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From: Dongdong Liu <liudongdong3@huawei.com>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: <hch@infradead.org>, <kw@linux.com>, <logang@deltatee.com>,
	<leon@kernel.org>, <linux-pci@vger.kernel.org>,
	<rajur@chelsio.com>, <hverkuil-cisco@xs4all.nl>,
	<linux-media@vger.kernel.org>, <netdev@vger.kernel.org>
Subject: Re: [PATCH V7 4/9] PCI: Enable 10-Bit Tag support for PCIe Endpoint devices
Date: Sat, 7 Aug 2021 14:19:10 +0800	[thread overview]
Message-ID: <c7fdb77d-8b93-f3ef-05d2-54daf67305e2@huawei.com> (raw)
In-Reply-To: <20210805195407.GA1763784@bjorn-Precision-5520>



On 2021/8/6 3:54, Bjorn Helgaas wrote:
> On Thu, Aug 05, 2021 at 03:47:31PM +0800, Dongdong Liu wrote:
>> Hi Bjorn
>>
>> Many thanks for your review.
>> On 2021/8/5 7:17, Bjorn Helgaas wrote:
>>> On Wed, Aug 04, 2021 at 09:47:03PM +0800, Dongdong Liu wrote:
>>>> 10-Bit Tag capability, introduced in PCIe-4.0 increases the total Tag
>>>> field size from 8 bits to 10 bits.
>>>>
>>>> PCIe spec 5.0 r1.0 section 2.2.6.2 "Considerations for Implementing
>>>> 10-Bit Tag Capabilities" Implementation Note.
>>>> For platforms where the RC supports 10-Bit Tag Completer capability,
>>>> it is highly recommended for platform firmware or operating software
>>>> that configures PCIe hierarchies to Set the 10-Bit Tag Requester Enable
>>>> bit automatically in Endpoints with 10-Bit Tag Requester capability. This
>>>> enables the important class of 10-Bit Tag capable adapters that send
>>>> Memory Read Requests only to host memory.
>>>
>>> Quoted material should be set off with a blank line before it and
>>> indented by two spaces so it's clear exactly what comes from the spec
>>> and what you've added.  For example, see
>>> https://git.kernel.org/linus/ec411e02b7a2
>> Good point, will fix.
>>>
>>> We need to say why we assume it's safe to enable 10-bit tags for all
>>> devices below a Root Port that supports them.  I think this has to do
>>> with switches being required to forward 10-bit tags correctly even if
>>> they were designed before 10-bit tags were added to the spec.
>>
>> PCIe spec 5.0 r1.0 section 2.2.6.2 "Considerations for Implementing
>> 10-Bit Tag Capabilities" Implementation Note:
>>
>>   Switches that lack 10-Bit Tag Completer capability are still able to
>>   forward NPRs and Completions carrying 10-Bit Tags correctly, since the
>>   two new Tag bits are in TLP Header bits that were formerly Reserved,
>>   and Switches are required to forward Reserved TLP Header bits without
>>   modification. However, if such a Switch detects an error with an NPR
>>   carrying a 10-Bit Tag, and that Switch handles the error by acting as
>>   the Completer for the NPR, the resulting Completion will have an
>>   invalid 10-Bit Tag. Thus, it is strongly recommended that Switches
>>   between any components using 10-Bit Tags support 10-Bit Tag Completer
>>   capability.  Note that Switches supporting 16.0 GT/s data rates or
>>   greater must support 10-Bit Tag Completer capability.
>>
>> This patch also consider to enable 10-Bit Tag for EP device need RP
>> and Switch device support 10-Bit Tag Completer capability.
>>>
>>> And it should call out any cases where it is *not* safe, e.g., if P2P
>>> traffic is an issue.
>> Yes, indeed.
>>>
>>> If there are cases where we don't want to enable 10-bit tags, whether
>>> it's to enable P2P traffic or merely to work around device defects,
>>> that ability needs to be here from the beginning.  If somebody needs
>>> to bisect with 10-bit tags disabled, we don't want a bisection hole
>>> between this commit and the commit that adds the control.
>> We provide sysfs file to disable 10-bit tag for P2P traffic when needed.
>> The details see PATCH 7/8/9.
>
> A mechanism for avoiding problems needs to be present from the very
> beginning so there's no bisection hole.  It should not be added by a
> future patch.
Yes, will adjust PATCH 7/8/9 before PATCH 4。
>
> The sysfs file is a start, but if we run into an issue, it could mean
> that we can't boot and run long enough to use sysfs to disable 10-bit
> tags.  So I think we might need a kernel parameter that disables it
> (and possibly other things like MPS optimization).

