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

[Hans Zhang <18255117159@163.com>]

On 2025/8/17 04:13, Lukas Wunner wrote:
> 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.


Dear Lukas,

Thank you very much for your reply.

I think the original static array will waste some memory space. 
Originally, we only needed a size of 6 bytes, but in reality, the size 
of this array is 26 bytes.

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,
};



What do you think if the first patch is modified as follows?


diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 34f65d69662e..d6c3333315a0 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -422,6 +422,28 @@ void pci_bus_put(struct pci_bus *bus);
          PCI_SPEED_UNKNOWN);                                            \
  })

+static inline u16 pcie_speed_to_lnkctl2_tls(enum pci_bus_speed speed)
+{
+       /*
+        * Convert PCIe speed enum to LNKCTL2_TLS value using
+        * direct arithmetic:
+        *
+        * Speed enum:  0x14 (2.5GT) -> TLS = 0x1
+        *              0x15 (5.0GT) -> TLS = 0x2
+        *              0x16 (8.0GT) -> TLS = 0x3
+        *              0x17 (16.0GT)-> TLS = 0x4
+        *              0x18 (32.0GT)-> TLS = 0x5
+        *              0x19 (64.0GT)-> TLS = 0x6
+        *
+        * Formula: TLS = (speed - PCIE_SPEED_2_5GT) + 1
+        */
+       if (!WARN_ON_ONCE(speed >= PCIE_SPEED_2_5GT ||
+                         speed <= PCIE_SPEED_64_0GT))
+               return 0;
+
+       return (speed - PCIE_SPEED_2_5GT) + PCI_EXP_LNKCTL2_TLS_2_5GT;
+}
+
  /* PCIe speed to Mb/s reduced by encoding overhead */
  #define PCIE_SPEED2MBS_ENC(speed) \
         ((speed) == PCIE_SPEED_64_0GT ? 64000*1/1 : \







If you think the above plan is feasible. Then, should all the following 
macro definitions be changed to inline functions?

drivers/pci/pci.h
#define PCIE_LNKCAP_SLS2SPEED(lnkcap)					\
({									\
	u32 lnkcap_sls = (lnkcap) & PCI_EXP_LNKCAP_SLS;			\
									\
	(lnkcap_sls == PCI_EXP_LNKCAP_SLS_64_0GB ? PCIE_SPEED_64_0GT :	\
	 lnkcap_sls == PCI_EXP_LNKCAP_SLS_32_0GB ? PCIE_SPEED_32_0GT :	\
	 lnkcap_sls == PCI_EXP_LNKCAP_SLS_16_0GB ? PCIE_SPEED_16_0GT :	\
	 lnkcap_sls == PCI_EXP_LNKCAP_SLS_8_0GB ? PCIE_SPEED_8_0GT :	\
	 lnkcap_sls == PCI_EXP_LNKCAP_SLS_5_0GB ? PCIE_SPEED_5_0GT :	\
	 lnkcap_sls == PCI_EXP_LNKCAP_SLS_2_5GB ? PCIE_SPEED_2_5GT :	\
	 PCI_SPEED_UNKNOWN);						\
})

/* PCIe link information from Link Capabilities 2 */
#define PCIE_LNKCAP2_SLS2SPEED(lnkcap2) \
	((lnkcap2) & PCI_EXP_LNKCAP2_SLS_64_0GB ? PCIE_SPEED_64_0GT : \
	 (lnkcap2) & PCI_EXP_LNKCAP2_SLS_32_0GB ? PCIE_SPEED_32_0GT : \
	 (lnkcap2) & PCI_EXP_LNKCAP2_SLS_16_0GB ? PCIE_SPEED_16_0GT : \
	 (lnkcap2) & PCI_EXP_LNKCAP2_SLS_8_0GB ? PCIE_SPEED_8_0GT : \
	 (lnkcap2) & PCI_EXP_LNKCAP2_SLS_5_0GB ? PCIE_SPEED_5_0GT : \
	 (lnkcap2) & PCI_EXP_LNKCAP2_SLS_2_5GB ? PCIE_SPEED_2_5GT : \
	 PCI_SPEED_UNKNOWN)

#define PCIE_LNKCTL2_TLS2SPEED(lnkctl2) \
({									\
	u16 lnkctl2_tls = (lnkctl2) & PCI_EXP_LNKCTL2_TLS;		\
									\
	(lnkctl2_tls == PCI_EXP_LNKCTL2_TLS_64_0GT ? PCIE_SPEED_64_0GT :	\
	 lnkctl2_tls == PCI_EXP_LNKCTL2_TLS_32_0GT ? PCIE_SPEED_32_0GT :	\
	 lnkctl2_tls == PCI_EXP_LNKCTL2_TLS_16_0GT ? PCIE_SPEED_16_0GT :	\
	 lnkctl2_tls == PCI_EXP_LNKCTL2_TLS_8_0GT ? PCIE_SPEED_8_0GT :	\
	 lnkctl2_tls == PCI_EXP_LNKCTL2_TLS_5_0GT ? PCIE_SPEED_5_0GT :	\
	 lnkctl2_tls == PCI_EXP_LNKCTL2_TLS_2_5GT ? PCIE_SPEED_2_5GT :	\
	 PCI_SPEED_UNKNOWN);						\
})




Best regards,
Hans