Re: [PATCH v5 1/3] PCI/ASPM: Add helper to encode L1SS T_POWER_ON fields

[Manivannan Sadhasivam]

On Wed, Apr 29, 2026 at 08:34:58AM +0530, Krishna Chaitanya Chundru wrote:
> 
> 
> On 4/28/2026 2:57 PM, Ilpo Järvinen wrote:
> > On Tue, 28 Apr 2026, 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.
> >>
> >> Tested-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
> >> Reviewed-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
> >> Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@xxxxxxxxxxxxxxxx>
> >> ---
> >>  drivers/pci/pci.h       |  6 ++++++
> >>  drivers/pci/pcie/aspm.c | 40 ++++++++++++++++++++++++++++++++++++++++
> >>  2 files changed, 46 insertions(+)
> >>
> >> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> >> index 4a14f88e543a..c379befe1ebe 100644
> >> --- a/drivers/pci/pci.h
> >> +++ b/drivers/pci/pci.h
> >> @@ -1110,6 +1110,7 @@ void pcie_aspm_pm_state_change(struct pci_dev *pdev, bool locked);
> >>  void pcie_aspm_powersave_config_link(struct pci_dev *pdev);
> >>  void pci_configure_ltr(struct pci_dev *pdev);
> >>  void pci_bridge_reconfigure_ltr(struct pci_dev *pdev);
> >> +void pcie_encode_t_power_on(u16 t_power_on_us, u8 *scale, u8 *value);
> >>  #else
> >>  static inline void pcie_aspm_remove_cap(struct pci_dev *pdev, u32 lnkcap) { }
> >>  static inline void pcie_aspm_init_link_state(struct pci_dev *pdev) { }
> >> @@ -1118,6 +1119,11 @@ static inline void pcie_aspm_pm_state_change(struct pci_dev *pdev, bool locked)
> >>  static inline void pcie_aspm_powersave_config_link(struct pci_dev *pdev) { }
> >>  static inline void pci_configure_ltr(struct pci_dev *pdev) { }
> >>  static inline void pci_bridge_reconfigure_ltr(struct pci_dev *pdev) { }
> >> +static inline void pcie_encode_t_power_on(u16 t_power_on_us, u8 *scale, u8 *value)
> >> +{
> >> +	*scale = 0;
> >> +	*value = 0;
> >> +}
> >>  #endif
> >>  
> >>  #ifdef CONFIG_PCIE_ECRC
> >> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> >> index 925373b98dff..457d469b8d49 100644
> >> --- a/drivers/pci/pcie/aspm.c
> >> +++ b/drivers/pci/pcie/aspm.c
> >> @@ -525,6 +525,46 @@ 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)
> > Hi,
> >
> > I don't know how the type for t_power_on_us was picked but if it was 
> > arbitrary decision, I suggest you just go with 32-bit input.
> The maximum value of the T power ON is 3100us, so we are using u16 here.
> 

FWIW, I made sure dt-binding restricts the max value to 3100us:
https://github.com/devicetree-org/dt-schema/commit/3579443c5d2f5a8bbedd1ad8e6cc634ce0aa02e3

- Mani

-- 
மணிவண்ணன் சதாசிவம்