linux-kernel - Re: [PATCH v4 2/4] PCI: of: Add API to retrieve equalization presets from device tree

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Message-ID: <653c86f8-30d3-4ffe-aa2d-ffb37b7b5bb2@oss.qualcomm.com>
Date: Sat, 25 Jan 2025 13:51:46 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>, Jingoo Han <jingoohan1@...il.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>
Cc: linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        quic_mrana@...cinc.com, quic_vbadigan@...cinc.com,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>
Subject: Re: [PATCH v4 2/4] PCI: of: Add API to retrieve equalization presets
 from device tree

On 25.01.2025 1:47 PM, Konrad Dybcio wrote:
> On 24.01.2025 12:22 PM, 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@....qualcomm.com>
>> ---
>>  drivers/pci/of.c  | 47 +++++++++++++++++++++++++++++++++++++++++++++++
>>  drivers/pci/pci.h | 24 +++++++++++++++++++++++-
>>  2 files changed, 70 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
>> index dacea3fc5128..7aa17c0042c5 100644
>> --- a/drivers/pci/of.c
>> +++ b/drivers/pci/of.c
>> @@ -826,3 +826,50 @@ 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
>> + * assign default value 0xff to indicate property is not present.
>> + *
>> + * If the property is not found or is invalid, returns 0.
>> + */
>> +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] = 0xff;

Also please #define 0xff as a reserved value

Konrad

>> +	if (of_property_present(dev->of_node, "eq-presets-8gts")) {
>> +		ret = of_property_read_u16_array(dev->of_node, "eq-presets-8gts",
>> +						 presets->eq_presets_8gts, num_lanes);
>> +		if (ret) {
>> +			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] = 0xff;
>> +		snprintf(name, sizeof(name), "eq-presets-%dgts", 8 << (i + 1));
>> +		if (of_property_present(dev->of_node, name)) {
> 
> of_property_count_u8_elems returns -EINVAL if the property does not exist
> 
> you can then drop a level of indentation:
> 
> ret = of_property_read_u8_array...;
> if (ret == -EINVAL) {
> 	continue;
> } else {
> 	...
> }
> 
> 
> similarly above for 8gts
> 
> Konrad