First of all, apologies for sending such a long email. Double
apologies
if
you're caught in the email chain and you don't care about the topic.
Just
don't click "reply all" when you send me an angry response, as the
thread will
be publicly logged.
The reason for you receiving this, is that you've been involved with
libiio
(https://github.com/analogdevicesinc/libiio) either directly or
indirectly,
or subscribed to the IIO kernel mailing list, or have shown
interest in
the
project, or have an homonym who does.
About five years ago libiio was born, as an easy-to-use library for
interfacing to Linux' Industrial Input/Output (IIO) kernel
subsystem,
either
with devices preset on the host platform, or on a remote target
platform.
I think we did a good job, as the API was never broken since its
first
release,
and the praise/hate ratio we received was very encouraging. We've
seen
people
using it in all kinds of applications, from car infotainment to
space
research,
from drones to software radio.
It's not perfect, though, and its current design has room for
improvements,
both inside and outside the library. However, we've hit a point
where
such
improvements would require a redesign of the library and kernel
bits,
and as a
consequence, a breakage of the API. It's time for the next-gen
libiio,
and a
overhaul of the whole IIO stack, really.
Your mission, should you choose to accept it, is to feed me with
ideas,
concerns, comments about what you think libiio was lacking or just
not
doing
right. We want to open the possibility for all interested parties to
help
sketch the future library.
I have compiled below a list of changes that we think should be
done,
comments
are very welcome. Note that emails in HTML form will probably be
bounced back
by the IIO mailing list, so please write responses in plain text.
Thank you for your time.
Kind regards,
Paul Cercueil
------
Kernel
------
* Support for buffer metadata
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Support tagging the buffers with any relevant information. Either
from the IIO
core for general accounting (e.g. timestamp) or from the driver
for
hardware-facing code (e.g. buffer underflow).
Metadata would be attached to a specific sample in the buffer
(metadata that
applies to the whole buffer can be attached to the first sample).
Several
entries could be attached to the same sample. The format would be
a
simple
key="value", I don't think we need anything more complex than
that.
For DACs, it should be possible to set metadata from userspace.
The
drivers
would then interpret the metadata tags if they need to.
* Facility to detect overflow/underflow
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There should be a way to detect overflows (for ADCs) and
underflows
(for
DACs), if the hardware supports it, and report them to userspace
(through a
IIO event, I suppose). This is something that could be done by the
drivers,
but maybe it would make sense to have this functionality in the
IIO
common
code?
* Delayed attribute write / Command queues
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We need a mechanism that allows us to change the value of an
attribute at a
very specific time (or sample number) after a capture or upload is
started;
some kind of script mechanism, or 'command list' analog to the
display lists
used in old GPUs. This would be used for instance in software
applications
doing time-divison duplexing (TDD).
* Better high-speed buffer mechanism
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The current buffer API is fine for low-speed devices, but we're
dealing with
ADCs and DACs of the GB/s class. ADI already contributed an
improved
(faster)
buffer mechanism, but which is incomplete (no support for DACs),
doesn't
integrate very well in the ecosystem, and is somewhat redundant
with
the old
one.
The idea would be to deprecate this API and propose an alternative
that
makes use of current technology, like dmabuf. The concept would
stay
the
same, each IIO device has a pool of DMA buffers, and userspace can
queue
and dequeue buffers.
The rationale behind this change, is that with the current two
APIs
it is not
possible to move data between IIO devices and a network or USB
card
without
having the CPU copy the data. This is an important problem, as
high-speed
ADCs and DACs are generally connected to FPGAs running Linux on a
softcore,
which stream their data to a workstation for further processing.
With
the
new API, the userspace software would simply obtain a pointer to a
dmabuf
from the IIO interface, and simply pass it to the network card
(this
means
the network stack and USB stack would also need to support
dmabuf).
An alternative would be to keep the current file-based buffer and
buffer-queue-based APIs, but enhance the former one with support
for
splicing (with splice()/vmsplice()). This might arguably be
easier to
do,
since the network stack already supports it. The problem with
splicing is
that the kernel swaps each data page with a fresh zeroed page in
order to
avoid leaking kernel memory. To be truely zero-copy, this requires
the page
cleaning mechanism to be offloaded to e.g. DMA, otherwise the
process
isn't
entirely CPU-free.
* Parsable attribute names
~~~~~~~~~~~~~~~~~~~~~~~~
IIO attributes follow a certain formatting that does not make it
possible to
reconstruct the devices/channels/attributes tree in userspace. The
channel
attributes are formatted as:
<direction>_<type><id>_<modifier>_<extended-name>_<attribute-name>
The problem is that <extended-name>, <modifier> and
<attribute-name>
can
contain underscores, which makes it impossible to parse. For
instance,
with an attribute named "in_voltage0_high_impedence_line_enable":
Is the extended name "high_impedence_line" and the attribute name
"enable",
or is the extended name "high_impedence" and the attribute name
"line_enable"?
Since the sysfs attributes is ABI, the solution could be to have a
separate
sysfs file, for instance <direction>_<type><id>_extended_name,
that
would
contain the extended name of the channel. This should be enough
for
the IIO
sysfs interface to be machine-readable.
-------
Libiio2
-------
* stdio redirection
~~~~~~~~~~~~~~~~~
Right now libiio prints its debug information unconditionally to
stdout/stderr, it should be modified to be able to output its log
to
any
valid file descriptor. While that looks simple on paper, it must
be
set up
before a library context is created, since the context creation
itself may
print debug information. This require the context allocation to be
separated
from its initialization, so a break of API.
