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

[Bjorn Helgaas]

On Wed, Jul 31, 2013 at 3:15 AM, Yijing Wang <wangyijing@xxxxxxxxxx> wrote:
> Hi Bjorn,
>    I didn't observe a performance difference between MPS=128 and MPS=512. I use ping $dest_ip -s 65500(large size packet)
> to test the different situations.

Interesting.  "ping" is probably not a good way to see performance
differences, but hopefully you could see a difference in *some*
scenario.  Otherwise, there's not much point in increasing MPS :)

>> I assume there are no AER or other errors logged by the root port?
> Yes, AER is not support in local machine.

Per the 5520/5500 spec, it does support AER (sec 19.11.5).  Maybe
there's some platform support required in addition.  You might still
be able to see some info just with "lspci -vv"

> Hmmm, PCIe Spec does not involve too much about MPS setting. So maybe different platform
> has different strategy.

I think there's enough in the spec to tell us what we need to do (this
is sec 2.2.2):

  - A Transmitter must not send a TLP larger than its Max_Payload_Size
  - A Receiver must treat TLPs larger than its Max_Payload_Size as malformed

The only way I can see to guarantee that is to set the MPS on both
ends of the link the same.

> Conservatively, as a improvement for mps setting after hotplug. I think update mps setting equal to its parent
> make sense. This is no harm to other devices, we only modify the hotplug device itself mps register.
>
> So if you agree, I will update my patch ,only try to modify hotplug device mps, make them equal to its parent.

Yes, I think that would be safe.  If the switch is set to a larger MPS
than the hot-added device supports, I don't think we can safely use
the device.

Bjorn
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/