From: Sinan Kaya <okaya@codeaurora.org>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Ron Yuan <ron.yuan@memblaze.com>,
Bjorn Helgaas <bhelgaas@google.com>,
Bo Chen <bo.chen@memblaze.com>,
William Huang <william.huang@memblaze.com>,
Fengming Wu <fengming.wu@memblaze.com>,
Jason Jiang <jason.jiang@microsemi.com>,
Radjendirane Codandaramane <radjendirane.codanda@microsemi.com>,
Ramyakanth Edupuganti <Ramyakanth.Edupuganti@microsemi.com>,
William Cheng <william.cheng@microsemi.com>,
"Kim Helper (khelper)" <khelper@micron.com>,
Linux PCI <linux-pci@vger.kernel.org>
Subject: Re: One Question About PCIe BUS Config Type with pcie_bus_safe or pcie_bus_perf On NVMe Device
Date: Mon, 22 Jan 2018 18:24:22 -0500 [thread overview]
Message-ID: <00b97a2a-981d-f1c7-abcd-b5ee686c5568@codeaurora.org> (raw)
In-Reply-To: <20180122225127.GC5317@bhelgaas-glaptop.roam.corp.google.com>
On 1/22/2018 5:51 PM, Bjorn Helgaas wrote:
> On Mon, Jan 22, 2018 at 05:04:03PM -0500, Sinan Kaya wrote:
>> On 1/22/2018 4:36 PM, Bjorn Helgaas wrote:
>>> Reducing MPS may be necessary if there are several devices in the
>>> hierarchy and one requires a smaller MPS than the others. That
>>> obviously reduces the maximum read and write performance.
>>>
>>> Reducing the MRRS may be useful to prevent one device from hogging
>>> a link, but of course, it reduces read performance for that device
>>> because we need more read requests.
>>
>> Maybe, a picture could help.
>>
>> root (MPS=256)
>> |
>> ------------------
>> / \
>> bridge0 (MPS=256) bridge1 (MPS=128)
>> / \
>> EP0 (MPS=256) EP1 (MPS=128)
>>
>> If I understood this right, code allows the configuration above with
>> the performance mode so that MPS doesn't have to be uniform across
>> the tree.
>
> Yes. In PERFORMANCE mode, we will set EP1's MRRS=128 and
> EP0's MRRS=256, just as you show.
>
>> It just needs to be consistent between the root port and endpoints.
>
> No, it doesn't need to be consistent. In PERFORMANCE mode, we'll set
> the root's MPS=256 and EP1's MPS=128.
>
> (I'm not actually 100% convinced that the PERFORMANCE mode approach of
> reducing MRRS is safe, necessary, and maintainable. I suspect that in
> many of the interesting cases, the device we care about is the only
> one below a Root Port, and we can get the performance we need by
> maximizing MPS and MRRS for that Root Port and its children,
> independent of the rest of the system.)
Maybe, I started seeing more and more NVMe devices behind a switch every
day. That's why, I'm concerned.
>
>> Why are we reducing MRRS in this case?
>
> We have to set EP1's MRRS=128 so it will never receive a completion
> larger than 128 bytes. If we set EP1's MRRS=256, it could receive
> 256-byte TLPs, which it would treat as malformed. (We also assume no
> peer-to-peer DMA that targets EP1.)
What if we were to keep root port MPS as 128? and not touch the BIOS
configured MRRS (4k) ?
Everybody should be happy, right?
I know there is a rule to check the completions against MPS. Root port
could generate transactions that is a multiple of 128 bytes for reads.
Is there any rule against checking incoming writes?
>
>> Are we assuming that root bus cannot handle more than 256 bytes and
>> bridge1 would be starved while root bus is passing the completions
>> to bridge0?
>
> We don't have to assume. Every device tells us via Dev Cap what size
> TLPs it can handle. In your example, I assume the root's Dev Cap
> tells us it supports 256-byte TLPs.
>
> PERFORMANCE mode reduces MRRS not because of a starvation issue, but
> because reducing EP1's MRRS allows EP0 to use a larger MPS.
>
--
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
next prev parent reply other threads:[~2018-01-22 23:24 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <SH2PR01MB106A1E21DEB5FE3FFB3D61C83E90@SH2PR01MB106.CHNPR01.prod.partner.outlook.cn>
[not found] ` <ef16a3cc-b641-a30d-644a-7940e340eb3e@codeaurora.org>
[not found] ` <SHAPR01MB173A5EA1677C2138CB528F2FEE90@SHAPR01MB173.CHNPR01.prod.partner.outlook.cn>
[not found] ` <5727b0b1-f0d5-7c78-373e-fc9d1bd662d2@codeaurora.org>
[not found] ` <CABhMZUU0643U-qVc9ymA+1PMZToSLFm2dg8-cu=iQ2LGw2Pi8Q@mail.gmail.com>
[not found] ` <SHAPR01MB173A36104635A8BFF9A83E1FEE80@SHAPR01MB173.CHNPR01.prod.partner.outlook.cn>
2018-01-18 16:24 ` One Question About PCIe BUS Config Type with pcie_bus_safe or pcie_bus_perf On NVMe Device Sinan Kaya
2018-01-19 20:51 ` Bjorn Helgaas
2018-01-20 19:20 ` Sinan Kaya
2018-01-20 19:29 ` Sinan Kaya
2018-01-22 21:36 ` Bjorn Helgaas
2018-01-22 22:04 ` Sinan Kaya
2018-01-22 22:51 ` Bjorn Helgaas
2018-01-22 23:24 ` Sinan Kaya [this message]
2018-01-23 0:16 ` Bjorn Helgaas
2018-01-23 2:27 ` Sinan Kaya
2018-01-23 13:25 ` Ron Yuan
2018-01-23 14:01 ` Ron Yuan
2018-01-23 17:48 ` Bjorn Helgaas
2018-01-23 18:07 ` Bjorn Helgaas
2018-01-23 14:38 ` Bjorn Helgaas
2018-01-23 23:50 ` Radjendirane Codandaramane
2018-01-24 16:29 ` Myron Stowe
2018-01-24 17:59 ` Ron Yuan
2018-01-24 18:01 ` Bjorn Helgaas
2018-01-31 8:40 ` Ron Yuan
2018-02-01 0:01 ` Myron Stowe
2018-02-01 0:13 ` Sinan Kaya
2018-02-01 3:37 ` Bjorn Helgaas
2018-02-01 15:14 ` Sinan Kaya
2018-02-05 1:02 ` Sinan Kaya
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