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

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Date:   Fri, 10 May 2019 10:56:55 +0200
From:   Bastien Nocera <hadess@...ess.net>
To:     "H. Nikolaus Schaller" <hns@...delico.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Eric Piel <eric.piel@...mplin-utc.net>,
        linux-input@...r.kernel.org, letux-kernel@...nphoenux.org,
        kernel@...a-handheld.com, Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org
Subject: Re: [RFC v3] iio: input-bridge: optionally bridge iio acceleometers
 to create a /dev/input interface

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@...ess.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.