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

[Konrad Dybcio]

On 3/29/25 7:30 AM, Manivannan Sadhasivam wrote:
> On Fri, Mar 28, 2025 at 10:53:19PM +0100, Konrad Dybcio wrote:
>> 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@xxxxxxxxxxxxxxxx>
>>>>>> ---
>>>>>>   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
>>
> 
> I have seen the worse... The problem is, if those controllers start to show up
> and define preset properties in DT, there will be no errors whatsoever to
> indicate that the preset values were not applied, resulting in hard to debug
> errors.

else {
	dev_warn(pci->dev, "Missing equalization presets programming sequence\n");
}

> 
> I'm not forseeing any issue in this part of the code to support higher GEN
> speeds though.

I would hope so as well, but both not programming and misprogramming are
equally hard to detect

Konrad