Re: [RFC 1/5] misc: introduce notify-device driver

[Matthias Schiffer]

On Wed, 2022-10-26 at 16:37 +0200, Greg Kroah-Hartman wrote:
> On Wed, Oct 26, 2022 at 03:15:30PM +0200, Matthias Schiffer wrote:
> > A notify-device is a synchronization facility that allows to query
> > "readiness" across drivers, without creating a direct dependency between
> > the driver modules. The notify-device can also be used to trigger deferred
> > probes.
> > 
> > Signed-off-by: Matthias Schiffer <matthias.schiffer@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/misc/Kconfig          |   4 ++
> >  drivers/misc/Makefile         |   1 +
> >  drivers/misc/notify-device.c  | 109 ++++++++++++++++++++++++++++++++++
> >  include/linux/notify-device.h |  33 ++++++++++
> >  4 files changed, 147 insertions(+)
> >  create mode 100644 drivers/misc/notify-device.c
> >  create mode 100644 include/linux/notify-device.h
> > 
> > diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
> > index 358ad56f6524..63559e9f854c 100644
> > --- a/drivers/misc/Kconfig
> > +++ b/drivers/misc/Kconfig
> > @@ -496,6 +496,10 @@ config VCPU_STALL_DETECTOR
> >  
> >  	  If you do not intend to run this kernel as a guest, say N.
> >  
> > +config NOTIFY_DEVICE
> > +	tristate "Notify device"
> > +	depends on OF
> > +
> >  source "drivers/misc/c2port/Kconfig"
> >  source "drivers/misc/eeprom/Kconfig"
> >  source "drivers/misc/cb710/Kconfig"
> > diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
> > index ac9b3e757ba1..1e8012112b43 100644
> > --- a/drivers/misc/Makefile
> > +++ b/drivers/misc/Makefile
> > @@ -62,3 +62,4 @@ obj-$(CONFIG_HI6421V600_IRQ)	+= hi6421v600-irq.o
> >  obj-$(CONFIG_OPEN_DICE)		+= open-dice.o
> >  obj-$(CONFIG_GP_PCI1XXXX)	+= mchp_pci1xxxx/
> >  obj-$(CONFIG_VCPU_STALL_DETECTOR)	+= vcpu_stall_detector.o
> > +obj-$(CONFIG_NOTIFY_DEVICE)	+= notify-device.o
> > diff --git a/drivers/misc/notify-device.c b/drivers/misc/notify-device.c
> > new file mode 100644
> > index 000000000000..42e0980394ea
> > --- /dev/null
> > +++ b/drivers/misc/notify-device.c
> > @@ -0,0 +1,109 @@
> > +// SPDX-License-Identifier: GPL-2.0-or-later
> > +
> > +#include <linux/device/class.h>
> > +#include <linux/kernel.h>
> > +#include <linux/module.h>
> > +#include <linux/notify-device.h>
> > +#include <linux/platform_device.h>
> > +#include <linux/slab.h>
> > +
> > +static void notify_device_release(struct device *dev)
> > +{
> > +	of_node_put(dev->of_node);
> > +	kfree(dev);
> > +}
> > +
> > +static struct class notify_device_class = {
> > +	.name = "notify-device",
> > +	.owner = THIS_MODULE,
> > +	.dev_release = notify_device_release,
> > +};
> > +
> > +static struct platform_driver notify_device_driver = {

[Pruning the CC list a bit, to avoid clogging people's inboxes]

> 
> Ick, wait, this is NOT a platform device, nor driver, so it shouldn't be
> either here.  Worst case, it's a virtual device on the virtual bus.

This part of the code is inspired by mac80211_hwsim, which uses a
platform driver in a similar way, for a plain struct device. Should
this rather use a plain struct device_driver?

Also, what's the virtual bus? Grepping the Linux code and documentation
didn't turn up anything.

> 
> But why is this a class at all?  Classes are a representation of a type
> of device that userspace can see, how is this anything that userspace
> cares about?

Makes sense, I will remove the class.

> 
> Doesn't the device link stuff handle all of this type of "when this
> device is done being probed, now I can" problems?  Why is a whole new
> thing needed?

The issue here is that (as I understand it) the device link and
deferred probing infrastructore only cares about whether the supplier
device has been probed successfully.

This is insuffient in the case of the dependency between mwifiex and
hci_uart/hci_mrvl that I want to express: mwifiex loads its firmware
asynchronously, so finishing the mwifiex probe is too early to retry
probing the Bluetooth driver.

While mwifiex does create a few devices (ieee80211, netdevice) when the
firmware has loaded, none of these bind to a driver, so they don't
trigger the deferred probes. Using their existence as a condition for
allowing the Bluetooth driver to probe also seems ugly too me
(ieee80211 currently can't be looked up by OF node, and netdevices can
be created and deleted dynamically).

Because of this, I came to the conclusion that creating and binding a
device specifically for this purpose is a good solution, as it solves
two problems at once:
- The driver bind triggers deferred probes
- The driver allows to look up the device by OF node

Integrating this with device links might make sense as well, but I
haven't looked much into that yet.

Thanks,
Matthias


> 
> thanks,
> 
> greg k-h