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

From: Cornelia Huck <cohuck@redhat.com>
To: Parav Pandit <parav@mellanox.com>
Cc: "alex.williamson@redhat.com" <alex.williamson@redhat.com>,
	Jiri Pirko <jiri@mellanox.com>,
	"kwankhede@nvidia.com" <kwankhede@nvidia.com>,
	"davem@davemloft.net" <davem@davemloft.net>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
Date: Tue, 27 Aug 2019 13:34:32 +0200	[thread overview]
Message-ID: <20190827133432.156f7db3.cohuck@redhat.com> (raw)
In-Reply-To: <AM0PR05MB4866BDA002F2C6566492244ED1A00@AM0PR05MB4866.eurprd05.prod.outlook.com>

On Tue, 27 Aug 2019 11:07:37 +0000
Parav Pandit <parav@mellanox.com> wrote:

> > -----Original Message-----
> > From: Cornelia Huck <cohuck@redhat.com>
> > Sent: Tuesday, August 27, 2019 4:17 PM
> > To: Parav Pandit <parav@mellanox.com>
> > Cc: alex.williamson@redhat.com; Jiri Pirko <jiri@mellanox.com>;
> > kwankhede@nvidia.com; davem@davemloft.net; kvm@vger.kernel.org; linux-
> > kernel@vger.kernel.org; netdev@vger.kernel.org
> > Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
> > 
> > On Mon, 26 Aug 2019 15:41:18 -0500
> > Parav Pandit <parav@mellanox.com> 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.

> 
> >   anyway.
> >   
> > > +
> > > +	return sprintf(buf, "%s\n", dev->alias); } static
> > > +DEVICE_ATTR_RO(alias);
> > > +
> > >  static const struct attribute *mdev_device_attrs[] = {
> > > +	&dev_attr_alias.attr,
> > >  	&dev_attr_remove.attr,
> > >  	NULL,
> > >  };  
> 