linux-kernel - Re: [PATCH v7 4/8] PCI/sysfs: Allow userspace to query and set device reset mechanism

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Date:   Mon, 21 Jun 2021 08:01:35 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Amey Narkhede <ameynarkhede03@...il.com>
Cc:     alex.williamson@...hat.com,
        Raphael Norwitz <raphael.norwitz@...anix.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        kw@...ux.com, Shanker Donthineni <sdonthineni@...dia.com>,
        Sinan Kaya <okaya@...nel.org>, Len Brown <lenb@...nel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH v7 4/8] PCI/sysfs: Allow userspace to query and set
 device reset mechanism

On Sat, Jun 19, 2021 at 07:29:20PM +0530, Amey Narkhede wrote:
> On 21/06/18 03:00PM, Bjorn Helgaas wrote:
> > On Tue, Jun 08, 2021 at 11:18:53AM +0530, Amey Narkhede wrote:
> > > Add reset_method sysfs attribute to enable user to
> > > query and set user preferred device reset methods and
> > > their ordering.

> > > +	if (sysfs_streq(options, "default")) {
> > > +		for (i = 0; i < PCI_RESET_METHODS_NUM; i++)
> > > +			reset_methods[i] = reset_methods[i] ? prio-- : 0;
> > > +		goto set_reset_methods;
> > > +	}
> >
> > If you use pci_init_reset_methods() here, you can also get this case
> > out of the way early.
> >
> The problem with alternate encoding is we won't be able to know if
> one of the reset methods was disabled previously. For example,
> 
> # cat reset_methods
> flr,bus 			# dev->reset_methods = [3, 5, 0, ...]
> # echo bus > reset_methods 	# dev->reset_methods = [5, 0, 0, ...]
> # cat reset_methods
> bus
> 
> Now if an user wants to enable flr
> 
> # echo flr > reset_methods 	# dev->reset_methods = [3, 0, 0, ...]
> OR
> # echo bus,flr > reset_methods 	# dev->reset_methods = [5, 3, 0, ...]
> 
> either they need to write "default" first then flr or we will need to
> reprobe reset methods each time when user writes to reset_method attribute.

Not sure I completely understand the problem here.  I think relying on
previous state that is invisible to the user is a little problematic
because it's hard for the user to predict what will happen.

If the user enables a method that was previously "disabled" because
the probe failed, won't the reset method itself just fail with
-ENOTTY?  Is that a problem?

> > > +	while ((name = strsep(&options, ",")) != NULL) {
> > > +		if (sysfs_streq(name, ""))
> > > +			continue;
> > > +
> > > +		name = strim(name);
> > > +
> > > +		for (i = 0; i < PCI_RESET_METHODS_NUM; i++) {
> > > +			if (reset_methods[i] &&
> > > +			    sysfs_streq(name, pci_reset_fn_methods[i].name)) {
> > > +				reset_methods[i] = prio--;
> > > +				break;
> > > +			}
> > > +		}
> > > +
> > > +		if (i == PCI_RESET_METHODS_NUM) {
> > > +			kfree(options);
> > > +			return -EINVAL;
> > > +		}
> > > +	}
> > > +
> > > +	if (reset_methods[0] &&
> > > +	    reset_methods[0] != PCI_RESET_METHODS_NUM)
> > > +		pci_warn(pdev, "Device specific reset disabled/de-prioritized by user");
> >
> > Is there a specific reason for this warning?  Is it just telling the
> > user that he might have shot himself in the foot?  Not sure that's
> > necessary.
> >
> I think generally presence of device specific reset method means other
> methods are potentially broken. Is it okay to skip this?

We might want a warning at reset-time if all the methods failed,
because that means we may leak state between users.  Maybe we also
want one here, if *all* reset methods are disabled.  I don't really
like special treatment of device-specific methods here because it
depends on the assumption that "device-specific means all other resets
are broken."  That's hard to maintain.

Bjorn