linux-kernel - Re: [PATCH] PCI: qcom: Add interconnect bandwidth for PCIe Gen4

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Message-ID: <09058de7-e207-414b-ab4c-88f0cbde9c22@linaro.org>
Date:   Mon, 25 Sep 2023 12:40:34 +0200
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Abel Vesa <abel.vesa@...aro.org>
Cc:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        lpieralisi@...nel.org, kw@...ux.com, andersson@...nel.org,
        bhelgaas@...gle.com, linux-arm-msm@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: qcom: Add interconnect bandwidth for PCIe Gen4

On 25.09.2023 12:37, Abel Vesa wrote:
> On 23-09-25 12:34:53, Konrad Dybcio wrote:
>> On 25.09.2023 12:33, Abel Vesa wrote:
>>> On 23-09-25 10:57:47, Konrad Dybcio wrote:
>>>> On 24.09.2023 18:07, Manivannan Sadhasivam wrote:
>>>>> PCIe Gen4 supports the interconnect bandwidth of 1969 MBps. So let's add
>>>>> the bandwidth support in the driver. Otherwise, the default bandwidth of
>>>>> 985 MBps will be used.
>>>>>
>>>>> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
>>>>> ---
>>>>>  drivers/pci/controller/dwc/pcie-qcom.c | 7 +++++--
>>>>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/pci/controller/dwc/pcie-qcom.c b/drivers/pci/controller/dwc/pcie-qcom.c
>>>>> index 297442c969b6..6853123f92c1 100644
>>>>> --- a/drivers/pci/controller/dwc/pcie-qcom.c
>>>>> +++ b/drivers/pci/controller/dwc/pcie-qcom.c
>>>>> @@ -1384,11 +1384,14 @@ static void qcom_pcie_icc_update(struct qcom_pcie *pcie)
>>>>>  	case 2:
>>>>>  		bw = MBps_to_icc(500);
>>>>>  		break;
>>>>> +	case 3:
>>>>> +		bw = MBps_to_icc(985);
>>>>> +		break;
>>>>>  	default:
>>>>>  		WARN_ON_ONCE(1);
>>>>>  		fallthrough;
>>>>> -	case 3:
>>>>> -		bw = MBps_to_icc(985);
>>>>> +	case 4:
>>>>> +		bw = MBps_to_icc(1969);
>>>>>  		break;
>>>> Are you adding case 4 under `default`? That looks.. bizzare..
>>>
>>> That's intentional. You want it to use 1969MBps if there is a different
>>> gen value. AFAIU.
>> Gah right, then the commit message is wrong.
> 
> Yep, should be: "Otherwise, the default bandwidth of 1969 MBps will be
> used."
> 
> But maybe we should not default to that. Maybe we should still default
> to 985 MBps.
Perhaps we shouldn't have a default at all..

E.g. if the gen5 bus may get clogged if we exceed gen4
limits

Konrad