Re: [PATCH v2 16/21] ipmi: kcs_bmc: Add a "raw" character device interface

From: Arnd Bergmann <arnd@kernel.org>
To: Andrew Jeffery <andrew@aj.id.au>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
	DTML <devicetree@vger.kernel.org>,
	"Chia-Wei, Wang" <chiawei_wang@aspeedtech.com>,
	Ryan Chen <ryan_chen@aspeedtech.com>,
	Tomer Maimon <tmaimon77@gmail.com>,
	Corey Minyard <minyard@acm.org>,
	Avi Fishman <avifishman70@gmail.com>,
	Patrick Venture <venture@google.com>,
	OpenBMC Maillist <openbmc@lists.ozlabs.org>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Tali Perry <tali.perry1@gmail.com>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	openipmi-developer@lists.sourceforge.net,
	Lee Jones <lee.jones@linaro.org>,
	Linus Walleij <linus.walleij@linaro.org>,
	linux-aspeed <linux-aspeed@lists.ozlabs.org>,
	Benjamin Fair <benjaminfair@google.com>
Subject: Re: [PATCH v2 16/21] ipmi: kcs_bmc: Add a "raw" character device interface
Date: Mon, 12 Apr 2021 10:48:14 +0200	[thread overview]
Message-ID: <CAK8P3a3RXr5CR7DJgD9rEkN8owpPxXRgzRnPB_5LuQcHkzc4LA@mail.gmail.com> (raw)
In-Reply-To: <ba63f830-4758-49aa-a63e-f204a8eec1b4@www.fastmail.com>

On Mon, Apr 12, 2021 at 3:33 AM Andrew Jeffery <andrew@aj.id.au> wrote:
> On Fri, 9 Apr 2021, at 17:25, Arnd Bergmann wrote:
> > On Fri, Mar 19, 2021 at 7:31 AM Andrew Jeffery <andrew@aj.id.au> wrote:
> > >
> > > The existing IPMI chardev encodes IPMI behaviours as the name suggests.
> > > However, KCS devices are useful beyond IPMI (or keyboards), as they
> > > provide a means to generate IRQs and exchange arbitrary data between a
> > > BMC and its host system.
> >
> > I only noticed the series after Joel asked about the DT changes on the arm
> > side. One question though:
> >
> > How does this related to the drivers/input/serio/ framework that also talks
> > to the keyboard controller for things that are not keyboards?
>
> I've taken a brief look and I feel they're somewhat closely related.
>
> It's plausible that we could wrangle the code so the Aspeed and Nuvoton
> KCS drivers move under drivers/input/serio. If you squint, the i8042
> serio device driver has similarities with what the Aspeed and Nuvoton
> device drivers are providing to the KCS IPMI stack.

After looking some more into it, I finally understood that the two are
rather complementary. While the  drivers/char/ipmi/kcs_bmc.c
is the other (bmc) end of drivers/char/ipmi/ipmi_kcs_sm.c, it seems
that the proposed kcs_bmc_cdev_raw.c interface would be
what corresponds to the other side of
drivers/input/serio/i8042.c+userio.c. Then again, these are also on
separate ports (0x60 for the keyboard controller, 0xca2 for the BMC
KCS), so they would never actually talk to one another.

> Both the KCS IPMI and raw chardev I've implemented in this patch need
> both read and write access to the status register (STR). serio could
> potentially expose its value through serio_interrupt() using the
> SERIO_OOB_DATA flag, but I haven't put any thought into it beyond this
> sentence. We'd need some extra support for writing STR via the serio
> API. I'm not sure that fits into the abstraction (unless we make
> serio_write() take a flags argument?).
>
> In that vein, the serio_raw interface is close to the functionality
> that the raw chardev provides in this patch, though again serio_raw
> lacks userspace access to STR. Flags are ignored in the ->interrupt()
> callback so all values received via ->interrupt() are exposed as data.
> The result is there's no way to take care of SERIO_OOB_DATA in the
> read() path. Given that, I think we'd have to expose an ioctl() to
> access the STR value after taking care of SERIO_OOB_DATA in
> ->interrupt().
>
> I'm not sure where that lands us.

Based on what I looked up, I think you can just forget about my original
question. We have two separate interfaces that use an Intel 8042-style
protocol, but they don't really interact.

      Arnd