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

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Message-ID: <20250203150849.nexmt3ywv7x4b7e3@thinkpad>
Date: Mon, 3 Feb 2025 20:38:49 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
Cc: 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>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	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 Fri, Jan 24, 2025 at 04:52:48PM +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@....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.

Use "Return:" prefix to define the return value. Like,

	"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] = 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);

As Konrad mentioned, you can use -EINVAL from these APIs to determine whether
the property exists or not.

> +		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)) {
> +			ret = of_property_read_u8_array(dev->of_node, name,
> +							presets->eq_presets_Ngts[i],
> +							num_lanes);
> +			if (ret) {
> +				dev_err(dev, "Error %s %d\n", name, ret);

"Error reading..."

- Mani

-- 
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