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

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Date:   Tue, 15 Jun 2021 17:58:25 -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 17:20 CDT 2021, Rob Herring wrote:

>  On Tue, Jun 15, 2021 at 3:57 PM Bjorn Andersson
> <bjorn.andersson@...aro.org> wrote:
> >
> > 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...
> 
> I think you could write a book on it. In the beginning, there was x86
> and it was strongly ordered...
> 

I guess it would allow me to ask people to RTFM (RTFB) instead then :)

Jokes aside, I think we came to the conclusion that writel() was better
than incorrect use of writel_relaxed() followed by wmb(). And in this
particular case it's definitely not happening in a hot code path...

> >
> > > > > +     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?
> 
> Only a PCI thing in the sense all the drivers happen to copy-n-paste
> it and are mostly built-in. The Android modules thing doesn't seem to
> have quite hit PCI yet. On the flipside, I'm sure there's lots of
> drivers we can unbind and fun will ensue.
> 
> There is some work needed as the remove() implementations that we do
> have are all unique (such as do we need a lock or not). I keep nudging
> people to look into it.
> 

Thanks, that confirms that my expectation. I would prefer to see this
tackled in a separate step then :)

Regards,
Bjorn