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

[Don Dutile <ddutile@redhat.com>]

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.

> --
> To unsubscribe from this list: send the line "unsubscribe linux-pci" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html