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Message-ID: <CAErSpo76egMNuVdE7er_DEXG5+Zpo-vAff8+TEQAzpuDhOmSaQ@mail.gmail.com>
Date: Tue, 24 Apr 2012 11:01:27 -0600
From: Bjorn Helgaas <bhelgaas@...gle.com>
To: Don Dutile <ddutile@...hat.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 Tue, Apr 24, 2012 at 10:03 AM, Don Dutile <ddutile@...hat.com> wrote:
> 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.
>
> 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.
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