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

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On Monday, April 23, 2012, Bjorn Helgaas wrote:
> On Sun, Apr 15, 2012 at 1:40 PM, Rafael J. Wysocki <rjw@xxxxxxx> wrote:
> > On Sunday, April 15, 2012, Linus Torvalds wrote:
> >> On Sun, Apr 15, 2012 at 11:47 AM, Rafael J. Wysocki <rjw@xxxxxxx> 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@xxxxxxx>
> > 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@xxxxxxxxx>
> > Signed-off-by: Rafael J. Wysocki <rjw@xxxxxxx>
> > ---
> >  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
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