Date:   Tue, 3 Apr 2018 19:24:06 +0100
From:   John Garry <john.garry@...wei.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
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 03/04/2018 18:53, Bjorn Helgaas wrote:
> 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.
>

Ok, thanks.

John

> 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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