linux-kernel - Re: [PATCH v2 2/3] PCI: dwc: Add Qualcomm PCIe Endpoint controller driver

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Message-ID: <YMkiSDxOhD7jun3x@builder.lan>
Date:   Tue, 15 Jun 2021 16:57:28 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>, devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Siddartha Mohanadoss <smohanad@...eaurora.org>
Subject: Re: [PATCH v2 2/3] PCI: dwc: Add Qualcomm PCIe Endpoint controller
 driver

On Tue 15 Jun 16:40 CDT 2021, Rob Herring wrote:

> On Sat, Jun 5, 2021 at 9:07 PM Bjorn Andersson
> <bjorn.andersson@...aro.org> wrote:
> >
> > On Thu 03 Jun 05:38 CDT 2021, Manivannan Sadhasivam wrote:
> >
> > > Add driver support for Qualcomm PCIe Endpoint controller driver based on
> > > the Designware core with added Qualcomm specific wrapper around the
> > > core. The driver support is very basic such that it supports only
> > > enumeration, PCIe read/write, and MSI. There is no ASPM and PM support
> > > for now but these will be added later.
> > >
> > > The driver is capable of using the PERST# and WAKE# side-band GPIOs for
> > > operation and written on top of the DWC PCI framework.
> > >
> > > Co-developed-by: Siddartha Mohanadoss <smohanad@...eaurora.org>
> > > Signed-off-by: Siddartha Mohanadoss <smohanad@...eaurora.org>
> > > [mani: restructured the driver and fixed several bugs for upstream]
> > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> >
> > Really nice to see this working!
> 
> [...]
> 
> > > +static void qcom_pcie_ep_configure_tcsr(struct qcom_pcie_ep *pcie_ep)
> > > +{
> > > +     writel_relaxed(0x0, pcie_ep->tcsr + TCSR_PCIE_PERST_EN);
> >
> > Please avoid _relaxed accessor unless there's a strong reason, and if so
> > document it.
> 
> Uhhh, what!? That's the wrong way around from what I've ever seen
> anyone say. Have you ever looked at the resulting code on arm32 with
> OMAP enabled? It's just a memory barrier and an indirect function call
> on every access.
> 
> Use readl/writel if you have an ordering requirement WRT DMA,
> otherwise use relaxed variants.
> 

That does make sense. Unfortunately I don't know where this started, but
I would guess it might have been a reaction to the fact that people
where just sprinkling wmb() all over the place to be on the safe side...

> > > +     writel_relaxed(0x0, pcie_ep->tcsr + TCSR_PERST_SEPARATION_ENABLE);
> > > +}
> > > +
> 
> [...]
> 
> > > +static struct platform_driver qcom_pcie_ep_driver = {
> > > +     .probe  = qcom_pcie_ep_probe,
> > > +     .driver = {
> > > +             .name           = "qcom-pcie-ep",
> >
> > Skip the indentation of the '='.
> >
> > > +             .suppress_bind_attrs = true,
> >
> > Why do we suppress_bind_attrs?
> 
> Because remove is not handled.
> 

Not handled in Mani's driver, or is this a PCI thing?

Regards,
Bjorn