linux-kernel - Re: [PATCH] usb: typec: Send uevent for num

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Date:   Thu, 7 Jan 2021 11:57:08 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Prashant Malani <pmalani@...omium.org>
Cc:     "open list:USB NETWORKING DRIVERS" <linux-usb@...r.kernel.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Benson Leung <bleung@...omium.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: typec: Send uevent for num_altmodes update

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@...uxfoundation.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@...ux.intel.com>
> > > Cc: Benson Leung <bleung@...omium.org>
> > > Signed-off-by: Prashant Malani <pmalani@...omium.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