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Date:	Tue, 24 Apr 2012 12:03:39 -0400
From:	Don Dutile <ddutile@...hat.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mikko Vinni <mmvinni@...oo.com>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Allen Kay <allen.m.kay@...el.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] PCI: Fix regression in pci_restore_state()

On 04/23/2012 06:33 PM, Bjorn Helgaas wrote:
> On Mon, Apr 23, 2012 at 2:07 PM, Don Dutile<ddutile@...hat.com>  wrote:
>> On 04/23/2012 03:53 PM, Rafael J. Wysocki wrote:
>>>
>>> On Monday, April 23, 2012, Bjorn Helgaas wrote:
>>>>
>>>> On Sun, Apr 15, 2012 at 1:40 PM, Rafael J. Wysocki<rjw@...k.pl>    wrote:
>>>>>
>>>>> On Sunday, April 15, 2012, Linus Torvalds wrote:
>>>>>>
>>>>>> On Sun, Apr 15, 2012 at 11:47 AM, Rafael J. Wysocki<rjw@...k.pl>
>>>>>>   wrote:
>>>>>>>
>>>>>>>
>>>>>>> mdelay(10) doesn't really look good either to me in this case, though.
>>>>>>
>>>>>>
>>>>>> Oh, I agree. What kind of ass-backwards device actually needs that
>>>>>> kind of crazy delays? It is almost certainly buggy.
>>>>>>
>>>>>> With retries, 10ms delays are totally unacceptable. There's something
>>>>>> wrong.
>>>>>>
>>>>>> A single ms *may* be ok.
>>>>>>
>>>>>> Anyway, can you also split the actual "write _one_ register with
>>>>>> retry" into a function of its own? The code looks like crap with those
>>>>>> multiple levels of looping, with conditionals inside them etc. With a
>>>>>> simple helper function, you could change the break into return, and it
>>>>>> would look much better, I bet.
>>>>>
>>>>>
>>>>> Sure.  It appears cleaner this way.
>>>>>
>>>>> ---
>>>>> From: Rafael J. Wysocki<rjw@...k.pl>
>>>>> Subject: PCI: Fix regression in pci_restore_state(), v3
>>>>>
>>>>> Commit 26f41062f28de65e11d3cf353e52d0be73442be1
>>>>>
>>>>>     PCI: check for pci bar restore completion and retry
>>>>>
>>>>> attempted to address problems with PCI BAR restoration on systems
>>>>> where FLR had not been completed before pci_restore_state() was
>>>>> called, but it did that in an utterly wrong way.
>>>>>
>>>>> First off, instead of retrying the writes for the BAR registers
>>>>> only, it did that for all of the PCI config space of the device,
>>>>> including the status register (whose value after the write quite
>>>>> obviously need not be the same as the written one).  Second, it
>>>>> added arbitrary delay to pci_restore_state() even for systems
>>>>> where the PCI config space restoration was successful at first
>>>>> attempt.  Finally, the mdelay(10) it added to every iteration of the
>>>>> writing loop was way too much of a delay for any reasonable device.
>>>>>
>>>>> All of this actually caused resume failures for some devices on
>>>>> the Mikko's system.
>>>>>
>>>>> To fix the regression, make pci_restore_state() only retry the
>>>>> writes for BAR registers and only wait if the first read from
>>>>> the register doesn't return the written value.  Additionaly, make
>>>>> it wait for 1 ms, instead of 10 ms, after every failing attempt
>>>>> to write into config space.
>>>>>
>>>>> Reported-by: Mikko Vinni<mmvinni@...oo.com>
>>>>> Signed-off-by: Rafael J. Wysocki<rjw@...k.pl>
>>>>> ---
>>>>>   drivers/pci/pci.c |   57
>>>>> ++++++++++++++++++++++++++++++++++++------------------
>>>>>   1 file changed, 39 insertions(+), 18 deletions(-)
>>>>>
>>>>> Index: linux/drivers/pci/pci.c
>>>>> ===================================================================
>>>>> --- linux.orig/drivers/pci/pci.c
>>>>> +++ linux/drivers/pci/pci.c
>>>>> @@ -967,16 +967,47 @@ pci_save_state(struct pci_dev *dev)
>>>>>         return 0;
>>>>>   }
>>>>>
>>>>> +static void pci_restore_config_dword(struct pci_dev *pdev, int offset,
>>>>> +                                    u32 saved_val, int retry)
>>>>> +{
>>>>> +       u32 val;
>>>>> +
>>>>> +       pci_read_config_dword(pdev, offset,&val);
>>>>> +       if (val == saved_val)
>>>>> +               return;
>>>>> +
>>>>> +       for (;;) {
>>>>> +               dev_dbg(&pdev->dev, "restoring config space at offset "
>>>>> +                       "%#x (was %#x, writing %#x)\n", offset, val,
>>>>> saved_val);
>>>>> +               pci_write_config_dword(pdev, offset, saved_val);
>>>>> +               if (retry--<= 0)
>>>>> +                       return;
>>>>> +
>>>>> +               pci_read_config_dword(pdev, offset,&val);
>>>>> +               if (val == saved_val)
>>>>> +                       return;
