Re: [PATCH] PCI: Fix regression in pci_restore_state()

[Don Dutile <ddutile@redhat.com>]

On 04/24/2012 01:01 PM, Bjorn Helgaas wrote:
> On Tue, Apr 24, 2012 at 10:03 AM, Don Dutile<ddutile@redhat.com>  wrote:
>> On 04/23/2012 06:33 PM, Bjorn Helgaas wrote:
>>>
>>> On Mon, Apr 23, 2012 at 2:07 PM, Don Dutile<ddutile@redhat.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@sisk.pl>
>>>>>>   wrote:
>>>>>>>
>>>>>>>
>>>>>>> On Sunday, April 15, 2012, Linus Torvalds wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On Sun, Apr 15, 2012 at 11:47 AM, Rafael J. Wysocki<rjw@sisk.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@sisk.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@yahoo.com>
>>>>>>> Signed-off-by: Rafael J. Wysocki<rjw@sisk.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.
>>
>> 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't we have the same possibility of causing AERs if we add this
> retry stuff in pci_restore_state()?  It sounds like the same
> possibility exists regardless of whether the config access happens in
> the FLR path or the restore path.


Dont know; have to re-read thread on where/when pci_restore_state() is recalled
wrt link synch/enable state(s).