linux-kernel - Re: [PATCH v10 0/9] Add the I3C subsystem

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Message-ID: <20181115200058.1869afdb@bbrezillon>
Date:   Thu, 15 Nov 2018 20:00:58 +0100
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     vitor <vitor.soares@...opsys.com>
Cc:     Wolfram Sang <wsa@...-dreams.de>, <linux-i2c@...r.kernel.org>,
        "Jonathan Corbet" <corbet@....net>, <linux-doc@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Przemyslaw Sroka <psroka@...ence.com>,
        Arkadiusz Golec <agolec@...ence.com>,
        Alan Douglas <adouglas@...ence.com>,
        Bartosz Folta <bfolta@...ence.com>,
        Damian Kos <dkos@...ence.com>,
        Alicja Jurasik-Urbaniak <alicja@...ence.com>,
        "Cyprian Wronka" <cwronka@...ence.com>,
        Suresh Punnoose <sureshp@...ence.com>,
        "Rafal Ciepiela" <rafalc@...ence.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Nishanth Menon <nm@...com>, Rob Herring <robh+dt@...nel.org>,
        Pawel Moll <pawel.moll@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ian Campbell <ijc+devicetree@...lion.org.uk>,
        "Kumar Gala" <galak@...eaurora.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Xiang Lin <Xiang.Lin@...aptics.com>,
        <linux-gpio@...r.kernel.org>, Sekhar Nori <nsekhar@...com>,
        Przemyslaw Gaj <pgaj@...ence.com>,
        Peter Rosin <peda@...ntia.se>,
        Mike Shettel <mshettel@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v10 0/9] Add the I3C subsystem

On Thu, 15 Nov 2018 18:03:47 +0000
vitor <vitor.soares@...opsys.com> wrote:

> Hi Boris,
> 
> 
> On 15/11/18 15:28, Boris Brezillon wrote:
> > On Thu, 15 Nov 2018 16:01:37 +0100
> > Wolfram Sang <wsa@...-dreams.de> wrote:
> >  
> >> Hi Boris,
> >>  
> >>> What we could do though, is expose I3C devices that do not have a
> >>> driver in kernel space, like spidev does.  
> >> ...
> >>  
> >>> Mark, Wolfram, Arnd, Greg, any opinion?  
> >> Is there a benefit for having drivers in userspace? My gut feeling is to
> >> encourage people to write kernel drivers. If this is, for some reason,
> >> not possible for some driver, then we have a use case at hand to test
> >> the then-to-be-developed userspace interface against. Until then, I
> >> personally wouldn't waste effort on designing it without a user in
> >> sight.  
> > I kind of agree with that. Vitor, do you have a use case in mind for
> > such userspace drivers? I don't think it's worth designing an API for
> > something we don't need (yet).  
> 
> My use case is a tool for tests, lets say like the i2c tools.

What would you like to test exactly?

> There is 
> other subsystems, some of them mentioned on this thread, that have and 
> ioctl system call or other method to change parameters or send data.

I don't think they added the /dev interface before having a real use
case for it.

> 
> 
> I rise this topic because I really think it worth to define now how this 
> should be design (and for me how to do the things right) to avoid future 
> issues.

Actually it should be done the other way around: you should have a real
need and the /dev interface should be designed to fulfill this need.
Based on this real use case we can discuss other potential usage that
might appear in the future and try to design something more
future-proof, but clearly, this userspace interface should be driven by
a real/well-defined use case.

Also, exposing things to userspace is way more risky than adding a new
in-kernel subsystem/framework, because it then becomes part of the
stable ABI.

To make things clearer, I'm not against the idea of exposing I3C
devices (or I3C buses) to userspace, but I'd like to understand what you
plan to do with that. If this is about testing, what kind of tests
you'd like to run. If this is about developing drivers in userspace,
why can't these be done in kernel space (license issues?), and what
would those drivers be allowed to do?