Re: Let's design the next-gen libiio!

[Paul Cercueil <paul@crapouillou.net>]

Hi Alexandru,

I wanted to delay my response to answer in a big email all the feature 
requests and concerns that came my way, but you've been the only one to 
answer so far and it's been more than one week now, so I'll answer your 
comments below.



Le mar., déc. 10, 2019 at 07:37, "Ardelean, Alexandru" 
<alexandru.Ardelean@analog.com> a écrit :
> On Mon, 2019-12-09 at 16:26 +0100, Paul Cercueil wrote:
>> 
>>  Hello beautiful people,
>> 
> 
> Hey,
> 
> Before going forward, looking at the current backlog [on Github] 
> would be
> an idea. There are some PRs there which are unresolved and have some 
> real
> value.
> Like the plugin-support for libiio, and the Android build support.
> Not saying we should resolve them now, but they should get resolved 
> in time
> [either implement or drop].

The plugin support for libiio was in my list, as "modular backends".
The PRs which are unresolved generally mean that more work or testing 
is needed.

> 
> There's also some static-code-checker that's running, and one of them
> [Codacy] seems to yield some interesting stuff for C# bindings.
> 
> https://app.codacy.com/manual/commodo/libiio/dashboard
> 
> So, going forward, a few things from my side that have cropped up in 
> some
> time.
> I did go quickly through your list and eliminated a few [1 or 2] that 
> were
> already there. If am repeating something, it is possible.
> 
> Bindings:
> - would be nice for libiio to be present on pypi (for the Python 
> bindings);
> I am not sure if this was ever a request, but the idea popped up 
> here-n-
> there
> - Java-bindings for Android; personally, I'm not a fan of Java, but 
> Android
> seems to be; and there was someone trying to integrate a heart-rate 
> sensor
> with libiio on Android; which makes sense, given all the sensors 
> people are
> putting into phones these day

About pypi: indeed, that would be a good idea. I didn't add these 
"small features" to my list as they don't depend on a redesign of the 
library to be brought to life, which is the focus here.

Kotlin is all the rage these days in the Android world, maybe it would 
make more sense than Java? Or both?

> 
> Deployment-wise:
> - there's been talk of trying to deploy libiio in Launchpad [for 
> Ubuntu]
> and something [similar to Launchpad] for CentOS/RedHat RPMs; the main
> benefit here is ease-of-deployment with the latest libiio; people 
> seem to
> try to get the latest, and have trouble with it

I think it's important to always have the latest version in the 
software managers of the distributions. It's the case for 
Debian/Ubuntu, but I don't think it's being packaged elsewhere. 
Besides, if releases are made more often (e.g. with patch-level 
revisions, 1.0.x versions), then there shouldn't be a need for an 
off-the-track solution to get updates.

> 
> Configuration-wise:
> - an alternative to XML seemed to be interesting at at time; JSON was
> preferred, but now I'm wondering if YAML would also be preferred 
> [these-
> days]; in any case, the jump from JSON to YAML may be not as big, so 
> JSON
> would be great as a first step; one argument against XML was 
> libxml2's size

XML is only used internally, as a software representation of an IIO 
context. It's never there to see or modify, so I don't really see the 
point of switching to another markup language, besides the libxml2 
problem you mention (but then again, there are other XML libraries we 
could switch to). Additionally, that would mean breaking the 
compatibility with the old libiio/IIOD, and I want to avoid that as 
much as possible.

> 
> Communication-wise:
> - I started getting this idea to maybe use websockets for data-stream
> communication [using libwebsockets]; the whole idea would be to maybe
> convert/add a web-server part to IIOD. This would allow people to 
> write
> web-pages [HTML + JS] that talk directly to libiio. This would 
> simplify
> [and maybe accelerate] some app-development; web-browsers have really 
> great
> capabilities to display animated data, and given that there are more 
> JS
> devs [than C++/desktop-app devs] in this world, getting things 
> displayed is
> much faster. Also, creating simple mobile-apps is really easy with JS.
> While it sounded that I am a JS fan, I'm actually not, but it's big 
> enough
> that it's hard to avoid it, and I do feel that it has become a gateway
> language for web & mobile [as is C for OS development].

I don't really like the idea of having IIOD exposed to the outside 
world. This has ramifications that I'm not comfortable with, as it 
means the server software must be fully secure, provide cryptographic 
functionalities, etc.

Support for server-side JavaScript (via NodeJS) or PHP would probably 
be a good alternative, I think. The server software would communicate 
the IIO data to the client page the way it wants to.

> 
> Misc:
> - throughput & loss-rate measurement: not sure if this should be part 
> of
> the library, or an example app [or both], but people have been asking 
> this
> same question many times: how do i measure my throughput and how do i 
> check
> for losses? at least, i was never able to give a good answer here
> - profile support: this is something from Lars; but for some more 
> complex
> devices, configuration is challenging; so, something to load all
> configuration params, and perhaps save them back, would be 
> interesting;
> this has been pushed to apps, but who knows? maybe libiio? i am not 
> sure
> how valid this is

The throughput and loss-rate measurement would be easy to obtain if 
there was a mechanism built into IIO to report overflows/underflows. 
This is definitely something that we want and need, yes.

About profile support, I didn't list it here, because I consider it a 
side project - it should be its own library, "libiioprofile" if you 
want, which would use libiio, and not a mechanism built into libiio.

> 
> I guess for now, this is mostly it.
> Not sure if I forgot anything, but these were collected from various 
> parts
> of memory, notes and some issue trackers.

Thank you very much for taking the time to answer.

Cheers,
-Paul


> 
> Thanks
> Alex
> 
>>  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.
>> 
>>