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

[Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>]

On 3/6/2025 8:52 AM, Manivannan Sadhasivam wrote:
> 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().
Can we add like this to make parsing logic easier otherwise while
deference the presets array we need to subtract -1.
currently we are using like this presets[EQ_PRESET_TYPE_16GTS] if
we want to keep in same way we need to use like below.

	EQ_PRESET_TYPE_8GTS,
	EQ_PRESET_TYPE_16GTS = 0,
> 
>> +	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?
> 
I will update in the next patch.

- Krishna Chaitanya.
> - Mani
>