linux-kernel - Re: [PATCH v4 1/5] PCI: designware: add memory barrier after enabling region

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Date:	Wed, 09 Dec 2015 10:52:27 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Pratyush Anand <pratyush.anand@...il.com>
Cc:	Stanimir Varbanov <stanimir.varbanov@...aro.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Bjorn Andersson <bjorn.andersson@...ymobile.com>
Subject: Re: [PATCH v4 1/5] PCI: designware: add memory barrier after enabling region

On Wednesday 09 December 2015 10:10:05 Pratyush Anand wrote:
> On Tue, Dec 8, 2015 at 2:31 PM, Stanimir Varbanov
> > > Signed-off-by: Stanimir Varbanov <stanimir.varbanov@...aro.org>
> > > ---
> > >  drivers/pci/host/pcie-designware.c |    5 +++++
> > >  1 file changed, 5 insertions(+)
> > >
> > > diff --git a/drivers/pci/host/pcie-designware.c b/drivers/pci/host/pcie-designware.c
> > > index 02a7452bdf23..ed4dc2e2553b 100644
> > > --- a/drivers/pci/host/pcie-designware.c
> > > +++ b/drivers/pci/host/pcie-designware.c
> > > @@ -164,6 +164,11 @@ static void dw_pcie_prog_outbound_atu(struct pcie_port *pp, int index,
> > >       dw_pcie_writel_rc(pp, upper_32_bits(pci_addr), PCIE_ATU_UPPER_TARGET);
> > >       dw_pcie_writel_rc(pp, type, PCIE_ATU_CR1);
> > >       dw_pcie_writel_rc(pp, PCIE_ATU_ENABLE, PCIE_ATU_CR2);
> > > +     /*
> > > +      * ensure that the ATU enable has been happaned before accessing
> > > +      * pci configuration/io spaces through dw_pcie_cfg_[read|write].
> > > +      */
> > > +     wmb();
> > >  }
> > >
> 
> 
> My understnading is that since writel() of dw_pcie_writel_rc() in
> above code and readl(), writel() of dw_pcie_cfg_[read|write]() (which
> will follow) goes through same device (ie PCIe host here). So, it is
> guaranteed that 1st writel() will be executed before later
> readl()/writel(). If that is true then we do not need any explicit
> barrier here.
> 
> Arnd, Russel: whats your opinion here.

I think the ordering is only enforced if the two register accesses are
on the same device as seen from the bus, and it's possible that the
RC registers and the config space registers are not considered the
same thing here.

For config write, this is not a problem, because the config space write
has a wmb() that enforces ordering, but it's possible that the config
space read may hit the device in parallel with the PCIE_ATU_ENABLE
write.

	Arnd
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