linux-kernel - Re: [PATCH v8 00/10] Add the I3C subsystem

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Date:   Wed, 17 Oct 2018 15:18:29 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     gregkh <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Wolfram Sang <wsa@...-dreams.de>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        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>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vitor Soares <Vitor.Soares@...opsys.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Xiang Lin <Xiang.Lin@...aptics.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Sekhar Nori <nsekhar@...com>,
        Przemyslaw Gaj <pgaj@...ence.com>,
        Peter Rosin <peda@...ntia.se>, mshettel@...eaurora.org,
        swboyd@...omium.org
Subject: Re: [PATCH v8 00/10] Add the I3C subsystem

Hi Greg,

On Mon, 8 Oct 2018 12:47:21 +0200
Arnd Bergmann <arnd@...db.de> wrote:

> On Wed, Oct 3, 2018 at 3:22 PM Boris Brezillon
> <boris.brezillon@...tlin.com> wrote:
> >
> > Sorry for the huge delay between v7 and v8 despite the small amount of
> > things I was asked to fix/rework.
> >
> > This patch series is adding a new subsystem to support I3C devices.
> >
> > This is just adding support for basic features. Extra features will
> > be added afterwards.
> >
> > There are a few design choices that are worth mentioning because they
> > impact the way I3C device drivers can interact with their devices:
> >
> > - all functions used to send I3C/I2C frames must be called in
> >   non-atomic context. Mainly done this way to ease implementation, but
> >   this is still open to discussion. Please let me know if you think it's
> >   worth considering an asynchronous model here
> > - the I3C bus and I3C master controller are now tightly coupled even
> >   though they're still allocated separately. There's now a 1:1
> >   relationship between these objects, and the I3C master is no longer
> >   represented under the I3C bus object.
> >   Arnd, let me know if you had something different in mind, and I'll
> >   rework the implementation accordingly.  
> 
> I looked at the entire series again and I'm rather happy with how
> it turned out. I've commented on a tiny issue about the readsl()
> that should be easy to resolve one way or another, with that
> you can add my
> 
> Reviewed-by: Arnd Bergmann <arnd@...db.de>
> 
> There is one additional issue that we've talked about previously
> and that I'd like to hear about from GregKH or maybe other
> subsystem maintainers: In the current version, you have a single
> 'bus_type' object, and this is used to represent both a 'host' and
> a 'device'. I think we concluded that this is done in other
> subsystems as well, and that this is fitting here because a
> host (master device) can hand over being a master to another
> device (slave), which then becomes the host and sees this
> one as a slave. Also a lot of the sysfs attributes are the same
> because of this relationship.
> 
> It also means that you get a mix of things in sysfs:
> 
> /sys/devices/i3c/<bus>
> /sys/devices/i3c/<device>
> /sys/devices/i3c/<bus>/<device>
> 
> which is a bit like what we have on USB where we can have hub
> devices that are again parents of other USB devices, but I don't
> think we can have i3c hubs or multiplexers in the same way, so
> it's only a single level.
> 
> I'm ok with this model after our previous discussion and couldn't
> come up with a better one. If anyone else still sees it as
> problematic and has a better idea, please let us know now.

I know you're quite busy with the 4.19 release, but if you find a bit
of time, that'd be great to have your feedback on this.

Thanks,

Boris