Re: [PATCH v2 1/3] PCI/ASPM: Add helper to encode L1SS T_POWER_ON fields

[Bjorn Helgaas]

On Mon, Feb 23, 2026 at 04:43:30PM +0530, Krishna Chaitanya Chundru wrote:
> Add a shared helper to encode the PCIe L1 PM Substates T_POWER_ON
> parameter into the T_POWER_ON_Scale and T_POWER_ON_Value fields.
> 
> This helper can be used by the controller drivers to change the
> default/wrong value of T_POWER_ON in L1ss capability register to
> avoid incorrect calculation of LTR_L1.2_THRESHOLD value.
> 
> The helper converts a T_POWER_ON time specified in microseconds into
> the appropriate scale/value encoding defined by the PCIe spec r7.0,
> sec 7.8.3.2. Values that exceed the maximum encodable range are clamped
> to the largest representable encoding.
> 
> Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@xxxxxxxxxxxxxxxx>
> ---
>  drivers/pci/pcie/aspm.c | 43 +++++++++++++++++++++++++++++++++++++++++++
>  include/linux/pci.h     |  2 ++
>  2 files changed, 45 insertions(+)
> 
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index 21f5d23e0b61bd7e1163cc869fe9356d1ab87b34..d7f9ae9e48c25dbc2d9b4887e2f74623688098e0 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -525,6 +525,49 @@ static u32 calc_l12_pwron(struct pci_dev *pdev, u32 scale, u32 val)
>  	return 0;
>  }
>  
> +/**
> + * pcie_encode_t_power_on - Encode T_POWER_ON into scale and value fields
> + * @t_power_on_us: T_POWER_ON time in microseconds
> + * @scale: Encoded T_POWER_ON_Scale (0..2)
> + * @value: Encoded T_POWER_ON_Value
> + *
> + * T_POWER_ON is encoded as:
> + *   T_POWER_ON(us) = scale_unit(us) * value
> + *
> + * where scale_unit is selected by @scale:
> + *   0: 2us
> + *   1: 10us
> + *   2: 100us
> + *
> + * If @t_power_on_us exceeds the maximum representable value, the result
> + * is clamped to the largest encodable T_POWER_ON.
> + *
> + * See PCIe r7.0, sec 7.8.3.2.
> + */
> +void pcie_encode_t_power_on(u16 t_power_on_us, u8 *scale, u8 *value)
> +{
> +	u8 maxv = FIELD_MAX(PCI_L1SS_CTL2_T_PWR_ON_VALUE);
> +
> +	/*
> +	 * T_POWER_ON_Value ("value") is a 5-bit field with max
> +	 * value of 31.
> +	 */
> +	if (t_power_on_us <= 2 * maxv) {
> +		*scale = 0; /* Value times 2us */
> +		*value = DIV_ROUND_UP(t_power_on_us, 2);
> +	} else if (t_power_on_us <= 10 * maxv) {
> +		*scale = 1; /* Value times 10us */
> +		*value = DIV_ROUND_UP(t_power_on_us, 10);
> +	} else if (t_power_on_us <= 100 * maxv) {
> +		*scale = 2; /* value times 100us */
> +		*value = DIV_ROUND_UP(t_power_on_us, 100);
> +	} else {
> +		*scale = 2;
> +		*value = maxv;
> +	}
> +}
> +EXPORT_SYMBOL(pcie_encode_t_power_on);
> +
>  /*
>   * Encode an LTR_L1.2_THRESHOLD value for the L1 PM Substates Control 1
>   * register.  Ports enter L1.2 when the most recent LTR value is greater
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 1c270f1d512301de4d462fe7e5097c32af5c6f8d..eec16fdcb9996ab0f663f4587a2367a676a49ce6 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -1911,6 +1911,7 @@ int pci_enable_link_state_locked(struct pci_dev *pdev, int state);
>  void pcie_no_aspm(void);
>  bool pcie_aspm_support_enabled(void);
>  bool pcie_aspm_enabled(struct pci_dev *pdev);
> +void pcie_encode_t_power_on(u16 t_power_on_us, u8 *scale, u8 *value);

This looks like it should go in drivers/pci/pci.h.  I don't think it's
needed outside drivers/pci/.

>  #else
>  static inline int pci_disable_link_state(struct pci_dev *pdev, int state)
>  { return 0; }
> @@ -1923,6 +1924,7 @@ static inline int pci_enable_link_state_locked(struct pci_dev *pdev, int state)
>  static inline void pcie_no_aspm(void) { }
>  static inline bool pcie_aspm_support_enabled(void) { return false; }
>  static inline bool pcie_aspm_enabled(struct pci_dev *pdev) { return false; }
> +static inline void pcie_encode_t_power_on(u16 t_power_on_us, u8 *scale, u8 *value) { }
>  #endif
>  
>  #ifdef CONFIG_HOTPLUG_PCI
> 
> -- 
> 2.34.1
>