Yes, We can add a pcie_tag_p2p kernel parameter just to use the 8-bit
tags, not to enable 10-bit tags for all PCIe devices.

Thanks,
Dongdong


  reply	other threads:[~2021-08-07  6:19 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-04 13:46 [PATCH V7 0/9] PCI: Enable 10-Bit tag support for PCIe devices Dongdong Liu
2021-08-04 13:47 ` [PATCH V7 1/9] PCI: Use cached Device Capabilities Register Dongdong Liu
2021-08-04 13:47 ` [PATCH V7 2/9] PCI: Use cached Device Capabilities 2 Register Dongdong Liu
2021-08-04 13:47 ` [PATCH V7 3/9] PCI: Add 10-Bit Tag register definitions Dongdong Liu
2021-08-04 13:47 ` [PATCH V7 4/9] PCI: Enable 10-Bit Tag support for PCIe Endpoint devices Dongdong Liu
2021-08-04 23:17   ` Bjorn Helgaas
2021-08-05  7:47     ` Dongdong Liu
2021-08-05 19:54       ` Bjorn Helgaas
2021-08-07  6:19         ` Dongdong Liu [this message]
2021-08-04 13:47 ` [PATCH V7 5/9] PCI/IOV: Enable 10-Bit tag support for PCIe VF devices Dongdong Liu
2021-08-04 23:29   ` Bjorn Helgaas
2021-08-05  8:03     ` Dongdong Liu
2021-08-06 22:59       ` Bjorn Helgaas
2021-08-07  7:46         ` Dongdong Liu
2021-08-04 13:47 ` [PATCH V7 6/9] PCI: Enable 10-Bit Tag support for PCIe RP devices Dongdong Liu
2021-08-04 23:38   ` Bjorn Helgaas
2021-08-05  8:25     ` Dongdong Liu
2021-08-09 17:26       ` Bjorn Helgaas
2021-08-10 11:59         ` Dongdong Liu
2021-08-04 13:47 ` [PATCH V7 7/9] PCI/sysfs: Add a 10-Bit Tag sysfs file Dongdong Liu
2021-08-04 15:51   ` Logan Gunthorpe
2021-08-05 13:14     ` Dongdong Liu
2021-08-05 13:53       ` Leon Romanovsky
2021-08-05 15:36       ` Logan Gunthorpe
2021-08-04 23:49   ` Bjorn Helgaas
2021-08-05  8:37     ` Dongdong Liu
2021-08-05 15:31       ` Bjorn Helgaas
2021-08-07  7:01         ` Dongdong Liu
2021-08-09 17:37           ` Bjorn Helgaas
2021-08-10 12:16             ` Dongdong Liu
2021-08-04 23:52   ` Bjorn Helgaas
2021-08-05  8:38     ` Dongdong Liu
2021-08-04 13:47 ` [PATCH V7 8/9] PCI/IOV: Add 10-Bit Tag sysfs files for VF devices Dongdong Liu
2021-08-05  0:05   ` Bjorn Helgaas
2021-08-05  8:47     ` Dongdong Liu
2021-08-05  9:39     ` Dongdong Liu
2021-08-04 13:47 ` [PATCH V7 9/9] PCI/P2PDMA: Add a 10-Bit Tag check in P2PDMA Dongdong Liu
2021-08-04 15:56   ` Logan Gunthorpe
2021-08-05  8:49     ` Dongdong Liu
2021-08-05 18:12   ` Bjorn Helgaas
2021-08-07  7:11     ` Dongdong Liu
2021-08-09 17:31       ` Bjorn Helgaas
2021-08-10 12:31         ` Dongdong Liu

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