>>>>> +
>>>>> +               mdelay(1);
>>>>> +       }
>>>>> +}
>>>>> +
>>>>> +static void pci_restore_config_space(struct pci_dev *pdev, int start,
>>>>> int end,
>>>>> +                                    int retry)
>>>>> +{
>>>>> +       int index;
>>>>> +
>>>>> +       for (index = end; index>= start; index--)
>>>>> +               pci_restore_config_dword(pdev, 4 * index,
>>>>> +
>>>>>   pdev->saved_config_space[index],
>>>>> +                                        retry);
>>>>> +}
>>>>> +
>>>>>   /**
>>>>>   * pci_restore_state - Restore the saved state of a PCI device
>>>>>   * @dev: - PCI device that we're dealing with
>>>>>   */
>>>>>   void pci_restore_state(struct pci_dev *dev)
>>>>>   {
>>>>> -       int i;
>>>>> -       u32 val;
>>>>> -       int tries;
>>>>> -
>>>>>         if (!dev->state_saved)
>>>>>                 return;
>>>>>
>>>>> @@ -984,24 +1015,14 @@ void pci_restore_state(struct pci_dev *d
>>>>>         pci_restore_pcie_state(dev);
>>>>>         pci_restore_ats_state(dev);
>>>>>
>>>>> +       pci_restore_config_space(dev, 10, 15, 0);
>>>>>         /*
>>>>>          * The Base Address register should be programmed before the
>>>>> command
>>>>>          * register(s)
>>>>>          */
>>>>> -       for (i = 15; i>= 0; i--) {
>>>>> -               pci_read_config_dword(dev, i * 4,&val);
>>>>> -               tries = 10;
>>>>> -               while (tries&&    val != dev->saved_config_space[i]) {
>>>>>
>>>>> -                       dev_dbg(&dev->dev, "restoring config "
>>>>> -                               "space at offset %#x (was %#x, writing
>>>>> %#x)\n",
>>>>> -                               i, val,
>>>>> (int)dev->saved_config_space[i]);
>>>>> -                       pci_write_config_dword(dev,i * 4,
>>>>> -                               dev->saved_config_space[i]);
>>>>> -                       pci_read_config_dword(dev, i * 4,&val);
>>>>> -                       mdelay(10);
>>>>> -                       tries--;
>>>>> -               }
>>>>> -       }
>>>>> +       pci_restore_config_space(dev, 4, 9, 10);
>>>>> +       pci_restore_config_space(dev, 0, 3, 0);
>>>>> +
>>>>>         pci_restore_pcix_state(dev);
>>>>>         pci_restore_msi_state(dev);
>>>>>         pci_restore_iov_state(dev);
>>>>
>>>>
>>>> I'd feel better about this if there were a way to delay in the FLR
>>>> path instead.  If we delay in the restore path, we're only fixing one
>>>> of the many ways config space can be written.  Other paths that write
>>>> config space will just silently fail.
>>>>
>>>> The PCIe spec (r3.0, sec 6.6.2) mentions waiting for the "pre-FLR
>>>> value for Completion Timeout," but I don't see anything that looks
>>>> like that in pcie_flr() or pci_af_flr().  Are there any other direct
>>>> ways we can detect when the FLR is complete?
>>>
>>>
>>> I'm not aware of any.
>>>
>>> Thanks,
>>> Rafael
>>
>> I don't think so, either.
>> I believe an ECN is being worked in the PCI-SIG
>> to add such a notification, though.
>> Even if adopted, need to wait for another crank of the hw before
>> the notification can be used.
>
> I agree, we can't do something that works only on new hardware -- we
> have to make the existing hardware in the field work.
>
> What about the "waiting for as much time as the pre-FLR value for
> Completion Timeout" part?
>
> Or can we do something like asserting FLR, sleeping 100ms, then
> attempting a write to something in config space and retrying until it
> sticks?  It's kludgy, but I'm not sure it's any worse than putting the
> retries in the restore path, and it would have the advantage that
> other writers of config space wouldn't have to worry.
>
> Bjorn

Depending on system config, reading a port that is being FLR'd
can cause AERs, which if a driver is registered for the endpoint,
it will get AERs reported to the driver and potentially complicate the
FLRhandling.

This implies a hook to temp-disable AER during FLR, then turning it
back on (hw &/or sw).

- Don

ps -- and there was a deadly embrace where an AER induced during
       an FLR that was initiated by userspace (libvirt writing to device
       reset file in sysfs) would lock up the system b/c the AER
       handler did a config space access, which used the same mutex.
       Not sure if that was cleaned up finally.... very corner-case-ish,
       but it shows how subtle multiple PCIe events can become complicated.

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