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

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Date:   Wed, 23 Jun 2021 11:21:09 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Amey Narkhede <ameynarkhede03@...il.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 Mon, 21 Jun 2021 08:01:35 -0500
Bjorn Helgaas <helgaas@...nel.org> wrote:

> 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:  
> > > > +	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.

I'd say the device specific reset is special.  The device itself can
support a number of resets and they're theoretically all equivalent,
it's a policy decision which to use.  But the device specific reset is
a software provided reset.  Someone has specifically gone to the
trouble to create a reset mechanism that is in some way better than the
other methods.  Not using that one by default sure feels like something
worthy of leaving a breadcrumb in dmesg for debugging.  Thanks,

Alex