On Thu, Oct 28, 2021 at 03:44:49PM +0800, Dongdong Liu wrote: > On 2021/10/28 6:28, Bjorn Helgaas wrote: > > On Sat, Oct 09, 2021 at 06:49:34PM +0800, Dongdong Liu wrote: > > > PCIe spec 5.0 r1.0 section 2.2.6.2 says: > > > > > > If an Endpoint supports sending Requests to other Endpoints (as > > > opposed to host memory), the Endpoint must not send 10-Bit Tag > > > Requests to another given Endpoint unless an implementation-specific > > > mechanism determines that the Endpoint supports 10-Bit Tag Completer > > > capability. > > > > > > Add a 10bit_tag sysfs file, write 0 to disable 10-Bit Tag Requester > > > when the driver does not bind the device. The typical use case is for > > > p2pdma when the peer device does not support 10-Bit Tag Completer. > > > Write 1 to enable 10-Bit Tag Requester when RC supports 10-Bit Tag > > > Completer capability. The typical use case is for host memory targeted > > > by DMA Requests. The 10bit_tag file content indicate current status of > > > 10-Bit Tag Requester Enable. > > > > Don't we have a hole here? We're adding knobs to control 10-Bit Tag > > usage, but don't we have basically the same issues with Extended > > (8-bit) Tags? > > All PCIe completers are required to support 8-bit tags > from the "[PATCH] PCI: enable extended tags support for PCIe endpoints" > (https://patchwork.kernel.org/project/linux-arm-msm/patch/1474769434-5756-1-git-send-email-okaya@xxxxxxxxxxxxxx/). > > I ask hardware colleagues, also says all PCIe devices should support > 8-bit tags completer default, so seems no need to do this for 8-bit tags. Oh, right, I forgot that, thanks for the reminder! Let's add a comment in pci_configure_extended_tags() to that effect so I'll remember next time. I think the appropriate reference is PCIe r5.0, sec 2.2.6.2, which says "Receivers/Completers must handle 8-bit Tag values correctly regardless of the setting of their Extended Tag Field Enable bit (see Section 7.5.3.4)." The Tag field was 8 bits all the way from PCIe r1.0, but until r2.1 it said that by default, only the lower 5 bits are used. The text about all Completers explicitly being required to support 8-bit Tags wasn't added until PCIe r3.0, which might explain some confusion and the presence of the Extended Tag Field Enable bit. At the same time, can you fold pci_configure_10bit_tags() directly into pci_configure_extended_tags()? It's pretty small and I think it will be easier if it's all in one place. > > I wonder if we should be adding a more general "tags" file that can > > manage both 8-bit and 10-bit tag usage. I'm still thinking that maybe a generic name (without "10") would be better, even though we don't need it to manage 8-bit tags. It's conceivable that there could be even more tag bits in the future, and it would be nice if we didn't have to add yet another file. Bjorn
