Ongoing remoteproc discussions

[Bjorn Andersson]

During discussions with various people interested in moving their
remoteproc-related out-of-tree patches towards mainline I have come
across a set of topics common among various teams. The purpose of this
email is to share some insight into these discussions and the current
ideas for moving forward.

== Auto-boot or always-on:
There are cases where we want to achieve the current auto-boot mechanism
without rpmsg and there are cases where we don't want to auto-boot a
remoteproc just because its resource table contains rpmsg entries. So we
need to decouple this logic from the vdev. I suggest that:

* We introduce a property in the rproc struct where drivers can toggle
  if they want always-on or not. We default this to true, as an estimate
  of backwards compatibility.

* We move the allocation of vrings to be done at the time of the other
  allocations and follow that with the registration of virtio devices,
  before actually booting the rproc. And we tear down the virtio devices
  as part of shutdown.

* We remove the rproc_boot()/shutdown() calls from the
  remoteproc_virtio.c and based on rproc->always_on we call these in the
  async firmware loader callback, in rproc_trigger_recovery() and
  rproc_del().


A side effect of this is that the async firmware loading only purpose is
to trigger the boot as the firmware becomes available (something we need
to tweak further). I therefor see no reason to mandate the firmware is
unchanged between boots, which simplifies the posed rproc_set_firmware()
function - as long as it's done on a remoteproc that's not always_on.


== Amended resources:
We need a way for the driver to amend resource definitions of the
firmware with e.g. physical addresses and size constraints from
devicetree, so I suggest that:

* We split the resource table parsing and allocation of resources in two
  steps; where the parse step updates the lists of resources and then at
  the time of boot we run through these and allocate the actual
  resources.

* We expose an API to the drivers so that they can register resources,
  as if they came from the table parser. We match each resource against
  previously registered resources based on name and merge or reject the
  updates. E.g. we merge a reference to the resource table offset and we
  reject incompatible size changes.


== Resource-less firmware:
To be able to use remoteproc with firmware either without a resource
table or resource data in other forms we today provide a resource table
stub in each driver, instead we could refactor the resource table
parsing code.

* We replace the find_rsc_table operation in rproc_fw_ops with a parse
  operation, that uses the newly created API (above) to register the
  resources with the core; largely decoupling the resource table format
  from the remoteproc core.

* We make the parse() function in rproc_fw_ops optional, allowing
  remoteproc drivers to specify that there's no resource parsing to be
  done (they can still provide resources programmatically between
  rproc_alloc() and rproc_add()).

This setup allows custom resource building functions to be implemented,
one such example is the Qualcomm firmware files where most resource data
is a combination of static information (DT) and data from the ELF
header.


== Resource-less firmware with installed resource table:
The now available list of resources that have been registered with the
core can serve as input for a function that generates a resource table
for the remote.  This gives us a mechanism for providing a remoteproc
with information about resources in cases where the firmware lacks a
resource table.

* We replace the rproc_find_loaded_rsc_table() with an new fw_op that is
  tasked with installing the resource table and update rproc->table_ptr.
  This op is made optional, for the remoteprocs with no installed
  resource table.

* We provide a helper function in the core that can be used in the
  fw_op, that builds a resource table in memory and copy this into the
  appropriate address of the remote, and  update rproc->table_ptr to
  this.

The install function will be tasked with making sure table_ptr is valid
and we make sure that error paths out of there and upon shutdown the
core will make table_ptr reference the core's version - which might be
NULL if no resource table exist.


== Supporting rpmsg alternatives:
For systems with other communication mechanisms than rpmsg we still want
to be able to instantiate and tear down devices representing these
communication channels, in a way that follows the life cycle of the
remoteproc. To do this I suggest that:

* Like other resources we split virtio device handling in the remoteproc
  core into adding rproc_vdevs to rproc->rvdevs and actually calling
  rproc_add_virtio_dev().

* We generalise the rproc->rvdev somewhat, to be a list of "subdevices"
  that are registered with the remoteproc; each providing callbacks for
  registering and unregistering themselves as we bring the remoteproc up
  and down.

I made a quick hack of this with the Qualcomm SMD code and it turned out
pretty good, but I believe it's too ugly to post until we get the
cleanups from above tasks sorted out.


== Ramdump:
Being able to acquire core dumps from a miss-behaving remoteproc is an
important feature in product development. I believe this snapshot should
be taken between the shutdown of the remote and freeing of resources. As
such I think it would be appropriate to:

* Split the inner working of rproc_shutdown() into the two steps of
  shutting down the remote and cleaning up its resources. Giving us a
  window of opportunity for snapshotting any resources.

In the generic case we will have to repopulate the resources with data
from the firmware file (in case of corruptions), but we're expecting to
load the same firmware again and as such I see no meaning in releasing
and reacquiring carveouts etc. As such if we split the inner working of
rproc_boot() into resource allocation, virtio device allocation and
booting we can make rproc_trigger_recovery() do:

    * Shutdown the remoteproc
    * Shutdown virtio devices
    * Take snapshot
    * Register virtio devices
    * Start the remoteproc

(The order of the top two is opposite of todays implementation, but
probably more appropriate for the case of getting an accurate snapshot
of the device).

We need to discuss the requirements for the format of what comes out of
this.  I've seen raw memory dumps of a fixed memory segments and I've
seen ELF generators with segments matching those of the loaded ELF in
the past.


== Firmware from late mounted file systems (needs discussion):
Typical for most remoteproc systems seems to be that the firmware files
tend to be way to large for being stored in a initramfs. The two
available mechanisms for dealing with this is to build remoteproc
drivers as modules and relying on CONFIG_FW_LOADER_USER_HELPER_FALLBACK;
the latter is being frowned upon by everyone but works fairly well for
our purpose.

Forcing systems to compile remoteproc drivers as modules to achieve the
delayed firmware load is not acceptable in my view.

So we need to come up with some mechanism for triggering auto-booting
when firmware files are stored in a lately mounted file system.



Interwoven in these discussions are of course topics related to rpmsg
and I will try to get back to these as things materialize.

Regards,
Bjorn