* Separate allocation from initialization
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
As stated above, functions to alloc/destroy and init/deinit the
various IIO
structures should be separate; this would allow e.g. to set some
parameters
to a iio_context before the context creation happens, for instance
where to
redirect the debug output.
* Modular backends
~~~~~~~~~~~~~~~~
Right now the backends of libiio are tightly coupled to the core.
It
would
be great if the users could install only the backends they're
interested in.
Of course, backends could still be backed into the core library,
and
that
would stay the default behaviour for non-Linux platforms.
This requires two things:
- The libiio backends must be able to be updated independently of
the
main
library, therefore they should only depend on the top-level API of
libiio.
Apart from a few exceptions here and there, this is already mostly
the
case.
- There needs to be a facility to load external backends based on
the
backend name. Thanksfully with URIs this becomes easy: creating a
context
from the URI "foo:" would result in the backend module "foo" loaded
from
the disk, if not already baked in.
* Event support
~~~~~~~~~~~~~
The IIO subsystem has support for events. This allows the
userspace
to get
notified for instance when a temperature gets over or under a
threshold.
Right now, libiio cannot easily support events, as the
client/server
protocol of the network backend doesn't really allow it.
Therefore,
the new
libiio2 library should be designed from the ground up with
support for
events, and the API should offer a way to register a callback that
would be
called when a IIO event occurs.
* Context change detection
~~~~~~~~~~~~~~~~~~~~~~~~
When a device is added, removed or the context becomes unavailable
(e.g. the
USB cable was unplugged), there should be a built-in mechanism to
notify the
applications using libiio2, maybe using the same mechanism as for
IIO
events.
The same would apply on the local backend, if for instance a new
device
appears, the library should be able to pick it up and report the
new
device
to the application.
* Asynchronous network communication
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The network communication between libiio and IIO is sub-par,
because
it uses
a synchronous request-response protocol. This causes the
throughput
to max
out way below the theorical maximum throughput of Gigabit
Ethernet.
The communication protocol should be modified in a way that fully
maximizes
the throughput.
The old communication protocol should still be available and used
by
default
unless the two parties agree to use the new protocol. This ensures
that
TinyIIOD (the microcontroller variant of IIOD, the server that
communicates
with the network backend of libiio) can still work with the
network
backend of
libiio2. Alternatively, TinyIIOD could be updated to the new
network
protocol.
That way, the old network protocol wouldn't have to stick around.
To implement the asynchronous network communication, we could
have a
look at
ZeroMQ (https://zeromq.org) which seems to be designed for that
particular
task, and is available under a LGPLv3 license. It is however
uncertain how
that would work on the IIOD side (because of the zero-copy
requirement - see
below).
Additionally, the network backend should part ways with its
current
ASCII-based protocol. This made sense when libiio was first
created,
but it
requires quite a complex parser on the server side that could go
away
by
resorting to a more classic protocol.
* Zero-copy
~~~~~~~~~
Apart from supporting the new network protocol, libiio should
provide
a way
to stream data between IIO devices and standard file descriptors
or
sockets
without having the CPU copy the data.
If a dmabuf-based IIO kernel interface is used, we need a way to
send
a
dmabuf to a socket, and I'm not sure that this is currently
possible.
But as dmabuf are internally just scatterlists, and the sockets
support
splice(), that should be somewhat doable.
To complicate things, IIOD (the network server) must be able to
stream the
same data to more than one client at a time, unless we decide that
this is
no more a requirement. This means that one single dmabuf would be
submitted
to more than one socket.
Streaming data to multiple clients using zero-copy techniques in
IIOD
also
implies that the data cannot be modified, which means that the
server
cannot
do the demuxing, and this task is devoted to the clients; so the
(currently
optional) server-side demuxing option would be removed.
* USB3
~~~~
Just like the network, the USB backend should be updated to be
able
to reach
out the maximum throughput offered by USB3.
The principal problem with USB is the short number of endpoints,
which limits
the functionality, as it may not be possible to stream data from
different
devices at the same time. Right now, a pair of endpoints is
reserved
for
generic commands (read attributes, open/close IIO device, etc.),
and
one pair
is reserved per IIO device streaming data.
There are several possibilities:
- the USB backend could multiplex accesses into one single pair of
endpoints, which basically do in software what USB controllers do
in
hardware;
- Rework the communication protocol so that only one endpoint is
needed per
streaming device instead of two;
- Negate the problem, and consider it okay that the number of USB
endpoints
is a limiting factor.
* PCIe backend
~~~~~~~~~~~~
There is a need for a backend to support the PCIe cards populated
with
various chips and a FPGA with a softcore running Linux. The
communication
between the remote OS and the host OS would be something similar
to
Xillybus.
* Command queues
~~~~~~~~~~~~~~
The libiio2 API bits to use the "delayed attribute write / command
queue"
feature of the kernel. A command queue would contain several
commands
(e.g.
submit buffer, change a parameter, submit another buffer), that
would
be
built in the upper layer of the library using a specific API, and
then
performed atomically on the remote device.
* Buffer overhaul
~~~~~~~~~~~~~~~
Instead of having one iio_buffer, that is either pushed or
refilled,
the idea
would be to provide an API that allows the client application to
request,
enqueue or dequeue the buffers itself. This would offer much
greater
control
on the buffer management to the application.
* Backwards-compatibility
~~~~~~~~~~~~~~~~~~~~~~~
Backwards compatibility is not a hard requirement, but it should
still be
possible to be done by implementing the libiio API on top of
libiio2.
Things
like buffer refill/push of libiio can be implemented on top of the
new buffer
queue system. New features, like buffer metadata support wouldn't
be
backported.
This should be considered low-priority - we're not yet at the
point
where
libiio1 is deprecated.
