Re: [RFC] What are the goals for the architecture of an in-kernel IR system?

Jon Smirl
2009-12-08T00:28:08Z

On Mon, Dec 7, 2009 at 6:44 PM, Mauro Carvalho Chehab
<mchehab@redhat.com> wrote:
> Let me add my view for those questions.
>
> Jon Smirl wrote:
>> On Sun, Dec 6, 2009 at 3:34 PM, Krzysztof Halasa <khc@pm.waw.pl> wrote:
>>> Jon Smirl <jonsmirl@gmail.com> writes:
>>>
>>>>> Once again: how about agreement about the LIRC interface
>>>>> (kernel-userspace) and merging the actual LIRC code first? In-kernel
>>>>> decoding can wait a bit, it doesn't change any kernel-user interface.
>>>> I'd like to see a semi-complete design for an in-kernel IR system
>>>> before anything is merged from any source.
>>> This is a way to nowhere, there is no logical dependency between LIRC
>>> and input layer IR.
>>>
>>> There is only one thing which needs attention before/when merging LIRC:
>>> the LIRC user-kernel interface. In-kernel "IR system" is irrelevant and,
>>> actually, making a correct IR core design without the LIRC merged can be
>>> only harder.
>>
>> Here's a few design review questions on the LIRC drivers that were posted....
>>
>> How is the pulse data going to be communicated to user space?
>
> lirc_dev will implement a revised version of the lirc API. I'm assuming that
> Jarod and Christoph will do this review, in order to be sure that it is stable
> enough for kernel inclusion (as proposed by Gerd).
>
>> Can the pulse data be reported via an existing interface without
>> creating a new one?
>
> Raw pulse data should be reported only via lirc_dev, but it can be converted
> into a keycode and reported via evdev as well, via an existing interface.
>
>> Where is the documentation for the protocol?
>
> I'm not sure what you're meaning here. I've started a doc about IR at the media

What is the format of the pulse stream data coming out of the lirc device?

> docbook. This is currently inside the kernel Documents/DocBook. If you want
> to browse, it is also available as:
>
>        http://linuxtv.org/downloads/v4l-dvb-apis/ch17.html
>
> For sure we need to better document the IR's, and explain the API's there.
>
>> Is it a device interface or something else?
>
> lirc_dev should create a device interface.
>
>> What about capabilities of the receiver, what frequencies?
>> If a receiver has multiple frequencies, how do you report what
>> frequency the data came in on?
>
> IMO, via sysfs.

Say you have a hardware device with two IR diodes, one at 38K and one
at 56K. Both of these receivers can get pulses. How do we tell the
user space app which frequency the pulses were received on? Seems to
me like there has to be a header on the pulse data indicating the
received carrier frequency. There is also baseband signaling. sysfs
won't work for this because of the queuing latency.

How is over-run signaled to the app? You'd get an over-run if the app
is too slow at reading the data out of the FIFO. If you ignore
over-run you'll be processing bad data because part of the message was
lost. An over-run signal tell the abort to abort the signal and start
over.

>> What about multiple apps simultaneously using the pulse data?
>
> IMO, the better is to limit the raw interface to just one open.
>
>> How big is the receive queue?
>
> It should be big enough to receive at least one keycode event. Considering that
> the driver will use kfifo (IMO, it is a good strategy, especially since you
> won't need any lock if just one open is allowed), it will require a power of two size.

How is end of a pulse train detected? timeout? without decoding the
protocol there is no way to tell the end of signal other than timeout.

>
>> How does access work, root only or any user?
>
> IMO, it should be the same requirement as used by an input interface.
>
>> How are capabilities exposed, sysfs, etc?
>
> IMO, sysfs.
>
>> What is the interface for attaching an in-kernel decoder?
>
> IMO, it should use the kfifo for it. However, if we allow both raw data and
> in-kernel decoders to read data there, we'll need a spinlock to protect the
> kfifo.
>
>> If there is an in-kernel decoder should the pulse data stop being
>> reported, partially stopped, something else?
>
> I don't have a strong opinion here, but, from the previous discussions, it
> seems that people want it to be double-reported by default. If so, I think
> we need to implement a command at the raw interface to allow disabling the
> in-kernel decoder, while the raw interface is kept open.

Data could be sent to the in-kernel decoders first and then if they
don't handle it, send it to user space.

>
>> What is the mechanism to make sure both system don't process the same pulses?
>
> I don't see a good way to avoid it.
>
>> Does it work with poll, epoll, etc?
>> What is the time standard for the data, where does it come from?
>> How do you define the start and stop of sequences?
>> Is receiving synchronous or queued?
>> What about transmit, how do you get pulse data into the device?
>> Transmitter frequencies?
>> Multiple transmitters?
>> Is transmitting synchronous or queued?
>> How big is the transmit queue?
>
> I don't have a clear answer for those. I'll let those to LIRC developers to answer.
>
>
> Cheers,
> Mauro
>
>
>

--
Jon Smirl
jonsmirl@gmail.com