Re: [PATCH v7 2/4] PCI: of: Add API to retrieve equalization presets from device tree

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Tue, Feb 25, 2025 at 05:15:05PM +0530, Krishna Chaitanya Chundru wrote:
> PCIe equalization presets are predefined settings used to optimize
> signal integrity by compensating for signal loss and distortion in
> high-speed data transmission.
> 
> As per PCIe spec 6.0.1 revision section 8.3.3.3 & 4.2.4 for data rates
> of 8.0 GT/s, 16.0 GT/s, 32.0 GT/s, and 64.0 GT/s, there is a way to
> configure lane equalization presets for each lane to enhance the PCIe
> link reliability. Each preset value represents a different combination
> of pre-shoot and de-emphasis values. For each data rate, different
> registers are defined: for 8.0 GT/s, registers are defined in section
> 7.7.3.4; for 16.0 GT/s, in section 7.7.5.9, etc. The 8.0 GT/s rate has
> an extra receiver preset hint, requiring 16 bits per lane, while the
> remaining data rates use 8 bits per lane.
> 
> Based on the number of lanes and the supported data rate, this function
> reads the device tree property and stores in the presets structure.
> 
> Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
> ---
>  drivers/pci/of.c  | 43 +++++++++++++++++++++++++++++++++++++++++++
>  drivers/pci/pci.h | 27 ++++++++++++++++++++++++++-
>  2 files changed, 69 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> index 7a806f5c0d20..9ebe7d0e4e0c 100644
> --- a/drivers/pci/of.c
> +++ b/drivers/pci/of.c
> @@ -851,3 +851,46 @@ u32 of_pci_get_slot_power_limit(struct device_node *node,
>  	return slot_power_limit_mw;
>  }
>  EXPORT_SYMBOL_GPL(of_pci_get_slot_power_limit);
> +
> +/**
> + * of_pci_get_equalization_presets - Parses the "eq-presets-Ngts" property.
> + *
> + * @dev: Device containing the properties.
> + * @presets: Pointer to store the parsed data.
> + * @num_lanes: Maximum number of lanes supported.
> + *
> + * If the property is present read and store the data in the preset structure
> + * else assign default value 0xff to indicate property is not present.

'If the property is present, read and store the data in the @presets structure.
Else, assign a default value of PCI_EQ_RESV.'

> + *
> + * Return: 0 if the property is not available or successfully parsed; errno otherwise.
> + */
> +int of_pci_get_equalization_presets(struct device *dev,
> +				    struct pci_eq_presets *presets,
> +				    int num_lanes)
> +{
> +	char name[20];
> +	int ret;
> +
> +	presets->eq_presets_8gts[0] = PCI_EQ_RESV;
> +	ret = of_property_read_u16_array(dev->of_node, "eq-presets-8gts",
> +					 presets->eq_presets_8gts, num_lanes);
> +	if (ret && ret != -EINVAL) {
> +		dev_err(dev, "Error reading eq-presets-8gts :%d\n", ret);
> +		return ret;
> +	}
> +
> +	for (int i = 0; i < EQ_PRESET_TYPE_MAX; i++) {
> +		presets->eq_presets_Ngts[i][0] = PCI_EQ_RESV;
> +		snprintf(name, sizeof(name), "eq-presets-%dgts", 8 << (i + 1));
> +		ret = of_property_read_u8_array(dev->of_node, name,
> +						presets->eq_presets_Ngts[i],
> +						num_lanes);
> +		if (ret && ret != -EINVAL) {
> +			dev_err(dev, "Error reading %s :%d\n", name, ret);
> +			return ret;
> +		}
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(of_pci_get_equalization_presets);
> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index 01e51db8d285..c8d44b21ef03 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -9,6 +9,8 @@ struct pcie_tlp_log;
>  /* Number of possible devfns: 0.0 to 1f.7 inclusive */
>  #define MAX_NR_DEVFNS 256
>  
> +#define MAX_NR_LANES 16
> +
>  #define PCI_FIND_CAP_TTL	48
>  
>  #define PCI_VSEC_ID_INTEL_TBT	0x1234	/* Thunderbolt */
> @@ -808,6 +810,20 @@ static inline u64 pci_rebar_size_to_bytes(int size)
>  
>  struct device_node;
>  
> +#define PCI_EQ_RESV	0xff
> +
> +enum equalization_preset_type {

For the sake of completeness, you should add EQ_PRESET_TYPE_8GTS also. You could
skip it while reading the of_property_read_u8_array().

> +	EQ_PRESET_TYPE_16GTS,
> +	EQ_PRESET_TYPE_32GTS,
> +	EQ_PRESET_TYPE_64GTS,
> +	EQ_PRESET_TYPE_MAX
> +};
> +
> +struct pci_eq_presets {
> +	u16 eq_presets_8gts[MAX_NR_LANES];
> +	u8 eq_presets_Ngts[EQ_PRESET_TYPE_MAX][MAX_NR_LANES];
> +};
> +
>  #ifdef CONFIG_OF
>  int of_get_pci_domain_nr(struct device_node *node);
>  int of_pci_get_max_link_speed(struct device_node *node);
> @@ -822,7 +838,9 @@ void pci_release_bus_of_node(struct pci_bus *bus);
>  
>  int devm_of_pci_bridge_init(struct device *dev, struct pci_host_bridge *bridge);
>  bool of_pci_supply_present(struct device_node *np);
> -
> +int of_pci_get_equalization_presets(struct device *dev,
> +				    struct pci_eq_presets *presets,
> +				    int num_lanes);
>  #else
>  static inline int
>  of_get_pci_domain_nr(struct device_node *node)
> @@ -867,6 +885,13 @@ static inline bool of_pci_supply_present(struct device_node *np)
>  {
>  	return false;
>  }
> +
> +static inline int of_pci_get_equalization_presets(struct device *dev,
> +						  struct pci_eq_presets *presets,
> +						  int num_lanes)
> +{

Don't you need to initialize presets to PCI_EQ_RESV?

- Mani

-- 
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