Re: [PATCH v7 1/3] PCI: AtomicOps: Do not enable requests by RCiEPs

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Mar 30, 2026 at 08:01:57PM -0400, Kuehling, Felix wrote:
> On 2026-03-30 17:42, Bjorn Helgaas wrote:
> > [+to amdgpu, bnxe_re, mlx5 IB, qedr, mlx5 maintainers]
> > 
> > On Mon, Mar 30, 2026 at 03:09:44PM +0200, Gerd Bayer wrote:
> > > Since root complex integrated end points (RCiEPs) attach to a bus that
> > > has no bridge device describing the root port, the capability to
> > > complete AtomicOps requests cannot be determined with PCIe methods.
> > > 
> > > Change default of pci_enable_atomic_ops_to_root() to not enable
> > > AtomicOps requests on RCiEPs.
> > I know I suggested this because there's nothing explicit that tells us
> > whether the RC supports atomic ops from RCiEPs [1].  But I'm concerned
> > that GPUs, infiniband HCAs, and NICs that use atomic ops may be
> > implemented as RCiEPs and would be broken by this.
> 
> FWIW, on AMD APUs our driver doesn't call pci_enable_atomic_ops_to_root. It
> just assumes that the GPU can do atomic accesses because it doesn't actually
> go through PCIe: https://elixir.bootlin.com/linux/v6.19.10/source/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c#L4785

What does this mean for the other branch that *does* use
pci_enable_atomic_ops_to_root()?  Can any of those devices be RCiEPs?

> > These drivers use pci_enable_atomic_ops_to_root():
> > 
> >    amdgpu
> >    bnxt_re (infiniband)
> >    mlx5 (infinband)
> >    qedr (infiniband)
> >    mlx5 (ethernet)
> > 
> > Maybe we should assume that because RCiEPs are directly integrated
> > into the RC, the RCiEP would only allow AtomicOp Requester Enable to
> > be set if the RC supports atomic ops?
> > 
> > I don't like making assumptions like that, but it'd be worse to break
> > these devices.
> > 
> > [1] https://lore.kernel.org/all/20260326164002.GA1325368@bhelgaas
> > 
> > > Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>
> > > ---
> > >   drivers/pci/pci.c | 5 ++---
> > >   1 file changed, 2 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > index 8479c2e1f74f1044416281aba11bf071ea89488a..135e5b591df405e87e7f520a618d7e2ccba55ce1 100644
> > > --- a/drivers/pci/pci.c
> > > +++ b/drivers/pci/pci.c
> > > @@ -3692,15 +3692,14 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
> > >   	/*
> > >   	 * Per PCIe r4.0, sec 6.15, endpoints and root ports may be
> > > -	 * AtomicOp requesters.  For now, we only support endpoints as
> > > -	 * requesters and root ports as completers.  No endpoints as
> > > +	 * AtomicOp requesters.  For now, we only support (legacy) endpoints
> > > +	 * as requesters and root ports as completers.  No endpoints as
> > >   	 * completers, and no peer-to-peer.
> > >   	 */
> > >   	switch (pci_pcie_type(dev)) {
> > >   	case PCI_EXP_TYPE_ENDPOINT:
> > >   	case PCI_EXP_TYPE_LEG_END:
> > > -	case PCI_EXP_TYPE_RC_END:
> > >   		break;
> > >   	default:
> > >   		return -EINVAL;
> > > 
> > > -- 
> > > 2.51.0
> > >