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

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Date:	Mon, 30 Nov 2009 11:24:41 -0200
From:	Mauro Carvalho Chehab <mchehab@...hat.com>
To:	Andy Walls <awalls@...ix.net>
CC:	Ray Lee <ray-lk@...rabbit.org>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jon Smirl <jonsmirl@...il.com>,
	Krzysztof Halasa <khc@...waw.pl>,
	Christoph Bartelmus <lirc@...telmus.de>,
	dmitry.torokhov@...il.com, j@...nau.net, jarod@...hat.com,
	jarod@...sonet.com, linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	stefanr@...6.in-berlin.de, superm1@...ntu.com
Subject: Re: [RFC] What are the goals for the architecture of an in-kernel
 IR  system?

Andy Walls wrote:
> On Mon, 2009-11-30 at 09:56 -0200, Mauro Carvalho Chehab wrote:
>> Andy Walls wrote:
>>> On Sun, 2009-11-29 at 09:49 -0800, Ray Lee wrote:
>>>> On Sun, Nov 29, 2009 at 9:28 AM, Maxim Levitsky <maximlevitsky@...il.com> wrote:
>>>>> This has zero advantages besides good developer feeling that "My system
>>>>> has one less daemon..."
>>>> Surely it's clear that having an unnecessary daemon is introducing
>>>> another point of failure?
>>> A failure in a userspace IR daemon is worst case loss of IR
>>> functionality.
>>>
>>> A failure in kernel space can oops or panic the machine.
>> If IR is the only interface between the user and the system (like in a TV
>> or a Set Top Box), both will give you the same practical result: the system
>> will be broken, if you got a crash at the IR driver.
> 
> Yes, true.  I had forgotten about the embedded space.
> 
> Nonetheless I'd still rather debug a problem with a dead process in
> userspace than an oops or panic (not that an end user cares) and avoid
> the risk of filesystem corruption.
> 
>> Userspace is much more flexible.
>>
>> Why? The flexibility about the same on both kernelspace and userspace,
>> except for the boot time.
> 
> I suppose my best answer to that is question back to you: Why does udev
> run in userspace versus a kernel thread?

udev relies on a kernel implementation: sysfs.

> My personal thoughts on why user space is more flexible:
> 
> 1. You have all of *NIX available to you to use as tools to achieve your
> requirements.
> 
> 2. You are not constrained to use C.
> 
> 3. You can link in libraries with functions that are not available in
> the kernel.  (udev has libudev IIRC to handle complexities)
> 
> 4. Reading a configuration file or other file from the filesystem is
> trivial - file access from usespace is easy.
> 
> 5. You don't have to be concerned about the running context (am I
> allowed to sleep here or not?).

You can do all the above steps with the proper API, just like udev does
with sysfs API.

After the boot, a device can open the raw API, disabling any in-kernel
decoding/handling and handle IR directly. Alternatively, an udev rule 
can load a different keymap based on some config written on a file. 

So, you won't loose anything by having an in-kernel driver or decoder.

>> A kernelspace input device driver can start working since boot time.
>> On the other hand, an userspace device driver will be available only 
>> after mounting the filesystems and starting the deamons 
>> (e. g. after running inittab). 
>>
>> So, you cannot catch a key that would be affecting the boot 
>> (for example to ask the kernel to run a different runlevel or entering
>> on some administrative mode).
> 
> Right.  That's another requirement that makes sense, if we're talking
> about systems that don't have any other keyboard handy to the user.
> 
> So are we optimizing for the embedded/STB and HTPC with no keyboard use
> case, or the desktop or HTPC with a keyboard for maintencance?

If we remove the in-kernel decoders/evdev handlers, you'l limit the usecase,
where if you keep them in kernel, both ways can be used.

Cheers,
Mauro.
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