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

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Message-ID: <20230927125850.GA19623@thinkpad>
Date:   Wed, 27 Sep 2023 14:58:50 +0200
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Konrad Dybcio <konrad.dybcio@...aro.org>
Cc:     Abel Vesa <abel.vesa@...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 Mon, Sep 25, 2023 at 12:40:34PM +0200, Konrad Dybcio wrote:
> 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
> 

So the idea here is that if we happen to run this driver on a new Gen supported
SoC, we have to let the user know that the interconnects are running at a lower
gen speed and it needs attention.

But I think we can simplify it by fixing a default bandwidth, say Gen3 and get
rid of the fallthrough. And yeah, the same needs to be done for the pcie-qcom-ep
driver as well.

- Mani

> Konrad

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