linux-kernel - Re: [PATCH v3 3/3] PCI/bwctrl: Replace legacy speed conversion with shared macro

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Message-ID: <aKDmW6l6uxZGr1Wl@wunner.de>
Date: Sat, 16 Aug 2025 22:13:15 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Hans Zhang <18255117159@....com>
Cc: bhelgaas@...gle.com, lpieralisi@...nel.org, kw@...ux.com,
	kwilczynski@...nel.org, mani@...nel.org,
	ilpo.jarvinen@...ux.intel.com, jingoohan1@...il.com,
	robh@...nel.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] PCI/bwctrl: Replace legacy speed conversion with
 shared macro

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