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

[Hans Zhang <hans.zhang@cixtech.com>]

On 2025/8/18 13:21, Manivannan Sadhasivam wrote:
> EXTERNAL EMAIL
> 
> On Sun, Aug 17, 2025 at 11:02:10PM GMT, Hans Zhang wrote:
>>
>>
>> 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.
>>
> 
> This is just one time allocation as the array is a 'static const', which is not
> a big deal.
> 
>> 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,
>> };
> 
> [...]
> 
>> 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);                                            \
>> })
> 
> No, these macros are terrible. They generate more assembly code than needed for
> a simple array based lookup. So in the end, they increase the binary size and
> also doesn't provide any improvement other than the unification in the textual
> form.
> 
> I have to take my Acked-by back as I sort of overlooked these factors. As Lukas
> rightly said, the pci_bus_speed2lnkctl2() does lookup in O(1), which is what we
> want here.
> 
> Code refactoring shouldn't come at the expense of the runtime overhead.
> 

Dear Mani,

Thank you very much for your reply.


Could you please share your views on modifying PCIE_SPEED2LNKCTL2_TLS to 
the pcie_speed_to_lnkctl2_tls inline function? Or simply put the 
pci_bus_speed2lnkctl2 in bwctrl.c into drivers/pci/pci.h?

Previous version modifications:

diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 12215ee72afb..b5a3ce6c239b 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -419,6 +419,15 @@ void pci_bus_put(struct pci_bus *bus);
  	 (lnkctl2) == PCI_EXP_LNKCTL2_TLS_2_5GT ? PCIE_SPEED_2_5GT : \
  	 PCI_SPEED_UNKNOWN)

+#define PCIE_SPEED2LNKCTL2_TLS(speed) \
+	((speed) == PCIE_SPEED_2_5GT ? PCI_EXP_LNKCTL2_TLS_2_5GT : \
+	 (speed) == PCIE_SPEED_5_0GT ? PCI_EXP_LNKCTL2_TLS_5_0GT : \
+	 (speed) == PCIE_SPEED_8_0GT ? PCI_EXP_LNKCTL2_TLS_8_0GT : \
+	 (speed) == PCIE_SPEED_16_0GT ? PCI_EXP_LNKCTL2_TLS_16_0GT : \
+	 (speed) == PCIE_SPEED_32_0GT ? PCI_EXP_LNKCTL2_TLS_32_0GT : \
+	 (speed) == PCIE_SPEED_64_0GT ? PCI_EXP_LNKCTL2_TLS_64_0GT : \
+	 0)
+
  /* PCIe speed to Mb/s reduced by encoding overhead */
  #define PCIE_SPEED2MBS_ENC(speed) \
  	((speed) == PCIE_SPEED_64_0GT ? 64000*1/1 : \




Current modifications:

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 : \




In the future, I will try to find a good way to modify these macro 
definitions.


Best regards,
Hans