Remoteproc, rpmsg, virtio...

[Edward Wingate]

I'm trying to understand more about how remoteproc, rpmsg, virtio and
virtqueues all relate to one another in OpenAMP. Can anyone correct
any misunderstandings I may have?

* Remoteproc is used from Linux to control the remote CPU, including:
- Shutting it down
- Relinquishing control of it from Linux
- Loading firmware for it
- Booting it with the new firmware
- Creates virtio device with tx/rx vring pair for communications with it

* Rpmsg is used to create a channel between the remote CPU and one
driver in Linux. It is possible for a remote CPU to create multiple
rpmsg channels, each going to a different Linux driver. Rpmsg
channels use the virtio device and its vrings to send/receive messages
to/from the remote CPU over shared memory.

* Virtqueue is the data structure underlying the tx and rx vrings. I
get the feeling it is more than just a mere data structure, but unsure
what more it might be.

* Virtio - I'm really unclear how virtio relates to things in OpenAMP.
All virtio documentation talks its use in VMs and hypervisors, and I
don't know how they relate in OpenAMP. Is there always only one
virtio device per remote CPU in OpenAMP? Or can multiple virtio
devices be defined for a given remote CPU?

Thanks for reading.

Ed

[Jiaying Liang]

Hi Ed,

Please see my comment inline.

Thanks,
Wendy

> -----Original Message-----
> From: open...@googlegroups.com [mailto:open...@googlegroups.com]
> On Behalf Of Edward Wingate
> Sent: Wednesday, October 12, 2016 12:03 PM
> To: open...@googlegroups.com
> Subject: [open-amp] Remoteproc, rpmsg, virtio...
>
> I'm trying to understand more about how remoteproc, rpmsg, virtio and
> virtqueues all relate to one another in OpenAMP. Can anyone correct any
> misunderstandings I may have?
>
> * Remoteproc is used from Linux to control the remote CPU, including:
> - Shutting it down
> - Relinquishing control of it from Linux
> - Loading firmware for it
> - Booting it with the new firmware
> - Creates virtio device with tx/rx vring pair for communications with it

[Wendy] Yes. It looks like you are talking about the remoteproc driver in the Linux kernel
OpenAMP also have remoteproc support for RTOS/baremetal, but you will need to implement the remoteproc driver
For your platform

>
> * Rpmsg is used to create a channel between the remote CPU and one driver
> in Linux. It is possible for a remote CPU to create multiple rpmsg channels,
> each going to a different Linux driver. Rpmsg channels use the virtio device
> and its vrings to send/receive messages to/from the remote CPU over
> shared memory.

[Wendy] The rpmsg kernel drivers requires Linux kernel is the RPMSG master.
It is the Linux kernel to start the communiticaton and create shared buffers for
Each RPMsg channels.
Each RPMsg channel has tx vring and rx vring
If you just want to have multiple application running on the same remote CPU
To talk to Linux, you can consider using different RPMsg endpoints.

If you want to have multiple RPMsg channels. The RPMsg initialization will need
Changes as it only support one channel at the moment.

>
> * Virtqueue is the data structure underlying the tx and rx vrings. I get the
> feeling it is more than just a mere data structure, but unsure what more it
> might be.

[Wendy] virtqueue is used to manage the vrings, providing API to get a buffer to use and return the buffer

>
> * Virtio - I'm really unclear how virtio relates to things in OpenAMP.
> All virtio documentation talks its use in VMs and hypervisors, and I don't
> know how they relate in OpenAMP. Is there always only one virtio device
> per remote CPU in OpenAMP? Or can multiple virtio devices be defined for a
> given remote CPU?

[Wendy] OpenAMP uses virtio for shared memory management.
At the moment, it only uses vrings for RPMsg channel. And each remoteproc
Only have one pair of tx/rx vrings.

We can consider to support one remoteproc having multiple virtio devices.
Could you share your use cases?

>
> Thanks for reading.
>
> Ed
>
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[Ed]

On Wednesday, October 12, 2016 at 2:24:35 PM UTC-7, Jiaying Liang wrote:

[Wendy] OpenAMP uses virtio for shared memory management.
At the moment, it only uses vrings for RPMsg channel. And each remoteproc
Only have one pair of tx/rx vrings.

We can consider to support one remoteproc having multiple virtio devices.
Could you share your use cases?

 

My nominal use case is just passing messages back and forth between Linux on CPU0 and FreeRTOS on CPU1, which the current single RPMSG and single VIRTIO covers.  I don't have a specific use case in mind for multiple virtio devices.  I was just trying to better understand how virtio fits in the OpenAMP architecture so I can know how to use it better.

I also just found this new fairly new wiki (created 9 days ago) which explains virtio a lot for me:

https://github.com/OpenAMP/open-amp/wiki/OpenAMP-RPMsg-Virtio-Implementation

Thanks, Wendy, for your prompt response.