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

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Message-ID: <79281aca-c275-4055-8d2c-d2407b0f9811@oss.qualcomm.com>
Date: Sat, 25 Jan 2025 13:47:27 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: 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,
        konrad.dybcio@....qualcomm.com, 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 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;
> +	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