Re: [PATCH] PCI: handle pci_sriov_set_totalvfs(dev, 0)

[Edward Cree <ecree@solarflare.com>]

On 31/07/14 19:13, Edward Cree wrote:
> On 31/07/14 18:53, Don Dutile wrote:
>> Ideally, when the device is configured in different modes, the SRIOV
>> cap structure
>> should be modified so the pci sriov code doesn't try to act or reflect
>> the
>> non-reality the device is in.
> That would be much nicer of course, and I'll ask our firmware team if
> that's possible.  But I don't think it will be.
As I feared, it's not practical for the device to do this.
(Unfortunately, the IMEM (instruction memory) on the device is rather
small and the firmware team are struggling to fit all the required
features into it.)
However, I also found out that in principle, the device _is_ capable of
supporting VFs in PF-IOV mode, by attaching them directly to the
underlying v-switch.  The firmware may or may not already do this; the
person who's likely to know this isn't in today.  The driver at present
doesn't support it because it assumes it always needs to attach VFs to
its own v-switch, but it could (again, in principle) be taught otherwise.

So at this point it becomes a policy question of whether we want to
support this, and our opinion is that this isn't a valid use-case (as
the only time you'd want PF-IOV is if your system doesn't support VFs)
and thus shouldn't be supported (as it creates extra testing and
maintenance work).  The driver should probe the device, detect PF-IOV
mode, recognise that it (the driver) doesn't have support for VFs in
that mode, and set totalvfs to 0.
Others may demur, of course, but that's likely to be decided when
sending the driver patches to davem.  It's my belief that, uncertainty
about my use case notwithstanding, pci_sriov_set_totalvfs(dev, 0) should
be a valid operation with the semantics I've implemented in my patch.

-Edward
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