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Re: [Xen-devel] [PATCH V3 1/2] Drivers/PCI: Export pcie_has_flr() interface

Thanks Bjorn and Christophfor your response. Please see below for my comments.

On 12/13/2017 3:24 PM, Bjorn Helgaas wrote:
[+cc Christoph]

On Wed, Dec 13, 2017 at 02:46:57PM -0600, Govinda Tatti wrote:
-static bool pcie_has_flr(struct pci_dev *dev)
+bool pcie_has_flr(struct pci_dev *dev)
        u32 cap;
@@ -3882,6 +3882,7 @@ static bool pcie_has_flr(struct pci_dev *dev)
        pcie_capability_read_dword(dev, PCI_EXP_DEVCAP, &cap);
        return cap & PCI_EXP_DEVCAP_FLR;
I'd rather change pcie_flr() so you could *always* call it, and it
would return 0, -ENOTTY, or whatever, based on whether FLR is
supported.  Is that feasible?
Sure, I will add pcie_has_flr() logic inside pcie_flr() and return
values as suggested by you. Do we still want to retain pcie_has_flr() and
its usage inside pci.c?.Otherwise, I will remove it and do required cleanup.
If you can restructure the code and remove pcie_has_flr() while
retaining the existing behavior of its callers, that would be great.
I checked the current usage of pcie_has_flr() and pcie_flr(). I have
a couple
of questions or need some clarification.

1. pcie_has_flr() usage inside pci_probe_reset_function().

    This function is only calling pcie_has_flr() but not pcie_flr().
    Rest of the code is trying to do specific type of reset except

         rc = pci_dev_specific_reset(dev, 1);
         if (rc != -ENOTTY)
                 return rc;
         if (pcie_has_flr(dev))
                 return 0;
         rc = pci_af_flr(dev, 1);
         if (rc != -ENOTTY)
                 return rc;

    In other-words, I can remove usage of pcie_has_flr() in all other places
    in pci.c except in above function.
I think we should keep the EXPORT_SYMBOL_GPL() part of a60a2b73ba69
("PCI: Export pcie_flr()"), but revert the restructuring part.

Prior to a60a2b73ba69, we had

   int pcie_flr(struct pci_dev *dev, int probe);

like all the other reset methods.  AFAICT, the addition of
pcie_has_flr() was to optimize the path slightly because when drivers
call pcie_flr(), they should already know that their hardware supports
FLR.  But I don't think that optimization is worth the extra code
complexity.  If we do need to optimize it, we can check this in the
core during enumeration and set PCI_DEV_FLAGS_NO_FLR_RESET

Christoph, chime in if I'm missing something here.
Not all code paths are aware of FLR capability and also, not
using pcie_flr().  For example,


So, we should consider one of these options.

- set PCI_DEV_FLAGS_NO_FLR_RESET if it is not supported.
- pcie_flr() should return if it is not supported

If we modify pcie_flr() to return error codes, then we need to modify
all existing modules that are calling this function.

Please let me know your preference, so that I can move accordingly. Thanks.


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