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

From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: Jon Smirl <jonsmirl@gmail.com>, Krzysztof Halasa <khc@pm.waw.pl>,
	hermann pitton <hermann-pitton@arcor.de>,
	Christoph Bartelmus <lirc@bartelmus.de>,
	awalls@radix.net, j@jannau.net, jarod@redhat.com,
	jarod@wilsonet.com, kraxel@redhat.com,
	linux-input@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-media@vger.kernel.org, superm1@ubuntu.com
Subject: Re: [RFC] What are the goals for the architecture of an in-kernel IR  system?
Date: Mon, 7 Dec 2009 20:23:40 -0800	[thread overview]
Message-ID: <20091208042340.GC11147@core.coreip.homeip.net> (raw)
In-Reply-To: <4B1D934E.7030103@redhat.com>

On Mon, Dec 07, 2009 at 09:44:14PM -0200, Mauro Carvalho Chehab 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
> Jerod 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
> 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.

We probably need to think what exactly we report through sysfs siunce it
is ABI of sorts.

> 
> > What about multiple apps simultaneously using the pulse data?
> 
> IMO, the better is to limit the raw interface to just one open.
> 

Why woudl we want to do this? Quite often there is a need for "observer"
that maybe does not act on data but allows capturing it. Single-user
inetrfaces are PITA. 

> > 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.
> 

Would not it be wither driver- or protocol-specific?

> > 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.
> 

I think Jon meant userspace interface for attaching particular decoder.

> > 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.

Why don't you simply let consumers decide where they will get their data?

> 
> > 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.
> 

-- 
Dmitry