Re: [PATCH v6 1/2] PCI: AtomicOps: Do not enable without support in root complex

Bjorn Helgaas
2026-03-25T20:08:37Z

On Wed, Mar 25, 2026 at 04:16:17PM +0100, Gerd Bayer wrote:
> When inspecting the config space of a Connect-X physical function in an
> s390 system after it was initialized by the mlx5_core device driver, we
> found the function to be enabled to request AtomicOps despite the
> system's root-complex lacking support for completing them:
>
> 1ed0:00:00.1 Ethernet controller: Mellanox Technologies MT2894 Family [ConnectX-6 Lx]
> Subsystem: Mellanox Technologies Device 0002
> [...]
> DevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis-
> AtomicOpsCtl: ReqEn+
> IDOReq- IDOCompl- LTR- EmergencyPowerReductionReq-
> 10BitTagReq- OBFF Disabled, EETLPPrefixBlk-
>
> Turns out the device driver calls pci_enable_atomic_ops_to_root() which
> defaulted to enable AtomicOps requests even if it had no information
> about the root-port that the PCIe device is attached to. Similarly,
> AtomicOps requests are enabled for root complex integrated endpoints
> (RCiEPs) unconditionally.
>
> Change the logic of pci_enable_atomic_ops_to_root() to fully traverse the
> PCIe tree upwards, check that the bridge devices support delivering
> AtomicOps transactions, and finally check that there is a root port at
> the end that does support completing AtomicOps - or that the support for
> completing AtomicOps at the root complex is announced through some other
> arch specific way.
>
> Introduce a new pcibios_connects_to_atomicops_capable_rc() function to
> implement the check - and default to always "true". This leaves the
> semantics for today's RCiEPs intact. Pass in the device in question and
> the requested capabilities for future expansions.
> For s390, override pcibios_connects_to_atomicops_capable_rc() to
> always return "false".
>
> Do not change the enablement of AtomicOps requests if there is no
> positive confirmation that the root complex can complete PCIe AtomicOps.
>
> Reported-by: Alexander Schmidt <alexs@linux.ibm.com>
> Cc: stable@vger.kernel.org
> Fixes: 430a23689dea ("PCI: Add pci_enable_atomic_ops_to_root()")
> Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>
> ---
> arch/s390/pci/pci.c | 5 +++++
> drivers/pci/pci.c | 48 +++++++++++++++++++++++++++++++-----------------
> include/linux/pci.h | 1 +
> 3 files changed, 37 insertions(+), 17 deletions(-)
>
> diff --git a/arch/s390/pci/pci.c b/arch/s390/pci/pci.c
> index 2a430722cbe415dd56c92fed2e513e524f46481a..a0bef77082a153a258fbe4abb1070b22e020888e 100644
> --- a/arch/s390/pci/pci.c
> +++ b/arch/s390/pci/pci.c
> @@ -265,6 +265,11 @@ static int zpci_cfg_store(struct zpci_dev *zdev, int offset, u32 val, u8 len)
> return rc;
> }
>
> +bool pcibios_connects_to_atomicops_capable_rc(struct pci_dev *dev, u32 cap_mask)
> +{
> + return false;
> +}
> +
> resource_size_t pcibios_align_resource(void *data, const struct resource *res,
> resource_size_t size,
> resource_size_t align)
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 8479c2e1f74f1044416281aba11bf071ea89488a..006aa589926cb290de43f152100ddaf9961407d1 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3660,6 +3660,19 @@ void pci_acs_init(struct pci_dev *dev)
> pci_disable_broken_acs_cap(dev);
> }
>
> +static bool pci_is_atomicops_capable_rp(struct pci_dev *dev, u32 cap, u32 cap_mask)
> +{
> + if (!dev || !(pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT))
> + return false;
> +
> + return (cap & cap_mask) == cap_mask;
> +}
> +
> +bool __weak pcibios_connects_to_atomicops_capable_rc(struct pci_dev *dev, u32 cap_mask)
> +{
> + return true;
> +}
> +
> /**
> * pci_enable_atomic_ops_to_root - enable AtomicOp requests to root port
> * @dev: the PCI device
> @@ -3676,8 +3689,9 @@ void pci_acs_init(struct pci_dev *dev)
> int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
> {
> struct pci_bus *bus = dev->bus;
> - struct pci_dev *bridge;
> - u32 cap, ctl2;
> + struct pci_dev *bridge = NULL;
> + u32 cap = 0;
> + u32 ctl2;
>
> /*
> * Per PCIe r5.0, sec 9.3.5.10, the AtomicOp Requester Enable bit
> @@ -3714,29 +3728,29 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
> switch (pci_pcie_type(bridge)) {
> /* Ensure switch ports support AtomicOp routing */
> case PCI_EXP_TYPE_UPSTREAM:
> - case PCI_EXP_TYPE_DOWNSTREAM:
> - if (!(cap & PCI_EXP_DEVCAP2_ATOMIC_ROUTE))
> - return -EINVAL;
> - break;
> -
> - /* Ensure root port supports all the sizes we care about */
> - case PCI_EXP_TYPE_ROOT_PORT:
> - if ((cap & cap_mask) != cap_mask)
> - return -EINVAL;
> - break;
> - }
> -
> - /* Ensure upstream ports don't block AtomicOps on egress */
> - if (pci_pcie_type(bridge) == PCI_EXP_TYPE_UPSTREAM) {
> + /* Upstream ports must not block AtomicOps on egress */
> pcie_capability_read_dword(bridge, PCI_EXP_DEVCTL2,
> &ctl2);
> if (ctl2 & PCI_EXP_DEVCTL2_ATOMIC_EGRESS_BLOCK)
> return -EINVAL;
> + fallthrough;
> + /* All switch ports need to route AtomicOps */
> + case PCI_EXP_TYPE_DOWNSTREAM:
> + if (!(cap & PCI_EXP_DEVCAP2_ATOMIC_ROUTE))
> + return -EINVAL;
> + break;
> }
> -
> bus = bus->parent;
> }
>
> + /*
> + * Finally, last bridge must be root port and support requested sizes
> + * or firmware asserts support
> + */
> + if (!(pci_is_atomicops_capable_rp(bridge, cap, cap_mask) ||
> + pcibios_connects_to_atomicops_capable_rc(dev, cap_mask)))
> + return -EINVAL;

Sashiko says:

Since the generic weak implementation of
pcibios_connects_to_atomicops_capable_rc() unconditionally returns
true, the logical OR expression pci_is_atomicops_capable_rp(...) ||
true will always evaluate to true. This makes the entire if
condition evaluate to false.

Because of this, it appears -EINVAL is never returned here, and any
standard endpoint behind a Root Port will successfully be granted
AtomicOps even if the Root Port lacks the capability in its
PCI_EXP_DEVCAP2 register.

> +
> pcie_capability_set_word(dev, PCI_EXP_DEVCTL2,
> PCI_EXP_DEVCTL2_ATOMIC_REQ);
> return 0;
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 1c270f1d512301de4d462fe7e5097c32af5c6f8d..ef90604c39859ea8e61e5392d0bdaa1b0e43874b 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -692,6 +692,7 @@ void pci_set_host_bridge_release(struct pci_host_bridge *bridge,
> void *release_data);
>
> int pcibios_root_bridge_prepare(struct pci_host_bridge *bridge);
> +bool pcibios_connects_to_atomicops_capable_rc(struct pci_dev *dev, u32 cap_mask);
>
> #define PCI_REGION_FLAG_MASK 0x0fU /* These bits of resource flags tell us the PCI region flags */
>
>
> --
> 2.51.0
>