linux-kernel - Re: [PATCH v7 3/4] PCI: dwc: Improve handling of PCIe lane configuration

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Message-ID: <69f4c303-bba1-7d68-7701-01820ce8701b@oss.qualcomm.com>
Date: Tue, 11 Mar 2025 16:32:41 +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 3/4] PCI: dwc: Improve handling of PCIe lane
 configuration



On 3/6/2025 9:14 AM, Manivannan Sadhasivam wrote:
> On Tue, Feb 25, 2025 at 05:15:06PM +0530, Krishna Chaitanya Chundru wrote:
>> Currently even if the number of lanes hardware supports is equal to
>> the number lanes provided in the devicetree, the driver is trying to
>> configure again the maximum number of lanes which is not needed.
>>
>> Update number of lanes only when it is not equal to hardware capability.
>>
> 
> 'Update max link width only...'
> 
>> And also if the num-lanes property is not present in the devicetree
>> update the num_lanes with the maximum hardware supports.
> 
> '...update 'pci->num_lanes' with the hardware supported maximum link width using
> the newly introduced dw_pcie_link_get_max_link_width() API.'
> 
>>
>> Introduce dw_pcie_link_get_max_link_width() to get the maximum lane
>> width the hardware supports.
>>
>> Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
>> ---
>>   drivers/pci/controller/dwc/pcie-designware-host.c |  3 +++
>>   drivers/pci/controller/dwc/pcie-designware.c      | 11 ++++++++++-
>>   drivers/pci/controller/dwc/pcie-designware.h      |  1 +
>>   3 files changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
>> index ffaded8f2df7..dd56cc02f4ef 100644
>> --- a/drivers/pci/controller/dwc/pcie-designware-host.c
>> +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
>> @@ -504,6 +504,9 @@ int dw_pcie_host_init(struct dw_pcie_rp *pp)
>>   
>>   	dw_pcie_iatu_detect(pci);
>>   
>> +	if (pci->num_lanes < 1)
>> +		pci->num_lanes = dw_pcie_link_get_max_link_width(pci);
>> +
>>   	/*
>>   	 * Allocate the resource for MSG TLP before programming the iATU
>>   	 * outbound window in dw_pcie_setup_rc(). Since the allocation depends
>> diff --git a/drivers/pci/controller/dwc/pcie-designware.c b/drivers/pci/controller/dwc/pcie-designware.c
>> index 145e7f579072..9fc5916867b6 100644
>> --- a/drivers/pci/controller/dwc/pcie-designware.c
>> +++ b/drivers/pci/controller/dwc/pcie-designware.c
>> @@ -737,12 +737,21 @@ static void dw_pcie_link_set_max_speed(struct dw_pcie *pci)
>>   
>>   }
>>   
>> +int dw_pcie_link_get_max_link_width(struct dw_pcie *pci)
>> +{
>> +	u8 cap = dw_pcie_find_capability(pci, PCI_CAP_ID_EXP);
>> +	u32 lnkcap = dw_pcie_readl_dbi(pci, cap + PCI_EXP_LNKCAP);
>> +
>> +	return FIELD_GET(PCI_EXP_LNKCAP_MLW, lnkcap);
>> +}
>> +
>>   static void dw_pcie_link_set_max_link_width(struct dw_pcie *pci, u32 num_lanes)
>>   {
>> +	int max_lanes = dw_pcie_link_get_max_link_width(pci);
>>   	u32 lnkcap, lwsc, plc;
>>   	u8 cap;
>>   
>> -	if (!num_lanes)
>> +	if (max_lanes == num_lanes)
> 
> This gives the assumption that the link width in PCIE_PORT_LINK_CONTROL and
> PCIE_LINK_WIDTH_SPEED_CONTROL registers are same as MLW. Is it really true as
> per the DWC spec?
> 
You are correct both the values are not matching and as we are not sure
side effect of not updating it I will revert this logic.

- Krishna Chaitanya.
> - Mani
>