On Sat, Aug 16, 2025 at 11:46:33PM +0800, Hans Zhang wrote:
> Remove obsolete pci_bus_speed2lnkctl2() function and utilize the common
> PCIE_SPEED2LNKCTL2_TLS() macro instead.
[...]
> +++ b/drivers/pci/pcie/bwctrl.c
> @@ -53,23 +53,6 @@ static bool pcie_valid_speed(enum pci_bus_speed speed)
> return (speed >= PCIE_SPEED_2_5GT) && (speed <= PCIE_SPEED_64_0GT);
> }
>
> -static u16 pci_bus_speed2lnkctl2(enum pci_bus_speed speed)
> -{
> - static const u8 speed_conv[] = {
> - [PCIE_SPEED_2_5GT] = PCI_EXP_LNKCTL2_TLS_2_5GT,
> - [PCIE_SPEED_5_0GT] = PCI_EXP_LNKCTL2_TLS_5_0GT,
> - [PCIE_SPEED_8_0GT] = PCI_EXP_LNKCTL2_TLS_8_0GT,
> - [PCIE_SPEED_16_0GT] = PCI_EXP_LNKCTL2_TLS_16_0GT,
> - [PCIE_SPEED_32_0GT] = PCI_EXP_LNKCTL2_TLS_32_0GT,
> - [PCIE_SPEED_64_0GT] = PCI_EXP_LNKCTL2_TLS_64_0GT,
> - };
> -
> - if (WARN_ON_ONCE(!pcie_valid_speed(speed)))
> - return 0;
> -
> - return speed_conv[speed];
> -}
> -
> static inline u16 pcie_supported_speeds2target_speed(u8 supported_speeds)
> {
> return __fls(supported_speeds);
> @@ -91,7 +74,7 @@ static u16 pcie_bwctrl_select_speed(struct pci_dev *port, enum pci_bus_speed spe
> u8 desired_speeds, supported_speeds;
> struct pci_dev *dev;
>
> - desired_speeds = GENMASK(pci_bus_speed2lnkctl2(speed_req),
> + desired_speeds = GENMASK(PCIE_SPEED2LNKCTL2_TLS(speed_req),
> __fls(PCI_EXP_LNKCAP2_SLS_2_5GB));
No, that's not good. The function you're removing above,
pci_bus_speed2lnkctl2(), uses an array to look up the speed.
That's an O(1) operation, it doesn't get any more efficient
than that. It was a deliberate design decision to do this
when the bandwidth controller was created.
Whereas the function you're using instead uses a series
of ternary operators. That's no longer an O(1) operation,
the compiler translates it into a series of conditional
branches, so essentially an O(n) lookup (where n is the
number of speeds). So it's less efficient and less elegant.
Please come up with an approach that doesn't make this
worse than before.
Thanks,
Lukas
