Re: [PATCH V11 7/8] PCI: Enable 10-Bit Tag support for PCIe Endpoint device

[Vidya Sagar <vidyas@nvidia.com>]

On 05/11/21 23:09, Bjorn Helgaas wrote:
> On Fri, Nov 05, 2021 at 04:24:24PM +0800, Dongdong Liu wrote:
>> On 2021/11/4 0:02, Bjorn Helgaas wrote:
>>> But it does remind me that if the RC doesn't support 10-bit tags, but
>>> we use sysfs to enable 10-bit tags for a reqester that intends to use
>>> P2PDMA to a peer that *does* support them, I don't think there's
>>> any check in the DMA API that prevents the driver from setting up DMA
>>> to the RC in addition to the peer.
>> Current we use sysfs to enable/disable 10-bit tags for a requester also
>> depend on the RP support 10-bit tag completer, so it will be ok.
> Ah, OK.  So we can never *enable* 10-bit tags unless the Root Port
> supports them.
>
> I misunderstood the purpose of this file.  When the Root Port doesn't
> support 10-bit tags, we won't enable them during enumeration.  I
> though the point was that if we want to do P2PDMA to a peer that
> *does* support them, we could use this file to enable them.
>
> But my understanding was wrong -- the real purpose of the file is to
> *disable* 10-bit tags for the case when a P2PDMA peer doesn't support
> them.
>
> It does support enabling 10-bit tags as well, but that's only because
> we need a way to get back to the default "enabled during enumeration"
> state without having to reboot.
>
> We might be able to highlight this a little more in the commit log.
>
>>> 10-bit tag support appeared in the spec four years ago (PCIe r4.0, in
>>> September, 2017).  Surely there is production hardware that supports
>>> this and could demonstrate a benefit from this.
>> I found the below introduction about "Number of tags needed to achieve
>> maximum throughput for PCIe 4.0 and PCIe 5.0 links"
>> https://www.synopsys.com/designware-ip/technical-bulletin/accelerating-32gtps-pcie5-designs.html
>>
>> It seems pretty clear.
> Yes, that's a start.  But we don't really need a white paper to tell
> us that more outstanding transactions is better.  That's obvious.  But
> this adds risk, and if we can't demonstrate a tangible, measurable
> benefit, there's no point in doing it.
>
> Bjorn
>
Hi Dongdong Liu,
Thanks for pushing this patch series.
I would like to know your plan to pursue this series in getting them merged.
Given that there is no action on this series since 2021, I would like to take it up if you don't have an imminent plan.
Please let me know your opinion on this.

Thanks & Regards,
Vidya Sagar