Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20190409144250.7237-1-TheSven73@gmail.com> <982e69c6-4e68-6f62-8bed-cd5a1802272b@metux.net>
 <CAGngYiUTHZFFY=H7xXHzZnN4pS0jAqWBTrcw04hjf5S-ykxC9w@mail.gmail.com>
 <c1831703-a476-8870-0a5f-9060bda0f669@metux.net> <CAGngYiXx2eKR7DnHm9sNWVC+B1F2N6uUNXqZAq4rey2yjU1RyA@mail.gmail.com>
 <23a25601-ed98-5348-9bac-bf8fc2baea5e@metux.net>
In-Reply-To: <23a25601-ed98-5348-9bac-bf8fc2baea5e@metux.net>
From:   Sven Van Asbroeck <thesven73@gmail.com>
Date:   Tue, 16 Apr 2019 17:49:23 -0400
Message-ID: <CAGngYiV4fj7TA1Apwc57oQMSjPW3mVKzfDoLbD2QJKw-GWNzBA@mail.gmail.com>
Subject: Re: [PATCH v10 0/7] Add Fieldbus subsystem + support HMS Profinet card
To:     "Enrico Weigelt, metux IT consult" <lkml@metux.net>
Cc:     Rob Herring <robh+dt@kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Lee Jones <lee.jones@linaro.org>, mark.rutland@arm.com,
        =?UTF-8?Q?Andreas_F=C3=A4rber?= <afaerber@suse.de>,
        treding@nvidia.com, David Lechner <david@lechnology.com>,
        noralf@tronnes.org, johan@kernel.org,
        Michal Simek <monstr@monstr.eu>, michal.vokac@ysoft.com,
        Arnd Bergmann <arnd@arndb.de>,
        Greg KH <gregkh@linuxfoundation.org>, john.garry@huawei.com,
        geert+renesas@glider.be, robin.murphy@arm.com,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        sebastien.bourdelin@savoirfairelinux.com, icenowy@aosc.io,
        Stuart Yoder <stuyoder@gmail.com>,
        "J. Kiszka" <jan.kiszka@siemens.com>, maxime.ripard@bootlin.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Apr 16, 2019 at 5:21 PM Enrico Weigelt, metux IT consult
<lkml@metux.net> wrote:
>
> Yeah, that sounds logical. By the way: could we also mmap() that device?

AFAIK mmap only makes sense if you have actual device memory, accessible
through some memory controller. The process memory on anybus-s devices
is accessed through many layers of indirection, which includes a handshaking/
synchronization protocol over anybus-s.

In addition, you'd lose timing information about writes. The current API blocks
until the write to the remote PLC's process memory has been
successfully completed.

Perhaps if someone ever adds a fieldbus_dev driver for a device with mappable
process memory, we can consider mmap() as an optional extension ?

>
> Okay that's just a purely academical idea here, but I'm curious whether
> that would make a notable difference in performance.

Perhaps on a device with memory-mappable process memory.
But it would have to be an incredibly fast fieldbus device for the
difference between read()/write() and mmap() to matter...

>
> Yet another question: does each fieldbus_dev instance talk to exactly
> one plc process memory, or can there be many ?
>
> Having separate device nodes per plc process memory seems a good idea
> for finer access control (via chown+friends).
>

Good question ! I'll have to get back to you on that...