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

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Message-ID: <CAErSpo5GUTXyo4GeCGvh_GauKUA4decbDWUnD47qKPr6E65g9w@mail.gmail.com>
Date:	Wed, 31 Jul 2013 11:53:30 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Yijing Wang <wangyijing@...wei.com>
Cc:	Jon Mason <jdmason@...zu.us>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Yijing Wang <wangyijing0307@...il.com>,
	Hanjun Guo <guohanjun@...wei.com>,
	Jiang Liu <jiang.liu@...wei.com>, Joe Jin <joe.jin@...cle.com>
Subject: Re: [PATCH] PCI: update device mps when doing pci hotplug

On Wed, Jul 31, 2013 at 3:15 AM, Yijing Wang <wangyijing@...wei.com> 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
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