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
