Re: [RFC v2 0/4] Introduce i3c device userspace interface

From: Arnd Bergmann <arnd@arndb.de>
To: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Vitor Soares <Vitor.Soares@synopsys.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-i3c@lists.infradead.org" <linux-i3c@lists.infradead.org>,
	Jose Abreu <Jose.Abreu@synopsys.com>,
	Joao Pinto <Joao.Pinto@synopsys.com>,
	Wolfram Sang <wsa@the-dreams.de>,
	gregkh <gregkh@linuxfoundation.org>,
	Boris Brezillon <bbrezillon@kernel.org>,
	Mark Brown <broonie@kernel.org>
Subject: Re: [RFC v2 0/4] Introduce i3c device userspace interface
Date: Mon, 17 Feb 2020 17:31:08 +0100	[thread overview]
Message-ID: <CAK8P3a2O23=Jjmj0xTQC7pePnwHBrcJ1YeRAKp-1hVf1GNmA5w@mail.gmail.com> (raw)
In-Reply-To: <20200217172309.26697082@collabora.com>

On Mon, Feb 17, 2020 at 5:23 PM Boris Brezillon
<boris.brezillon@collabora.com> wrote:
> On Mon, 17 Feb 2020 15:55:08 +0000 Vitor Soares <Vitor.Soares@synopsys.com> wrote:
>
> Okay, I have clearly not read the code carefully enough. I thought you
> were declaring a new i3c_device_driver and were manually binding all
> orphan devices to this driver. Looks like the solution is more subtle
> than that, and i3cdevs are actually subdevices that are automatically
> created/removed when the I3C device is unbound/bound. That means the
> 'on-demand driver loading' logic is not impacted by this new layer. I'm
> still not convinced this is needed (I expect i3cdev to be used mostly
> for experiment, and in that case, having a udev rule, or manually
> binding the device to the i3cdev driver shouldn't be a problem).

I'm fairly sure it's not needed, other approaches could be used to
provide user space access, but it's not clear if any other way is
better either. It also took me a while to figure out what is going on
when I read the code.

One thought that I had was that this could be better integrated into
the core, with user space being there implicitly through sysfs rather
than a /dev file.

> I'm also not sure what happens if the device is still used when
> i3cdev_detach() is called, can transfers still be done after the device
> is attached to its in-kernel driver?

I think this is still an open issue that I also pointed out. The driver
binding/unbinding and user space access definitely needs to
be properly serialized, whichever method is used to implement the
user access.

       Arnd