vtscmi: Add a virtio-scmi driver

[cristian.marussi_arm.com]

Summary

Add a new virtio backend to support SCMI VirtIO devices (type 32) as
defined by the VirtIO specification since version v1.2.

https://docs.oasis-open.org/virtio/virtio/v1.2/cs01/virtio-v1.2-cs01.pdf

Sponsored by: Arm Ltd

[bryanv]

Just a few minor comments. Note though that I only have time to do a brief review of this and haven't read the relevant section of the spec.

sys/dev/virtio/scmi/virtio_scmi.c

278

Nit: M_WAITOK means this cannot fail. May be slightly clearer to use mallocarray() instead.

514

A virtqueue_poll() variant with a timeout param could be useful in general.

[cristian.marussi_arm.com]

Thanks for the review @bryanv.
Regarding:

"A virtqueue_poll() variant with a timeout param could be useful in general."

I was thinking exactly the same while doing this, but being a first time contributor to freeBSD, I was not sure how easily acceptable would have been to post a change related to some core system (like virtio) inside this series,
so I went for a custom solution at first. I can certainly post subsequently a patch to add a virtqueue_poll timeout variant to the virtio-core and make use of it in here, once this is merged/landed.

[bryanv]

In D43047#998921, @cristian.marussi_arm.com wrote:

Thanks for the review @bryanv.
Regarding:

"A virtqueue_poll() variant with a timeout param could be useful in general."

I was thinking exactly the same while doing this, but being a first time contributor to freeBSD, I was not sure how easily acceptable would have been to post a change related to some core system (like virtio) inside this series,
so I went for a custom solution at first. I can certainly post subsequently a patch to add a virtqueue_poll timeout variant to the virtio-core and make use of it in here, once this is merged/landed.

Sure, a separate follow on commit is fine.

[cristian.marussi_arm.com]

Removed needless checks on malloc, using mallocarray

Using mallocarray() and dropping the retval check since invoked as M_WAITOK

[freebsd_igalic.co]

I'm a bit late to the party, but is there a reason why this module wasn't hooked into the build system?

see https://cgit.freebsd.org/src/tree/sys/modules/virtio/Makefile
and for example, https://cgit.freebsd.org/src/tree/sys/modules/virtio/network/Makefile

[andrew]

It's not very useful as a module as it provides infrastructure other devices need, e.g. their clock, or a reset line. If it was a module these drivers would fail to attach if they were built into the kernel.