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

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Date:   Mon, 17 Feb 2020 17:34:53 +0100
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Vitor Soares <Vitor.Soares@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-i3c@...ts.infradead.org,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Wolfram Sang <wsa@...-dreams.de>,
        gregkh <gregkh@...uxfoundation.org>,
        Boris Brezillon <bbrezillon@...nel.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [RFC v2 0/4] Introduce i3c device userspace interface

On Mon, 17 Feb 2020 17:19:57 +0100
Arnd Bergmann <arnd@...db.de> wrote:

> On Mon, Feb 17, 2020 at 4:36 PM Boris Brezillon
> <boris.brezillon@...labora.com> wrote:
> > 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.  
> 
> As I understand it, this is handled by the patches: when a new device
> shows up, this triggers the creation of the userspace interface and
> also the event that leads to the kernel driver to get loaded. If there
> is a kernel driver for the device, that should still load and bind to the
> device, at which point the user space interface will go away again.

Yep, that's what I figured after having a closer look at the code.

> 
> This may waste CPU cycles for first creating and then destroying
> the user space interface, but I don't see how it requires extra work.
> If it does require manual configuration or unbinding, that would
> indeed be a bad design.

To be honest, I had something less invasive in mind. Something closer
to what spidev provides (a driver that can expose I3C devices to
userspace when explicitly requested). I see now that the USB subsystem
does something similar to what's done here, but I'm wondering if it's
really worth it in the I3C case. As I said in my previous reply, I
expect i3cdev to be used when experimenting or when kernel-space driver
is not an option (licensing/security issues).