On Sat, Jun 25, 2011 at 6:11 PM, Ohad Ben-Cohen [off-list ref] wrote:

Hi Stephen,

On Fri, Jun 24, 2011 at 11:16 PM, Stephen Boyd [off-list ref] wrote:

quoted

This sounds a lot like SMD (shared memory driver) on MSM. The main
difference I see is that SMD uses the platform bus instead of the virtio
bus and it has its own protocol for channel allocation.

Yeah, virtio is a key factor in this work; it was suggested to us by
Arnd at the AMP plumbers discussions last year, where it was apparent
that many vendors have their own IPC drivers/buses/channels over
shared memory with some vendor-ish binary protocol. I must say we
really liked virtio: it considerably simplified the code (we're not
adding any new binary protocol), it's very nicely optimized and
flexible, and it comes with a set of virtio drivers (e.g. network,
console, block) so we don't have to write our own.

We also cared about adding this functionality as an IPC bus, so the
driver core will help matching drivers to channels - it simplified the
code (in both setup and tear down of channels) and kept it flexible.
It will also facilitate error recovery (on remote crash, we just
remove the virtio device, and then the driver core will in turn start
->remove()ing the rpmsg drivers) and power management (via runtime
PM).

About SMD: I'm not familiar with it too much, but Brian naturally is
(just for the sake of everyone who are not reading headers - Brian
Swetland wrote the Linux SMD driver, and is also an author of this
Google+TI joint work).

rpmsg definitely shares some design features with SMD (given that I
wrote the linux SMD driver and was involved in the design of rpmsg,
this is maybe unsurprising), but whereas in SMD we had to be
compatible with existing AMSS modems (to a degree), we had more
flexibility in the "wire protocol" on rpmsg and virtio looked like a
really nice fit that already was in the kernel.

Ohad had a number of great ideas for making the dynamic discovery,
publishing, and binding of remote services very clean and
straightforward -- I wish I had a chance to pick his brain about this
stuff back when building the SMD interfaces, which started out fairly
static, but then evolved into publishing platform devices, etc.  Of
course some of this is the benefit of hindsight.  It's easier to get
it right (er?) the second or third time around.

The TI SYS/BIOS folks were quite helpful and patient as we redesigned
the wire protocol and publishing/resource model several times along
the way here.

Brian