linux-kernel - Re: [PATCH 2/2] PCI: qcom-ep: Implement dbi_cs2_access() function callback for DBI CS2 access

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Date:   Tue, 17 Oct 2023 23:11:00 +0530
From:   Manivannan Sadhasivam <mani@...nel.org>
To:     Bjorn Andersson <quic_bjorande@...cinc.com>
Cc:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH 2/2] PCI: qcom-ep: Implement dbi_cs2_access() function
 callback for DBI CS2 access

On Tue, Oct 17, 2023 at 09:56:09AM -0700, Bjorn Andersson wrote:
> On Tue, Oct 17, 2023 at 09:51:29PM +0530, Manivannan Sadhasivam wrote:
> > On Tue, Oct 17, 2023 at 07:24:31AM -0700, Bjorn Andersson wrote:
> > > On Tue, Oct 17, 2023 at 11:47:55AM +0530, Manivannan Sadhasivam wrote:
> > > > From: Manivannan Sadhasivam <mani@...nel.org>
> > > 
> > > Your S-o-b should match this.
> > > 
> > 
> > I gave b4 a shot for sending the patches and missed this. Will fix it in next
> > version.
> > 
> > > > 
> > > > Qcom EP platforms require enabling/disabling the DBI CS2 access while
> > > > programming some read only and shadow registers through DBI. So let's
> > > > implement the dbi_cs2_access() callback that will be called by the DWC core
> > > > while programming such registers like BAR mask register.
> > > > 
> > > > Without DBI CS2 access, writes to those registers will not be reflected.
> > > > 
> > > > Fixes: f55fee56a631 ("PCI: qcom-ep: Add Qualcomm PCIe Endpoint controller driver")
> > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > > > ---
> > > >  drivers/pci/controller/dwc/pcie-qcom-ep.c | 14 ++++++++++++++
> > > >  1 file changed, 14 insertions(+)
> > > > 
> > > > diff --git a/drivers/pci/controller/dwc/pcie-qcom-ep.c b/drivers/pci/controller/dwc/pcie-qcom-ep.c
> > > > index 32c8d9e37876..4653cbf7f9ed 100644
> > > > --- a/drivers/pci/controller/dwc/pcie-qcom-ep.c
> > > > +++ b/drivers/pci/controller/dwc/pcie-qcom-ep.c
> > > > @@ -124,6 +124,7 @@
> > > >  
> > > >  /* ELBI registers */
> > > >  #define ELBI_SYS_STTS				0x08
> > > > +#define ELBI_CS2_ENABLE				0xa4
> > > >  
> > > >  /* DBI registers */
> > > >  #define DBI_CON_STATUS				0x44
> > > > @@ -262,6 +263,18 @@ static void qcom_pcie_dw_stop_link(struct dw_pcie *pci)
> > > >  	disable_irq(pcie_ep->perst_irq);
> > > >  }
> > > >  
> > > > +static void qcom_pcie_dbi_cs2_access(struct dw_pcie *pci, bool enable)
> > > > +{
> > > > +	struct qcom_pcie_ep *pcie_ep = to_pcie_ep(pci);
> > > > +
> > > > +	writel_relaxed(enable, pcie_ep->elbi + ELBI_CS2_ENABLE);
> > > 
> > > Don't you want to maintain the ordering of whatever write came before
> > > this?
> > > 
> > 
> > Since this in a dedicated function, I did not care about the ordering w.r.t
> > previous writes. Even if it gets inlined, the order should not matter since it
> > only enables/disables the CS2 access for the forthcoming writes.
> > 
> 
> The wmb() - in a non-relaxed writel -  would ensure that no earlier
> writes are reordered and end up in your expected set of "forthcoming
> writes".
> 

I was under the impression that the readl_relaxed() here serves as an implicit
barrier. But reading the holy memory-barriers documentation doesn't explicitly
say so. So I'm going to add wmb() to be on the safe side as you suggested.

Thanks for pointing it out.

- Mani

> Not sure that the code is wrong, I just want you to be certain that this
> isn't a problem.
> 
> Thanks,
> Bjorn

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