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

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Date:   Fri, 15 Dec 2017 09:48:02 -0600
From:   Govinda Tatti <Govinda.Tatti@...cle.COM>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        Christoph Hellwig <hch@...radead.org>
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
Subject: 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;
>>>>>>   }
>>>>>> +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.
Not all code paths are aware of FLR capability and also, not
using pcie_flr().  For example,

arch/powerpc/platforms/powernv/eeh-powernv.c
drivers/crypto/cavium/nitrox/nitrox_main.c
drivers/net/ethernet/cavium/liquidio/octeon_mailbox.c

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.

Cheers
GOVINDA