Re: [PATCH] PCI: update device mps when doing pci hotplug

[Keith Busch <keith.busch@intel.com>]

On Tue, 29 Jul 2014, Alex Williamson wrote:
> On Tue, 2014-07-29 at 10:30 -0600, Keith Busch wrote:
>> On Tue, 29 Jul 2014, Alex Williamson wrote:
>>> On Tue, 2014-07-29 at 16:17 +0800, Yijing Wang wrote:
>>>> Currently we don't update device's mps value when doing
>>>> pci device hot-add. The hot-added device's mps will be set
>>>> to default value (128B). But the upstream port device's mps
>>>> may be larger than 128B which was set by firmware during
>>>> system bootup. In this case the new added device may not
>>>> work normally.
>>>
>>> Apologies if we rehash some previously discussed topics while I try to
>>> cover for Bjorn while he's out.  By "normally", do you mean "optimally"?
>>> The device should be functional with a lower mps setting, right?
>>
>> You'd think so, but some platforms don't work. A pci-e trace showed
>> TLPs exceeding MPS when parent device at 256B and the end device left
>> at 128B. Even if that's a platform bug, I think we still want it to work.
>
> But if it's a platform bug for a non-compliant device, should it be
> handled as a quirk rather than standard configuration?  Thanks,

I'm not even sure it is a platform bug, but that was just a guess. The
way the devices are configured, it appears they behave inline with the
spec (from table 7-13):

   "
   Max_Payload_size -- This field sets maximum TLP payload size for the
   Function. As a Receiver, the Function must handle TLPs as large as the set
   value. As a Transmitter, the Function must not generate TLPs exceeding the
   set value.
   "

It sounds like it allows a transmitter to generate a TLP that the receiver
can't handle, but I don't know if that was the intent. :)