linux-kernel - Re: [PATCH] PCI: rework error checking in the reset path

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Message-ID: <20171025221046.GA25858@bhelgaas-glaptop.roam.corp.google.com>
Date:   Wed, 25 Oct 2017 17:10:46 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Sinan Kaya <okaya@...eaurora.org>, linux-pci@...r.kernel.org,
        timur@...eaurora.org, linux-arm-msm@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] PCI: rework error checking in the reset path

On Wed, Oct 25, 2017 at 11:28:05PM +0200, Alex Williamson wrote:
> On Wed, 25 Oct 2017 08:45:11 -0500
> Bjorn Helgaas <helgaas@...nel.org> wrote:
> 
> > [+cc Alex]
> > 
> > On Mon, Oct 23, 2017 at 05:36:48PM -0400, Sinan Kaya wrote:
> > > The return codes from various reset types are not consistent. The code is
> > > assuming that all reset types will return -ENOTTY when things go wrong.
> > > Instead of relying on negative error status, let's bail out if the
> > > operation is successful instead.  
> > 
> > I like this (no surprise since I suggested something similar at
> > http://lkml.kernel.org/r/20171011210057.GU25517@bhelgaas-glaptop.roam.corp.google.com),
> > but I'd like Alex's opinion before merging it.
> > 
> > Previously, we only tried the next reset method if one method failed
> > with -ENOTTY.  With this patch, we'll try the next reset method if one
> > method fails for any reason, not just -ENOTTY.
> 
> Hmm, I thought the return codes were pretty consistent.  -ENOTTY means
> that the reset callback doesn't handle the device, move on.  Many
> ioctls use the same return code to indicate an unknown ioctl.  This
> allows us to differentiate success vs error vs unhandled.  In the code
> below we lose the ability to, for instance, have a device specific
> reset that returns -EINVAL to prevent the PCI core for triggering
> further reset mechanisms which might be broken on the device.  So, I
> don't see that this patch specifically fixes anything, but it does
> remove what seems like useful functionality...  I'd veto it.  Thanks,

I didn't understand the  intention of -EINVAL vs -ENOTTY, so
that might be a reasonable argument.  The knowledge about mechanisms
being broken on a specific device seems like it would belong in
pci_dev_specific_reset() and not really applicable to other methods,
though.

But I'm not sure the current usage makes a lot of sense.  The only
places I found that return an error other than -ENOTTY are
reset_ivb_igd() and pci_pm_reset().  In reset_ivb_igd(), we return
-ENOMEM if an ioremap() fails.  That's not a case of "other reset
mechanisms are broken and we shouldn't try them."

pci_pm_reset() returns -EINVAL if the device is not in D0.  Maybe it
makes sense to not try any other reset methods in that case, but I
really don't know.

If we leave it as-is, maybe a comment like the following would be
useful.

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index f0d68066c726..2c98f309bc8a 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -4170,6 +4170,13 @@ int __pci_reset_function_locked(struct pci_dev *dev)
 
 	might_sleep();
 
+	/*
+	 * Reset method return values:
+	 *   0:		    Device was successfully reset
+	 *   -ENOTTY:	    Method doesn't support resetting this device;
+	 *		    try the next method
+	 *   anything else: Reset failed; don't try any other mechanisms
+	 */
 	rc = pci_dev_specific_reset(dev, 0);
 	if (rc != -ENOTTY)
 		return rc;