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Date:   Mon, 17 Feb 2020 17:03:45 +0100
From:   gregkh <gregkh@...uxfoundation.org>
To:     Vitor Soares <Vitor.Soares@...opsys.com>
Cc:     Boris Brezillon <boris.brezillon@...labora.com>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i3c@...ts.infradead.org" <linux-i3c@...ts.infradead.org>,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Boris Brezillon <bbrezillon@...nel.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [RFC v2 0/4] Introduce i3c device userspace interface

On Mon, Feb 17, 2020 at 03:55:08PM +0000, Vitor Soares wrote:
> Hi,
> 
> From: Boris Brezillon <boris.brezillon@...labora.com>
> Date: Mon, Feb 17, 2020 at 15:36:22
> 
> > On Mon, 17 Feb 2020 16:06:45 +0100
> > Arnd Bergmann <arnd@...db.de> wrote:
> > 
> > > On Mon, Feb 17, 2020 at 3:51 PM Boris Brezillon
> > > <boris.brezillon@...labora.com> wrote:
> > > > Sorry for taking so long to reply, and thanks for working on that topic.
> > > >
> > > > On Wed, 29 Jan 2020 13:17:31 +0100
> > > > Vitor Soares <Vitor.Soares@...opsys.com> wrote:
> > > >  
> > > > > For today there is no way to use i3c devices from user space and
> > > > > the introduction of such API will help developers during the i3c device
> > > > > or i3c host controllers development.
> > > > >
> > > > > The i3cdev module is highly based on i2c-dev and yet I tried to address
> > > > > the concerns raised in [1].
> > > > >
> > > > > NOTES:
> > > > > - The i3cdev dynamically request an unused major number.
> > > > >
> > > > > - The i3c devices are dynamically exposed/removed from dev/ folder based
> > > > >   on if they have a device driver bound to it.  
> > > >
> > > > May I ask why you need to automatically bind devices to the i3cdev
> > > > driver when they don't have a driver matching the device id
> > > > loaded/compiled-in? If we get the i3c subsystem to generate proper
> > > > uevents we should be able to load the i3cdev module and bind the device
> > > > to this driver using a udev rule.  
> > > 
> > > I think that would require manual configuration to ensure that the correct
> > > set of devices get bound to either the userspace driver or an in-kernel
> > > driver.
> > 
> > Hm, isn't that what udev is supposed to do anyway? Remember that
> > I3C devices expose a manufacturer and part-id (which are similar to the
> > USB vendor and product ids), so deciding when an I3C device should be
> > bound to the i3cdev driver should be fairly easy, and that's a
> > per-device decision anyway.
> > 
> > > The method from the current patch series is more complicated,
> > > but it means that any device can be accessed by the user space driver
> > > as long as it's not already owned by a kernel driver.
> > 
> > Well, I'm more worried about the extra churn this auto-binding logic
> > might create for the common 'on-demand driver loading' use case. At
> > first, there's no driver matching a specific device, but userspace
> > might load one based on the uevents it receives. With the current
> > approach, that means we'd first have to unbind the device before
> > loading the driver. AFAICT, no other subsystem does that.
> 
> I'm about to finish v3 (today or tomorrow) and I think I fixed all 
> concerns rise during v2. I would like you to see that version before any 
> change.

Why are there so many "RFC" series here?  I treat "RFC" as "I don't
really like this patch but I'm throwing it out there for others to look
at if they care".  No RFC should ever go on beyond a v1 as obviously you
think this is good enough by now, right?

Also, I almost never review RFC patches, we have enough "real" patches
to review as it is :)

thanks,

greg k-h

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