RE: [PATCH v2 2/3] PCI: Create new reset method to force SBR for CXL

Dan Williams
2024-03-28T01:53:26Z

Dave Jiang wrote:
> CXL spec r3.1 8.1.5.2
> By default Secondary Bus Reset (SBR) is masked for CXL ports. Introduce a
> new PCI reset method "cxl_bus_force" to force SBR on CXL ports by setting
> the unmask SBR bit in the CXL DVSEC port control register before performing
> the bus reset and restore the original value of the bit post reset. The
> new reset method allows the user to intentionally perform SBR on a CXL
> device without needing to set the "Unmask SBR" bit via a user tool.
>
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> ---
> v2:
> - Use pci_upstream_bridge() instead of dev->bus->self.
> - Return -ENOTTY as error for reset function
> ---
> drivers/pci/pci.c | 52 +++++++++++++++++++++++++++++++++++++++++++--
> include/linux/pci.h | 2 +-
> 2 files changed, 51 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 259e5d6538bb..cbcad8f0880d 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -4933,6 +4933,12 @@ static bool pci_is_cxl(struct pci_dev *dev)
> CXL_DVSEC_PCIE_DEVICE);
> }
>
> +static int cxl_port_dvsec(struct pci_dev *dev)
> +{
> + return pci_find_dvsec_capability(dev, PCI_DVSEC_VENDOR_ID_CXL,
> + CXL_DVSEC_PCIE_PORT);
> +}
> +
> static bool is_cxl_port_sbr_masked(struct pci_dev *dev)
> {
> int dvsec;
> @@ -4942,8 +4948,7 @@ static bool is_cxl_port_sbr_masked(struct pci_dev *dev)
> /*
> * No DVSEC found, must not be CXL port.
> */
> - dvsec = pci_find_dvsec_capability(dev, PCI_DVSEC_VENDOR_ID_CXL,
> - CXL_DVSEC_PCIE_PORT);

Once applied, those 2 lines had a very short life in mainline. Perhaps
just define cxl_port_dvsec() in patch1?

> + dvsec = cxl_port_dvsec(dev);
> if (!dvsec)
> return false;
>
> @@ -4982,6 +4987,48 @@ static int pci_reset_bus_function(struct pci_dev *dev, bool probe)
> return pci_parent_bus_reset(dev, probe);
> }
>
> +static int cxl_reset_bus_function(struct pci_dev *dev, bool probe)
> +{
> + struct pci_dev *bridge;
> + int dvsec;
> + int rc;
> + u16 reg, val;
> +
> + if (!pci_is_cxl(dev))
> + return -ENOTTY;
> +
> + bridge = pci_upstream_bridge(dev);
> + if (!bridge)
> + return -ENOTTY;
> +
> + dvsec = cxl_port_dvsec(bridge);
> + if (!dvsec)
> + return -ENOTTY;
> +
> + if (probe)
> + return 0;
> +
> + rc = pci_read_config_word(bridge, dvsec + CXL_DVSEC_PORT_CONTROL,
> + &reg);
> + if (rc)
> + return -ENOTTY;
> +
> + if (!(reg & CXL_DVSEC_PORT_CONTROL_UNMASK_SBR)) {
> + val = reg | CXL_DVSEC_PORT_CONTROL_UNMASK_SBR;
> + pci_write_config_word(bridge,
> + dvsec + CXL_DVSEC_PORT_CONTROL, val);
> + } else {
> + val = reg;
> + }
> +
> + rc = pci_reset_bus_function(dev, probe);
> +
> + if (reg != val)
> + pci_write_config_word(bridge, dvsec + CXL_DVSEC_PORT_CONTROL, reg);

Doesn't this whole sequence need to be wrapped in pci_cfg_access_lock()?
Otherwise userspace can get confused if it races to access
CXL_DVSEC_PCIE_PORT while the link is down, or if it races to write
Unmask SBR and messes up the saved value.

I took a quick look and did not see this lock taken from
reset_method_store().

> +
> + return rc;
> +}
> +
> void pci_dev_lock(struct pci_dev *dev)
> {
> /* block PM suspend, driver probe, etc. */
> @@ -5066,6 +5113,7 @@ static const struct pci_reset_fn_method pci_reset_fn_methods[] = {
> { pci_af_flr, .name = "af_flr" },
> { pci_pm_reset, .name = "pm" },
> { pci_reset_bus_function, .name = "bus" },
> + { cxl_reset_bus_function, .name = "cxl_bus_force" },

Why include "_force" in the name? "cxl_bus" already implies "do what is
needed to bus reset this CXL link".