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

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Message-ID: <CH2PR12MB421610BDA54B7292BB439824AE160@CH2PR12MB4216.namprd12.prod.outlook.com>
Date:   Mon, 17 Feb 2020 16:12:41 +0000
From:   Vitor Soares <Vitor.Soares@...opsys.com>
To:     gregkh <gregkh@...uxfoundation.org>
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

From: gregkh <gregkh@...uxfoundation.org>
Date: Mon, Feb 17, 2020 at 16:03:45

> 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 

My bad ☹.

> as obviously you
> think this is good enough by now, right?

Yes and I really appreciate the feedback that leads me to this v3.

> 
> Also, I almost never review RFC patches, we have enough "real" patches
> to review as it is :)
> 
> thanks,
> 
> greg k-h

Thanks for your advice.


Best regards,
Vitor Soares