = <&mb_tx 0 0>, <&mb_rx 0 0>;
	shmem = <&a2p_mem>;

 3. system with a TX 'a2p' defined over a pair of unidirectional channels
    AND an RX 'p2a' defined over a unidirectional channel (similar to 2):

	mboxes = <&mb_tx 0 0>, <&mb_rx 0 0>, <&mb_rx 0 1>;
	shmem = <&a2p_shmem>, <&p2a_shmem>;

The SCMI mailbox transport logic has been modified to select and make a
proper use of the needed channels depending on the combination of found
mboxes/shmem descriptors:

  a) 1 mbox / 1 shmem => SCMI TX over 1 mailbox bidirectional channel
  b) 2 mbox / 2 shmem => SCMI TX and RX over 2 mailbox bidirectional chans

  c) 2 mbox / 1 shmem => SCMI TX over 2 mailbox unidirectional chans
  d) 3 mbox / 2 shmem => SCMI TX and RX over 3 mailbox unidirectional chans

with any other combination considered invalid.

Note that, up until the changes in this series, the only valid configs
accepted by the SCMI mailbox transport are a) and b): this ensures backward
compatibility even in the case in which a DT sporting the new format (c,d)
is, wrongly, deployed with an old kernel still not supporting this new
logic: in such a case c) and d) configs will be simply rejected. (wrongly
deployed because installing a c) or d) styled-DT would be required only if
the underlying mailbox HW had effectively changed and used unidir chans)

I have tested this on a JUNO board (MHUv1 bidirectional) and TotalCompute
TC2 reference design (MHUv2 unidirectional). [1]

The series is based on v6.3-rc4.

Having said that, I am not completely sure if all of the above constraints
should (and/or could) be expressed in a more formal way also in the YAML
binding itself.

Any feedback or suggestion in these regards is highly appreciated.

Thanks,
Cristian

[1]: https://gitlab.arm.com/arm-reference-solutions/arm-reference-solutions-docs/-/blob/master/docs/totalcompute/tc2/tc2_sw_stack.rst

----
v2 --> v3
 - coalesced oneOf mbox-names entries using minItems
 - removed unidirectional channel DT example
v1 --> v2
 - added mbox-names unidirectional definitions and example

Cristian Marussi (2):
  dt-bindings: firmware: arm,scmi: Support mailboxes unidirectional
    channels
  firmware: arm_scmi: Add support for unidirectional mailbox channels

 .../bindings/firmware/arm,scmi.yaml           | 48 ++++++++--
 drivers/firmware/arm_scmi/mailbox.c           | 95 ++++++++++++++++---
 2 files changed, 122 insertions(+), 21 deletions(-)

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2.34.1