Re: [PATCH v8 2/4] PCI: of: Add of_pci_get_equalization_presets() API

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

On 3/28/2025 10:09 AM, Manivannan Sadhasivam wrote:
> On Sun, Mar 16, 2025 at 09:39:02AM +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,
>> of_pci_get_equalization_presets() 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  | 44 ++++++++++++++++++++++++++++++++++++++++++++
>>   drivers/pci/pci.h | 32 +++++++++++++++++++++++++++++++-
>>   2 files changed, 75 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
>> index 7a806f5c0d20..18691483e108 100644
>> --- a/drivers/pci/of.c
>> +++ b/drivers/pci/of.c
>> @@ -851,3 +851,47 @@ 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 @presets structure.
>> + * Else, assign a default value of PCI_EQ_RESV.
>> + *
>> + * Return: 0 if the property is not available or successfully parsed else
>> + * 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);
> 
> 'Error reading eq-presets-8gts: %d'
> 
>> +		return ret;
>> +	}
>> +
>> +	for (int i = 0; i < EQ_PRESET_TYPE_MAX - 1; 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);
> 
> 'Error reading %s: %d'
> 
>> +			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..78c9cc0ad8fa 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
> 
> Why did you limit to 16?
> 
As per PCIe spec we support maximum of 16 lanes only right

- Krishna Chaitanya.
> - Mani
>