linux-kernel - Re: [PATCH v8 4/4] PCI: dwc: Add support for configuring lane equalization presets

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Message-ID: <5fece4ac-2899-4e7d-8205-3b1ebba4b56b@oss.qualcomm.com>
Date: Fri, 28 Mar 2025 22:53:19 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
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 v8 4/4] PCI: dwc: Add support for configuring lane
 equalization presets

On 3/28/25 7:45 AM, Manivannan Sadhasivam wrote:
> On Fri, Mar 28, 2025 at 11:04:11AM +0530, Krishna Chaitanya Chundru wrote:
>>
>>
>> On 3/28/2025 10:23 AM, Manivannan Sadhasivam wrote:
>>> On Sun, Mar 16, 2025 at 09:39:04AM +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.
>>>>
>>>> Based upon the number of lanes and the data rate supported, write
>>>> the preset data read from the device tree in to the lane equalization
>>>> control registers.
>>>>
>>>> Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
>>>> ---
>>>>   drivers/pci/controller/dwc/pcie-designware-host.c | 60 +++++++++++++++++++++++
>>>>   drivers/pci/controller/dwc/pcie-designware.h      |  3 ++
>>>>   include/uapi/linux/pci_regs.h                     |  3 ++
>>>>   3 files changed, 66 insertions(+)
>>>>
>>>> diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
>>>> index dd56cc02f4ef..7c6e6a74383b 100644
>>>> --- a/drivers/pci/controller/dwc/pcie-designware-host.c
>>>> +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
>>>> @@ -507,6 +507,10 @@ int dw_pcie_host_init(struct dw_pcie_rp *pp)
>>>>   	if (pci->num_lanes < 1)
>>>>   		pci->num_lanes = dw_pcie_link_get_max_link_width(pci);
>>>> +	ret = of_pci_get_equalization_presets(dev, &pp->presets, pci->num_lanes);
>>>> +	if (ret)
>>>> +		goto err_free_msi;
>>>> +
>>>>   	/*
>>>>   	 * Allocate the resource for MSG TLP before programming the iATU
>>>>   	 * outbound window in dw_pcie_setup_rc(). Since the allocation depends
>>>> @@ -808,6 +812,61 @@ static int dw_pcie_iatu_setup(struct dw_pcie_rp *pp)
>>>>   	return 0;
>>>>   }
>>>> +static void dw_pcie_program_presets(struct dw_pcie_rp *pp, enum pci_bus_speed speed)
>>>> +{
>>>> +	struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
>>>> +	u8 lane_eq_offset, lane_reg_size, cap_id;
>>>> +	u8 *presets;
>>>> +	u32 cap;
>>>> +	int i;
>>>> +
>>>> +	if (speed == PCIE_SPEED_8_0GT) {
>>>> +		presets = (u8 *)pp->presets.eq_presets_8gts;
>>>> +		lane_eq_offset =  PCI_SECPCI_LE_CTRL;
>>>> +		cap_id = PCI_EXT_CAP_ID_SECPCI;
>>>> +		/* For data rate of 8 GT/S each lane equalization control is 16bits wide*/
>>>> +		lane_reg_size = 0x2;
>>>> +	} else if (speed == PCIE_SPEED_16_0GT) {
>>>> +		presets = pp->presets.eq_presets_Ngts[EQ_PRESET_TYPE_16GTS - 1];
>>>> +		lane_eq_offset = PCI_PL_16GT_LE_CTRL;
>>>> +		cap_id = PCI_EXT_CAP_ID_PL_16GT;
>>>> +		lane_reg_size = 0x1;
>>>> +	} else {
>>>
>>> Can you add conditions for other data rates also? Like 32, 64 GT/s. If
>>> controller supports them and if the presets property is defined in DT, then you
>>> should apply the preset values.
>>>
>>> If the presets property is not present in DT, then below 'PCI_EQ_RESV' will
>>> safely return.
>>>
>> I am fine to add it, but there is no GEN5 or GEN6 controller support
>> added in dwc, isn't it best to add when that support is added and
>> tested.
>>
> 
> What is the guarantee that this part of the code will be updated once the
> capable controllers start showing up? I don't think there will be any issue in
> writing to these registers.

Let's not make assumptions about the spec of a cross-vendor mass-deployed IP

Konrad