usb: typec: Send uevent for num_altmodes update

Generate a change uevent when the "number_of_alternate_modes" sysfs file
for partners and plugs is updated by a port driver.

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Benson Leung <bleung@chromium.org>
Signed-off-by: Prashant Malani <pmalani@chromium.org>
---
 drivers/usb/typec/class.c | 2 ++
 1 file changed, 2 insertions(+)

On Wed, Jan 06, 2021 at 07:49:04PM -0800, Prashant Malani wrote:
> Generate a change uevent when the "number_of_alternate_modes" sysfs file
> for partners and plugs is updated by a port driver.
> 
> Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> Cc: Benson Leung <bleung@chromium.org>
> Signed-off-by: Prashant Malani <pmalani@chromium.org>
> ---
>  drivers/usb/typec/class.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/usb/typec/class.c b/drivers/usb/typec/class.c
> index ebfd3113a9a8..8f77669f9cf4 100644
> --- a/drivers/usb/typec/class.c
> +++ b/drivers/usb/typec/class.c
> @@ -766,6 +766,7 @@ int typec_partner_set_num_altmodes(struct typec_partner *partner, int num_altmod
>  		return ret;
>  
>  	sysfs_notify(&partner->dev.kobj, NULL, "number_of_alternate_modes");
> +	kobject_uevent(&partner->dev.kobj, KOBJ_CHANGE);

Shouldn't the sysfs_notify() handle the "something has changed" logic
good enough for userspace, as obviously someone is polling on the thing
(otherwise we wouldn't be calling sysfs_notify...)

The kobject itself hasn't "changed", but rather an individual attribute
has changed.  We don't want to create uevents for every individual sysfs
attribute changing values, do we?

What is preventing a normal "monitor the sysfs file" logic from working
here for anyone who wants to know that the alternate modes have changed?

thanks,

greg k-h

On Thu, Jan 07, 2021 at 01:50:53AM -0800, Prashant Malani wrote:
> Hi Greg,
> 
> Thanks for taking a look at the patch.
> 
> On Thu, Jan 7, 2021 at 1:16 AM Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Wed, Jan 06, 2021 at 07:49:04PM -0800, Prashant Malani wrote:
> > > Generate a change uevent when the "number_of_alternate_modes" sysfs file
> > > for partners and plugs is updated by a port driver.
> > >
> > > Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> > > Cc: Benson Leung <bleung@chromium.org>
> > > Signed-off-by: Prashant Malani <pmalani@chromium.org>
> > > ---
> > >  drivers/usb/typec/class.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/drivers/usb/typec/class.c b/drivers/usb/typec/class.c
> > > index ebfd3113a9a8..8f77669f9cf4 100644
> > > --- a/drivers/usb/typec/class.c
> > > +++ b/drivers/usb/typec/class.c
> > > @@ -766,6 +766,7 @@ int typec_partner_set_num_altmodes(struct typec_partner *partner, int num_altmod
> > >               return ret;
> > >
> > >       sysfs_notify(&partner->dev.kobj, NULL, "number_of_alternate_modes");
> > > +     kobject_uevent(&partner->dev.kobj, KOBJ_CHANGE);
> >
> > Shouldn't the sysfs_notify() handle the "something has changed" logic
> > good enough for userspace, as obviously someone is polling on the thing
> > (otherwise we wouldn't be calling sysfs_notify...)
> >
> > The kobject itself hasn't "changed", but rather an individual attribute
> > has changed.  We don't want to create uevents for every individual sysfs
> > attribute changing values, do we?
> 
> Fair point. I noticed other attributes in this source file use a
> similar approach (sysfs_notify + kobject_uevent)
> and took guidance from there in an attempt to remain consistent
> (though, of course, your point still stands).
> 
> I'm guessing it is for processes that rely on udev events
> (subsystem=typec) rather than polling.
> 
> >
> > What is preventing a normal "monitor the sysfs file" logic from working
> > here for anyone who wants to know that the alternate modes have changed?
> 
> One limitation I can think of is that this sysfs file is hidden till
> it has a valid value (i.e >= 0), so a user-space process might not
> be able to poll on the file till it is visible (I suppose even then
> one could poll on the parent).

If the file is being added at this point in time, then yes, it is ok to
send a KOBJ_CHANGE event as that is needed.  Is that what is happening
here?

thanks,

greg k-h