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Message-ID: <35f6d6f3-b857-48cc-b3cb-11a27675adfd@163.com>
Date: Sun, 17 Aug 2025 23:02:10 +0800
From: Hans Zhang <18255117159@....com>
To: Lukas Wunner <lukas@...ner.de>
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 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
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