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

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Message-ID: <7dd7ecd2-c8a1-4800-8746-2b6166d3ae1c@oss.qualcomm.com>
Date: Sat, 16 Nov 2024 11:59:38 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Krishna chaitanya chundru <quic_krichai@...cinc.com>,
        quic_mrana@...cinc.com, quic_vbadigan@...cinc.com, kernel@...cinc.com,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, 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
Subject: Re: [PATCH 2/4] PCI: of: Add API to retrieve equalization presets
 from device tree

On 16.11.2024 2:37 AM, 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 <quic_krichai@...cinc.com>
> ---
>  drivers/pci/of.c  | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  drivers/pci/pci.h | 17 +++++++++++++--
>  2 files changed, 77 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> index dacea3fc5128..0d37bc231956 100644
> --- a/drivers/pci/of.c
> +++ b/drivers/pci/of.c
> @@ -826,3 +826,65 @@ 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);
> +
> +int of_pci_get_equalization_presets(struct device *dev,
> +				    struct pci_eq_presets *presets,
> +				    int num_lanes)
> +{
> +	int ret;
> +
> +	if (of_property_present(dev->of_node, "eq-presets-8gts")) {
> +		presets->eq_presets_8gts = devm_kzalloc(dev, sizeof(u16) * num_lanes, GFP_KERNEL);
> +		if (!presets->eq_presets_8gts)

If you make this an array with enum indices, you can make a for loop and
read "eq-presets-%ugts", (8 << i)

Konrad