Date:   Tue, 3 Apr 2018 12:53:11 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     John Garry <john.garry@...wei.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        mika.westerberg@...ux.intel.com, rafael@...nel.org,
        lorenzo.pieralisi@....com, rjw@...ysocki.net,
        hanjun.guo@...aro.org, robh+dt@...nel.org, bhelgaas@...gle.com,
        arnd@...db.de, mark.rutland@....com, olof@...om.net,
        dann.frazier@...onical.com, andy.shevchenko@...il.com,
        robh@...nel.org, andriy.shevchenko@...ux.intel.com,
        joe@...ches.com, benh@...nel.crashing.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, linuxarm@...wei.com, minyard@....org,
        devicetree@...r.kernel.org, linux-arch@...r.kernel.org,
        rdunlap@...radead.org, gregkh@...uxfoundation.org,
        akpm@...ux-foundation.org, frowand.list@...il.com, agraf@...e.de,
        linux-tegra@...r.kernel.org
Subject: Re: [PATCH v17 01/10] LIB: Introduce a generic PIO mapping method

On Tue, Apr 03, 2018 at 06:02:43PM +0100, John Garry wrote:
> On 03/04/2018 17:37, Thierry Reding wrote:
> > On Tue, Apr 03, 2018 at 05:01:37PM +0100, John Garry wrote:
> > > > > > +int logic_pio_register_range(struct logic_pio_hwaddr *new_range)
> > > > > > +{
> > > > > > +	struct logic_pio_hwaddr *range;
> > > > > > +	resource_size_t start = new_range->hw_start;
> > > > > > +	resource_size_t end = new_range->hw_start + new_range->size;
> > > > > > +	resource_size_t mmio_sz = 0;
> > > > > > +	resource_size_t iio_sz = MMIO_UPPER_LIMIT;
> > > > > > +	int ret = 0;
> > > > > > +
> > > > > > +	if (!new_range || !new_range->fwnode || !new_range->size)
> > > > > > +		return -EINVAL;
> > > > > > +
> > > > > > +	mutex_lock(&io_range_mutex);
> > > > > > +	list_for_each_entry_rcu(range, &io_range_list, list) {
> > > > > > +		if (range->fwnode == new_range->fwnode) {
> > > > > > +			/* range already there */
> > > > > > +			ret = -EFAULT;
> > > > > > +			goto end_register;
> > > > > > +		}
> > > > > 
> > > 
> > > Hi Thierry,
> > > 
> > > > > This is the -EFAULT that propagates to pci-tegra.c's ->probe() and fails
> > > > > to bind the driver.
> > > > > 
> > > > > I'm not exactly sure what's causing the duplicate here because it's
> > > > > rather difficult to get at something useful from just the ->fwnode, but
> > > > > I'm fairly sure that the reason this breaks is because the Tegra driver
> > > > > will defer probe due to some regulators that aren't available on the
> > > > > first try. Given the above code and the rest of this file, I can't see a
> > > > > way to "fix" the driver and remove the I/O range on failure.
> > > > > 
> > > > > This is doubly bad because this doesn't only leak the ranges on probe
> > > > > deferral, but also on driver unload, and we just added support for
> > > > > building the Tegra driver as a loadable module, so these are actually
> > > > > cases that can happen in regular uses of the driver.
> > > > > 
> > > > > I have no idea on how to fix this. Anyone know of a quick fix to restore
> > > > > PCI for Tegra other than reverting all of these changes?
> > > > > 
> > > > > I suppose an API could be added to unregister the range, but the calling
> > > > > sequence is rather obfuscated, so removing the range will look totally
> > > > > asymmetric, I'm afraid.
> > > > > 
> > > > > Here's the call stack:
> > > > > 
> > > > > 	tegra_pcie_probe()
> > > > > 	tegra_pcie_parse_dt()
> > > > > 	of_pci_range_to_resource()
> > > > > 	pci_register_io_range()
> > > > > 	logic_pio_register_range()
> > > > > 
> > > > > So the range here is registered as part of a resource parsing function,
> > > > > which is supposed to not have any side-effects. There's no equivalent of
> > > > > that parsing routine (i.e. no "unparse" function that would undo the
> > > > > effects of parsing).
> > > > > 
> > > > > Perhaps a cleaner way would be to decouple the parsing from the actual
> > > > > request step that has the side-effect.
> > > 
> > > This could be added if we agreed that it would be useful.
> > 
> > I guess in most cases these ranges will be static at least during one
> > boot. But it still feels like this should be removed when the driver
> > goes away. While this may not depend on data by the driver, and hence
> > won't cause a crash or anything, it just seems wrong to leave it
> > around when the driver no longer isn't.
> 
> That sounds reasonable, considering we do unmap the iospace when we release
> - so it looks like currently we're leaving some IO range reserved which does
> not have a mapping.
> 
> However this change seems non-trivial, considering we're now even coupling
> the PIO range registration into DT parsing.
> 
> > 
> > > > > Going back in history a little, it looks like even before this commit
> > > > > the I/O range registration was triggered by the parsing code and even
> > > > > the range leak was there, except that it caused pci_register_io_range()
> > > > > to return 0 rather than -EFAULT. Perhaps the quickest fix for this would
> > > > > be to do the same in the new code and restore drivers that accidentally
> > > > > depend on this behaviour.
> > > > 
> > > > I can confirm that the following fixes the issue for me, though I don't
> > > > think it's a very clean fix given that the range will remain requested
> > > > forever, even if the driver is gone. But since that's already been the
> > > > case for quite a while, probably something that can be fixed separately.
> > > > 
> > > 
> > > Right, there was no way to deregister the range previously.  From looking at
> > > the history here I see no reason to not support it.
> > > 
> > > As for this patch, as you said, the only difference is that we fault on
> > > trying to register the same range again. So this solution seems reasonable.
> > 
> > Okay, I can turn this into a proper patch to fix this up. I suspect that
> > other drivers may be subject to the same regression. For the longer term
> > I think it'd be better to properly undo the registration on failure and
> > removal, but I suspect that it'd be quite a bit of work and not suitable
> > for v4.17 anymore.
> 
> Thanks, I had started to put the patch together but if you're happy to
> continue then that's fine. Please let me know.

Since you seem to agree this is the right short-term fix and I would
squash it into the original commit anyway, I went ahead and did that
so we could get this into linux-next as soon as possible.

Here's the diff from my previous "next" branch with respect to this
series:

diff --git a/lib/logic_pio.c b/lib/logic_pio.c
index 29cedeadb397..4664b87e1c5f 100644
--- a/lib/logic_pio.c
+++ b/lib/logic_pio.c
@@ -46,7 +46,6 @@ int logic_pio_register_range(struct logic_pio_hwaddr *new_range)
 	list_for_each_entry_rcu(range, &io_range_list, list) {
 		if (range->fwnode == new_range->fwnode) {
 			/* range already there */
-			ret = -EFAULT;
 			goto end_register;
 		}
 		if (range->flags == LOGIC_PIO_CPU_MMIO &&

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