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Message-ID: <9be6ce8e-f0e2-7226-e900-3a0c2506a16a@oss.qualcomm.com>
Date: Tue, 11 Mar 2025 16:31:33 +0530
From: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc: 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>,
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,
quic_mrana@...cinc.com, quic_vbadigan@...cinc.com
Subject: Re: [PATCH v7 2/4] PCI: of: Add API to retrieve equalization presets
from device tree
On 3/6/2025 8:52 AM, Manivannan Sadhasivam wrote:
> On Tue, Feb 25, 2025 at 05:15:05PM +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 | 43 +++++++++++++++++++++++++++++++++++++++++++
>> drivers/pci/pci.h | 27 ++++++++++++++++++++++++++-
>> 2 files changed, 69 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
>> index 7a806f5c0d20..9ebe7d0e4e0c 100644
>> --- a/drivers/pci/of.c
>> +++ b/drivers/pci/of.c
>> @@ -851,3 +851,46 @@ 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
>> + * else assign default value 0xff to indicate property is not present.
>
> 'If the property is present, read and store the data in the @presets structure.
> Else, assign a default value of PCI_EQ_RESV.'
>
>> + *
>> + * 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] = PCI_EQ_RESV;
>> + ret = of_property_read_u16_array(dev->of_node, "eq-presets-8gts",
>> + presets->eq_presets_8gts, num_lanes);
>> + if (ret && ret != -EINVAL) {
>> + 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] = PCI_EQ_RESV;
>> + snprintf(name, sizeof(name), "eq-presets-%dgts", 8 << (i + 1));
>> + ret = of_property_read_u8_array(dev->of_node, name,
>> + presets->eq_presets_Ngts[i],
>> + num_lanes);
>> + if (ret && ret != -EINVAL) {
>> + dev_err(dev, "Error reading %s :%d\n", name, ret);
>> + return ret;
>> + }
>> + }
>> +
>> + return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(of_pci_get_equalization_presets);
>> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
>> index 01e51db8d285..c8d44b21ef03 100644
>> --- a/drivers/pci/pci.h
>> +++ b/drivers/pci/pci.h
>> @@ -9,6 +9,8 @@ struct pcie_tlp_log;
>> /* Number of possible devfns: 0.0 to 1f.7 inclusive */
>> #define MAX_NR_DEVFNS 256
>>
>> +#define MAX_NR_LANES 16
>> +
>> #define PCI_FIND_CAP_TTL 48
>>
>> #define PCI_VSEC_ID_INTEL_TBT 0x1234 /* Thunderbolt */
>> @@ -808,6 +810,20 @@ static inline u64 pci_rebar_size_to_bytes(int size)
>>
>> struct device_node;
>>
>> +#define PCI_EQ_RESV 0xff
>> +
>> +enum equalization_preset_type {
>
> For the sake of completeness, you should add EQ_PRESET_TYPE_8GTS also. You could
> skip it while reading the of_property_read_u8_array().
Can we add like this to make parsing logic easier otherwise while
deference the presets array we need to subtract -1.
currently we are using like this presets[EQ_PRESET_TYPE_16GTS] if
we want to keep in same way we need to use like below.
EQ_PRESET_TYPE_8GTS,
EQ_PRESET_TYPE_16GTS = 0,
>
>> + EQ_PRESET_TYPE_16GTS,
>> + EQ_PRESET_TYPE_32GTS,
>> + EQ_PRESET_TYPE_64GTS,
>> + EQ_PRESET_TYPE_MAX
>> +};
>> +
>> +struct pci_eq_presets {
>> + u16 eq_presets_8gts[MAX_NR_LANES];
>> + u8 eq_presets_Ngts[EQ_PRESET_TYPE_MAX][MAX_NR_LANES];
>> +};
>> +
>> #ifdef CONFIG_OF
>> int of_get_pci_domain_nr(struct device_node *node);
>> int of_pci_get_max_link_speed(struct device_node *node);
>> @@ -822,7 +838,9 @@ void pci_release_bus_of_node(struct pci_bus *bus);
>>
>> int devm_of_pci_bridge_init(struct device *dev, struct pci_host_bridge *bridge);
>> bool of_pci_supply_present(struct device_node *np);
>> -
>> +int of_pci_get_equalization_presets(struct device *dev,
>> + struct pci_eq_presets *presets,
>> + int num_lanes);
>> #else
>> static inline int
>> of_get_pci_domain_nr(struct device_node *node)
>> @@ -867,6 +885,13 @@ static inline bool of_pci_supply_present(struct device_node *np)
>> {
>> return false;
>> }
>> +
>> +static inline int of_pci_get_equalization_presets(struct device *dev,
>> + struct pci_eq_presets *presets,
>> + int num_lanes)
>> +{
>
> Don't you need to initialize presets to PCI_EQ_RESV?
>
I will update in the next patch.
- Krishna Chaitanya.
> - Mani
>
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