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Date: Mon, 29 Jul 2013 17:33:52 -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 Mon, May 27, 2013 at 9:15 PM, Yijing Wang <wangyijing@...wei.com> wrote:
> Hi Bjorn and Jon,
> I'm sorry to disturb you. This patch is sent so long, but nobody seems had comment about it.
> Do you have any comment with this patch?
>
> This patch try to update device mps in following case:
> 1) target device under root port
> Because root port can split TLP, so target device mps greatr than root port mps is ok.
> But if root port mps greater than target device mps, it's bad, because target device cannot
> receive TLP payload size greater than its MPS. So if a target device under a root port, I think
> we should assign its mps greater than or equal root port mps.
> 2) target device under non root port
> We assume the target device both is a transmitter and receiver, so the safest way is to assign target
> device mps equal to its parent device.
Thanks, I just started reviewing this patch, and your notes above are
exactly the question I was going to ask. The comments in
pcie_bus_update_set() only tell me what the code does. I can read the
C code just fine; what we need there is the explanation about *why* we
handle devices below root ports differently than others. Maybe we can
adapt some of your notes as comments in the code.
Do you have references to the spec where it talks about this
difference? I want to make sure we can rely on the fact that a root
port can accept TLPs larger than its MPS.
Bjorn
> On 2013/2/5 11:55, Yijing Wang wrote:
>> Currently we dont't update device's mps vaule 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.
>>
>> The reference discussion at
>> http://marc.info/?l=linux-pci&m=135420434508910&w=2
>> and
>> http://marc.info/?l=linux-pci&m=134815603407842&w=2
>>
>> Reported-by: Joe Jin <joe.jin@...cle.com>
>> Reported-by: Yijing Wang <wangyijing@...wei.com>
>> Signed-off-by: Yijing Wang <wangyijing@...wei.com>
>> Cc: Jon Mason <jdmason@...zu.us>
>> ---
>> drivers/pci/probe.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
>> 1 files changed, 49 insertions(+), 0 deletions(-)
>>
>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>> index bbe4be7..57d9a5b 100644
>> --- a/drivers/pci/probe.c
>> +++ b/drivers/pci/probe.c
>> @@ -1556,6 +1556,52 @@ static int pcie_bus_configure_set(struct pci_dev *dev, void *data)
>> return 0;
>> }
>>
>> +static int pcie_bus_update_set(struct pci_dev *dev, void *data)
>> +{
>> + int mps, p_mps;
>> +
>> + if (!pci_is_pcie(dev) || !dev->bus->self)
>> + return 0;
>> +
>> + mps = pcie_get_mps(dev);
>> + p_mps = pcie_get_mps(dev->bus->self);
>> +
>> + if (pci_pcie_type(dev->bus->self) != PCI_EXP_TYPE_ROOT_PORT) {
>> + /* update mps when current device mps is not equal to upstream mps */
>> + if (mps != p_mps)
>> + goto update;
>> + } else {
>> + /* update mps when current device mps is smaller than upstream mps */
>> + if (mps < p_mps)
>> + goto update;
>> + }
>> +
>> + return 0;
>> +
>> +update:
>> + /* If current mpss is lager than upstream, use upstream mps to update
>> + * current mps, otherwise print warning info.
>> + */
>> + if ((128 << dev->pcie_mpss) >= p_mps)
>> + pcie_write_mps(dev, p_mps);
>> + else
>> + dev_warn(&dev->dev, "MPS %d MPSS %d both smaller than upstream MPS %d\n"
>> + "If necessary, use \"pci=pcie_bus_peer2peer\" boot parameter to avoid this problem\n",
>> + mps, 128 << dev->pcie_mpss, p_mps);
>> + return 0;
>> +}
>> +
>> +static void pcie_bus_update_setting(struct pci_bus *bus)
>> +{
>> +
>> + /*
>> + * After hot added a pci device, the device's mps will set to default
>> + * vaule(128 bytes). But the upstream port mps may be larger than 128B.
>> + * In this case, we should update this device's mps for better performance.
>> + */
>> + pci_walk_bus(bus, pcie_bus_update_set, NULL);
>> +}
>> +
>> /* pcie_bus_configure_settings requires that pci_walk_bus work in a top-down,
>> * parents then children fashion. If this changes, then this code will not
>> * work as designed.
>> @@ -1566,6 +1612,9 @@ void pcie_bus_configure_settings(struct pci_bus *bus, u8 mpss)
>>
>> if (!pci_is_pcie(bus->self))
>> return;
>> +
>> + /* update mps setting for newly hot added device */
>> + pcie_bus_update_setting(bus);
>>
>> if (pcie_bus_config == PCIE_BUS_TUNE_OFF)
>> return;
>>
>
>
> --
> Thanks!
> Yijing
>
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