linux-kernel - Re: [Xen-devel] [PATCH V3 1/2] Drivers/PCI: Export pcie_has

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Message-ID: <20171213212420.GH30595@bhelgaas-glaptop.roam.corp.google.com>
Date:   Wed, 13 Dec 2017 15:24:21 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Govinda Tatti <Govinda.Tatti@...cle.COM>
Cc:     jgross@...e.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, JBeulich@...e.com,
        bhelgaas@...gle.com, xen-devel@...ts.xenproject.org,
        boris.ostrovsky@...cle.COM, roger.pau@...rix.com,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [Xen-devel] [PATCH V3 1/2] Drivers/PCI: Export pcie_has_flr()
 interface

[+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;
> >>>>  }
> >>>>+EXPORT_SYMBOL_GPL(pcie_has_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
> >>appropriate
> >>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
> pcie_flr().
> 
>         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
accordingly.

Christoph, chime in if I'm missing something here.

> 2. W.r.t pcie_flr(), I am planning to return error code. Currently,
> the following
>    file/modules are calling this function. My plan is to add a check
> for return
>    code and print a WANRING message if return code is NON-ZERO. I
> hope this is
>    sufficient for this patch.
> 
>    drivers/crypto/qat/qat_common/adf_aer.c
>    drivers/infiniband/hw/hfi1/chip.c (2 places)
>    drivers/net/ethernet/cavium/liquidio/lio_vf_main.c
>    drivers/net/ethernet/intel/ixgbe/ixgbe_main.c (2 places)
>    drivers/pci/quirks.c (2 places)

Checking the return code is probably overkill, since pcie_flr() is
void and returns no errors now.  The only point of the return value is
to tell whether the device supports FLR.  If we call it with "probe ==
0" there's no useful error to return.

Bjorn