linux-kernel - Re: [RFC 1/1] s390/pci: expose UID checking state in sysfs

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Date:   Wed, 13 Jan 2021 12:55:00 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, Pierre Morel <pmorel@...ux.ibm.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Viktor Mihajlovski <mihajlov@...ux.ibm.com>
Subject: Re: [RFC 1/1] s390/pci: expose UID checking state in sysfs

On Wed, Jan 13, 2021 at 08:47:58AM +0100, Niklas Schnelle wrote:
> On 1/12/21 10:50 PM, Bjorn Helgaas wrote:
> > On Mon, Jan 11, 2021 at 10:38:57AM +0100, Niklas Schnelle wrote:
> >> We use the UID of a zPCI adapter, or the UID of the function zero if
> >> there are multiple functions in an adapter, as PCI domain if and only if
> >> UID Checking is turned on.
> >> Otherwise we automatically generate domains as devices appear.
> >>
> >> The state of UID Checking is thus essential to know if the PCI domain
> >> will be stable, yet currently there is no way to access this information
> >> from userspace.
> >> So let's solve this by showing the state of UID checking as a sysfs
> >> attribute in /sys/bus/pci/uid_checking

> >> +/* Global zPCI attributes */
> >> +static ssize_t uid_checking_show(struct kobject *kobj,
> >> +				 struct kobj_attribute *attr, char *buf)
> >> +{
> >> +	return sprintf(buf, "%i\n", zpci_unique_uid);
> >> +}
> >> +
> >> +static struct kobj_attribute sys_zpci_uid_checking_attr =
> >> +	__ATTR(uid_checking, 0444, uid_checking_show, NULL);
> > 
> > Use DEVICE_ATTR_RO instead of __ATTR.
> 
> It's my understanding that DEVICE_ATTR_* is only for
> per device attributes. This one is global for the entire
> Z PCI. I just tried with BUS_ATTR_RO instead
> and that works but only if I put the attribute at
> /sys/bus/pci/uid_checking instead of with a zpci
> subfolder. This path would work for us too, we
> currently don't have any other global attributes
> that we are planning to expose but those could of
> course come up in the future.

Ah, I missed the fact that this is a kobj_attribute, not a
device_attribute.  Maybe KERNEL_ATTR_RO()?  Very few uses so far, but
seems like it might fit?

Bjorn