RE: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree

[Parav Pandit]

> -----Original Message-----
> From: Cornelia Huck <cohuck@xxxxxxxxxx>
> Sent: Tuesday, August 27, 2019 5:25 PM
> To: Parav Pandit <parav@xxxxxxxxxxxx>
> Cc: alex.williamson@xxxxxxxxxx; Jiri Pirko <jiri@xxxxxxxxxxxx>;
> kwankhede@xxxxxxxxxx; davem@xxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
> 
> On Tue, 27 Aug 2019 11:52:21 +0000
> Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> 
> > > -----Original Message-----
> > > From: Cornelia Huck <cohuck@xxxxxxxxxx>
> > > Sent: Tuesday, August 27, 2019 5:05 PM
> > > To: Parav Pandit <parav@xxxxxxxxxxxx>
> > > Cc: alex.williamson@xxxxxxxxxx; Jiri Pirko <jiri@xxxxxxxxxxxx>;
> > > kwankhede@xxxxxxxxxx; davem@xxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> > > linux- kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> > > Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
> > >
> > > On Tue, 27 Aug 2019 11:07:37 +0000
> > > Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> > >
> > > > > -----Original Message-----
> > > > > From: Cornelia Huck <cohuck@xxxxxxxxxx>
> > > > > Sent: Tuesday, August 27, 2019 4:17 PM
> > > > > To: Parav Pandit <parav@xxxxxxxxxxxx>
> > > > > Cc: alex.williamson@xxxxxxxxxx; Jiri Pirko <jiri@xxxxxxxxxxxx>;
> > > > > kwankhede@xxxxxxxxxx; davem@xxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> > > > > linux- kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> > > > > Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
> > > > >
> > > > > On Mon, 26 Aug 2019 15:41:18 -0500 Parav Pandit
> > > > > <parav@xxxxxxxxxxxx> wrote:
> > >
> > > > > > +static ssize_t alias_show(struct device *device,
> > > > > > +			  struct device_attribute *attr, char *buf) {
> > > > > > +	struct mdev_device *dev = mdev_from_dev(device);
> > > > > > +
> > > > > > +	if (!dev->alias)
> > > > > > +		return -EOPNOTSUPP;
> > > > >
> > > > > I'm wondering how to make this consumable by userspace in the
> > > > > easiest
> > > way.
> > > > > - As you do now (userspace gets an error when trying to read)?
> > > > > - Returning an empty value (nothing to see here, move along)?
> > > > No. This is confusing, to return empty value, because it says that
> > > > there is an
> > > alias but it is some weird empty string.
> > > > If there is alias, it shows exactly what it is.
> > > > If no alias it returns an error code = unsupported -> inline with
> > > > other widely
> > > used subsystem.
> > > >
> > > > > - Or not creating the attribute at all? That would match what userspace
> > > > >   sees on older kernels, so it needs to be able to deal with
> > > > > that
> > > > New sysfs files can appear. Tool cannot say that I was not
> > > > expecting this file
> > > here.
> > > > User space is supposed to work with the file they are off interest.
> > > > Mdev interface has option to specify vendor specific files, though
> > > > in usual
> > > manner it's not recommended.
> > > > So there is no old user space, new kernel issue here.
> > >
> > > I'm not talking about old userspace/new kernel, but new userspace/old
> kernel.
> > > Code that wants to consume this attribute needs to be able to cope
> > > with its absence anyway.
> > >
> > Old kernel doesn't have alias file.
> > If some tool tries to read this file it will fail to open non existing file; open()
> system call is already taking care of it.
> 
> Yes, that was exactly my argument?
I misunderstood probably.
I re-read all 3 options you posted.
I do not see any issue in reporting error code similar to other widely used netdev subsystem, hence propose what is posted in the patch.