Re: [PATCH v2 1/2] PCI: Configure root port MPS to hardware maximum during host probing

[Niklas Cassel <cassel@kernel.org>]

On Fri, Apr 25, 2025 at 05:57:07PM +0800, Hans Zhang wrote:
> Current PCIe initialization logic may leave root ports operating with
> non-optimal Maximum Payload Size (MPS) settings. While downstream device
> configuration is handled during bus enumeration, root port MPS values
> inherited from firmware or hardware defaults might not utilize the full
> capabilities supported by the controller hardware. This can result in
> suboptimal data transfer efficiency across the PCIe hierarchy.
> 
> During host controller probing phase, when PCIe bus tuning is enabled,
> the implementation now configures root port MPS settings to their
> hardware-supported maximum values. By iterating through bridge devices
> under the root bus and identifying PCIe root ports, each port's MPS is set
> to 128 << pcie_mpss to match the device's maximum supported payload size.
> The Max Read Request Size (MRRS) is subsequently adjusted through existing
> companion logic to maintain compatibility with PCIe specifications.
> 
> Explicit initialization at host probing stage ensures consistent PCIe
> topology configuration before downstream devices perform their own MPS
> negotiations. This proactive approach addresses platform-specific
> requirements where controller drivers depend on properly initialized root
> port settings, while maintaining backward compatibility through
> PCIE_BUS_TUNE_OFF conditional checks. Hardware capabilities are fully
> utilized without altering existing device negotiation behaviors.
> 
> Signed-off-by: Hans Zhang <18255117159@163.com>

Perhaps Mani deserves a Suggested-by tag?


> ---
>  drivers/pci/probe.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 364fa2a514f8..3973c593fdcf 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -3206,6 +3206,7 @@ EXPORT_SYMBOL_GPL(pci_create_root_bus);
>  int pci_host_probe(struct pci_host_bridge *bridge)
>  {
>  	struct pci_bus *bus, *child;
> +	struct pci_dev *dev;
>  	int ret;
>  
>  	pci_lock_rescan_remove();
> @@ -3228,6 +3229,17 @@ int pci_host_probe(struct pci_host_bridge *bridge)
>  	 */
>  	pci_assign_unassigned_root_bus_resources(bus);
>  
> +	if (pcie_bus_config != PCIE_BUS_TUNE_OFF) {
> +		/* Configure root ports MPS to be MPSS by default */
> +		for_each_pci_bridge(dev, bus) {
> +			if (pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)
> +				continue;
> +
> +			pcie_write_mps(dev, 128 << dev->pcie_mpss);
> +			pcie_write_mrrs(dev);

The comment says configure MPS, but the code also calls pcie_write_mrrs().

Should we update the comment or should we remove the call to pcie_write_mrrs()?

Note that even when calling pcie_write_mrrs(), it will not update MRRS for the
common case (pcie_bus_config == PCIE_BUS_DEFAULT).


Kind regards,
Niklas