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

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Message-ID: <7d905563-137d-5c0e-42a1-8b93f4d630ce@quicinc.com>
Date: Wed, 27 Nov 2024 07:28:10 +0530
From: Krishna Chaitanya Chundru <quic_krichai@...cinc.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.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 11/16/2024 4:29 PM, Konrad Dybcio wrote:
> 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
as "eq-presets-8gts" is u16 array and other properties are u8 array.
I will use as it is for "eq-presets-8gts", but for remaining properties
I will use for loop as suggested by you.

- Krishna Chaitanya.