Re: [RFC v3] iio: input-bridge: optionally bridge iio acceleometers to create a /dev/input interface

From: Bastien Nocera <hadess@hadess.net>
To: "H. Nikolaus Schaller" <hns@goldelico.com>
Cc: Jonathan Cameron <jic23@kernel.org>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Eric Piel <eric.piel@tremplin-utc.net>,
	linux-input@vger.kernel.org, letux-kernel@openphoenux.org,
	kernel@pyra-handheld.com, Hartmut Knaack <knaack.h@gmx.de>,
	Lars-Peter Clausen <lars@metafoo.de>,
	Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
	linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org
Subject: Re: [RFC v3] iio: input-bridge: optionally bridge iio acceleometers to create a /dev/input interface
Date: Fri, 10 May 2019 10:56:55 +0200	[thread overview]
Message-ID: <55468fdd88da9f487789b2073a1babb14bc7a282.camel@hadess.net> (raw)
In-Reply-To: <AA2B43FE-E196-4FEC-B4C5-897D0F44A76F@goldelico.com>

On Tue, 2019-04-16 at 21:33 +0200, H. Nikolaus Schaller wrote:
> Hi Bastien,
> 
> > Am 16.04.2019 um 18:04 schrieb Bastien Nocera <hadess@hadess.net>:
> > This can be done in user-space, reading the data from the IIO driver,
> > and using uinput to feed it back. Why is doing this at the kernel level
> > better?
> 
> Well, I'd estimate that >80% of the current kernel could be done in user-space
> (but not at the same speed/quality).
> 
> E.g. TCP could most likely be done by directly accessing the Ethernet layer and
> providing other processes access through named pipes instead of sockets.
> 
> But usually a user-space daemon feeding things back into the kernel is slower
> (because it is scheduled differently) and needs more resources for running the
> process and IPC and is less protected against hickups and deadlocks.

This is mostly irrelevant for the amount of data we're treating, but it
doesn't matter too much.

> Two more aspects come to my mind from reading your project page:
> 
> a) "It requires libgudev and systemd"
> b) "Note that a number of kernel bugs will prevent it from working correctly"
> 
> a) this makes quite significant assumptions about the user-space while a kernel
>    driver can be kept independent of this

It's made for modern desktop OSes/"traditional" Linux. I don't think
that those 2 libraries are problematic dependencies unless you're on
Android, where a replacement could be implemented or iio-sensor-proxy
modified for that use case.

> b) if it is in-kernel it will be kept in sync with kernel changes and such bugs
>    are less likely

No they're not. This warning was because 1) drivers sometimes have bugs
2) user-space sometimes has bugs 3) user-space sometimes causes the
kernel to have bugs.

The 2 significant breakages for iio-sensor-proxy were caused by runtime
PM bugs in the hid-sensor-hub driver, and in the USB core. I doubt a
kernel-space implementation would have been able to magically fix those
bugs unfortunately.