Re: [RFC PATCH 1/7] drivers: Add motion control subsystem

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Mar 06, 2025 at 09:20:13AM +0100, David Jander wrote:
> On Thu, 6 Mar 2025 08:18:46 +0100
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> 
> > On Thu, Mar 06, 2025 at 12:21:22AM +0100, Uwe Kleine-König wrote:
> > > Hello David,
> > > 
> > > On Wed, Mar 05, 2025 at 04:40:45PM +0100, David Jander wrote:  
> > > > On Fri, 28 Feb 2025 17:44:27 +0100
> > > > Uwe Kleine-König <u.kleine-koenig@baylibre.com> wrote:  
> > > > > On Thu, Feb 27, 2025 at 05:28:17PM +0100, David Jander wrote:
> > > > > [...]  
> > > > > > +static int motion_open(struct inode *inode, struct file *file)
> > > > > > +{
> > > > > > +	int minor = iminor(inode);
> > > > > > +	struct motion_device *mdev = NULL, *iter;
> > > > > > +	int err;
> > > > > > +
> > > > > > +	mutex_lock(&motion_mtx);    
> > > > > 
> > > > > If you use guard(), error handling gets a bit easier.  
> > > > 
> > > > This looks interesting. I didn't know about guard(). Thanks. I see the
> > > > benefits, but in some cases it also makes the locked region less clearly
> > > > visible. While I agree that guard() in this particular place is nice,
> > > > I'm hesitant to try and replace all mutex_lock()/_unlock() calls with guard().
> > > > Let me know if my assessment of the intended use of guard() is incorrect.  
> > > 
> > > I agree that guard() makes it harder for non-trivial functions to spot
> > > the critical section. In my eyes this is outweight by not having to
> > > unlock in all exit paths, but that might be subjective. Annother
> > > downside of guard is that sparse doesn't understand it and reports
> > > unbalanced locking.
> > >    
> > > > > > +	list_for_each_entry(iter, &motion_list, list) {
> > > > > > +		if (iter->minor != minor)
> > > > > > +			continue;
> > > > > > +		mdev = iter;
> > > > > > +		break;
> > > > > > +	}    
> > > > > 
> > > > > This should be easier. If you use a cdev you can just do
> > > > > container_of(inode->i_cdev, ...);  
> > > > 
> > > > Hmm... I don't yet really understand what you mean. I will have to study the
> > > > involved code a bit more.  
> > > 
> > > The code that I'm convinced is correct is
> > > https://lore.kernel.org/linux-pwm/00c9f1181dc351e1e6041ba6e41e4c30b12b6a27.1725635013.git.u.kleine-koenig@baylibre.com/
> > > 
> > > This isn't in mainline because there is some feedback I still have to
> > > address, but I think it might serve as an example anyhow.
> > >   
> > > > > > [...]
> > > > > > +
> > > > > > +static const struct class motion_class = {
> > > > > > +	.name		= "motion",
> > > > > > +	.devnode	= motion_devnode,    
> > > > > 
> > > > > IIRC it's recommended to not create new classes, but a bus.  
> > > > 
> > > > Interesting. I did some searching, and all I could find was that the chapter
> > > > in driver-api/driver-model about classes magically vanished between versions
> > > > 5.12 and 5.13. Does anyone know where I can find some information about this?
> > > > Sorry if I'm being blind...  
> > > 
> > > Half knowledge on my end at best. I would hope that Greg knows some
> > > details (which might even be "no, classes are fine"). I added him to Cc:  
> > 
> > A class is there for when you have a common api that devices of
> > different types can talk to userspace (i.e. the UAPI is common, not the
> > hardware type).  Things like input devices, tty, disks, etc.  A bus is
> > there to be able to write different drivers to bind to for that hardware
> > bus type (pci, usb, i2c, platform, etc.)
> > 
> > So you need both, a bus to talk to the hardware, and a class to talk to
> > userspace in a common way (ignore the fact that we can also talk to
> > hardware directly from userspace like raw USB or i2c or PCI config
> > space, that's all bus-specific stuff).
> 
> Thanks for chiming in. Let me see if I understand this correctly: In this
> case, I have a UAPI that is common to different types of motion control
> devices. So I need a class. check.

Correct.

> Do I need a bus? If one can conceive other drivers or kernel parts that talk to
> motion drivers, I would need a bus. If that doesn't make sense, I don't. Right?

Correct.

> I actually can think of a new motion device that acts as an aggregator of
> several single-channel motion devices into a single "virtual" multi-channel
> device... so do I need also a bus? I suppose...?

Nope, that should just be another class driver.  Think about how input
does this, some input /dev/ nodes are the sum of ALL input /dev/ nodes
together, while others are just for individual input devices.

> Then the question remains: why did the chapter about classes vanish?

What are you specifically referring to?  I don't remember deleting any
documentation, did files move around somehow and the links not get
updated?

thanks,

greg k-h