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

[Lukas Wunner <lukas@wunner.de>]

On Sat, Jan 24, 2026 at 02:35:12PM +0000, Vidya Sagar wrote:
> On 05/11/21 23:09, Bjorn Helgaas wrote:
> > On Fri, Nov 05, 2021 at 04:24:24PM +0800, Dongdong Liu wrote:
> > > 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.
[...]
> 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.

Smita (+cc) took another stab at 10-bit tag support in 2023:

https://lore.kernel.org/all/20230828095429.GA17864@wunner.de/

The approach proposed by Dongdong Liu -- to disable 10-bit tag support
via sysfs -- doesn't seem right.  The kernel has all the information
it needs to automatically determine when to enable or disable 10-bit tags.
User space has no business overriding that.  There is an in-kernel API
to set up P2PDMA and it can automatically disable or re-enable 10-bit tags
when it is appropriate.  I think the only reason why a sysfs interface
would be needed is to support setting up P2PDMA behind the kernel's back
for proprietary technologies such as Nvidia GPU Direct.  I hereby
preemptively NAK patches that add a sysfs interface for this purpose.
Please help improve the existing in-kernel P2PDMA support instead
by plumbing 10-bit tag awareness into it.

Thanks,

Lukas